WO2013162902A1 - Synergistic combination of a fluid loss additive and rheology modifier - Google Patents

Description

SYNERGISTIC COMBINATION OF A FLUID LOSS ADDITIVE AND RHEOLOGY

MODIFIER

FIELD OF THE INVENTION

[0001] The present application is directed to compositions containing a synergistic combination of a fluid loss additive and a rheology modifier, in particular, to a synergistic combination of polymers for oil-field drilling applications. In accordance with one aspect of the invention, the fluid loss additive comprises a terpolymer of acrylamide (AM), 2- acrylamido-2-methyl-propanesulfonic acid (AMPS) and a cationic monomer such as acrylamidopropyl-trimethyl ammonium chloride (APTAC) and/or

methacrylamidopropyltrimethyl ammonium chloride (MAPTAC) and the rheology modifier comprises a terpolymer of acrylamide, 2-acrylamido-2-methyl-propanesulfonic acid and a long-chain alkyl acrylate.

[0002] In accordance with particularly useful embodiments, the fluid loss additive comprises from about 20 -80 wt. % acrylamide, 80 -20 wt. % AMPS, and 2 -50 wt. % APTAC and/or MAPTAC and the rheology modifier comprises from about 30 -70 wt. % acrylamide, 70-30 wt. % AMPS, and 0.01 - 5 wt. % long-chain alkyl acrylate. AMPS can be in either acid or neutralized form used in the polymerization process.

BACKGROUND

[0003] Wellbore fluids are utilized in the construction, repair or treatment of wellbores such as those that are drilled through earth formations in order to access reservoirs of oil, gas or water, or to access geothermal heat.

[0004] The term "wellbore fluid" as used herein means any liquid that serves a useful function when it is placed in a well during the processes of well construction, well treatment, or the repair of a well.

[0005] The term "rheology modifier" as used herein refers to a polymer that provides significant thickening effect at relatively low concentrations. Typically, a rheology modifier polymer when dissolved in fresh water at a concentration of 1 % active by weight provides a Brookfield viscosity of at least 200 cps at room temperature. [0006] The wellbore fluids of the present invention are suitable for use in a variety of wellbores including wellbores assigned to oil and/or gas production, water or gas injection wellbores, water production wellbores, and geothermal wellbores.

[0007] Wellbore fluids used during well construction include drilling fluids, lost circulation control fluids, spotting fluids such as those used to help free drill pipe that has become stuck in the well, under-reaming fluids, completion fluids such as brines used to control formation pressures, perforating pills, brine loss-control pills (a "pill" is a relatively small volume of wellbore fluid, usually less than 200 barrels, that is pumped into the desired position in a wellbore to accomplish its function), fluids used during gravel-packing operations, cement slurries, and packer fluids.

[0008] Wellbore fluids typically used as well-treatment fluids include clean-up fluids that are pumped to effect the removal of residues from the well, acidic treatment fluids, fracturing fluids, and viscous fluids pumped into a permeable formation for the purpose of diverting flow into other formations or for shutting-off the flow of produced water.

[0009] Wellbore fluids used during well repair ("workover") operations include workover fluids such as a kill fluid that is pumped into a well, the kill fluid having sufficient density to stop ("kill") the production of formation fluids. Workover operations include the milling out of old downhole hardware, and can use any of the fluids listed above as required to effect the repair or re-completion of the well.

[0010] Drilling fluids are utilized when drilling a wellbore through rock formations in order to sweep the rock cuttings created at the bit up to the surface where they are removed. To control downhole pressures, the fluid's density is usually increased by the addition of a powdered dense mineral. The fluid should therefore exhibit sufficient viscosity to provide efficient cuttings removal (hole-cleaning), and sufficient gel strength for the stable suspension of the mineral. Drilling Fluids should also exhibit a low filtration rate (Fluid Loss) in order to lessen the possibility of differential sticking.

[0011] Completion fluids are utilized during operations that take place in the so-called completion phase of wellbore construction, which is after drilling the wellbore and before commencement of production of fluids into the wellbore (or before injection of fluids from the wellbore into a rock formation). Frequently, a completion fluid will need to be viscosified to transport or suspend dispersed solid particles, and water-soluble polymers are also used to minimize the loss of completion fluid or filtrate into the permeable formation.

[0012] Treatment fluids may be utilized intermittently during the life of a wellbore, for example, when conducting stimulation or remedial operations in a rock formation penetrated by the wellbore. For example, where the treatment fluid is a fracturing fluid, it is highly desirable that the solid proppant particles that may be added to the fracturing fluid are swept efficiently along the length of the induced fractures so that the fracture remains propped open when the pumping pressure ceases. This often requires a viscosifying agent to be added to the fluid. It is also beneficial if the polymer solution reduces the rate of leak-off of the fracturing fluid into the permeable formation so that the hydraulic pressure is most effectively transmitted to the tip of the growing fracture.

[0013] Where a fracturing or other wellbore fluid is pumped at high flow rates it can enter a turbulent flow regime causing unwanted high pressure gradients. The turbulent flow and the pressure losses can be minimized by adding relatively smaller amounts (than used for viscosification) of a friction reducer.

[0014] After tubular steel liner or casing is run into a well, cement is pumped to seal the annular gap between the steel and the formation. Polymers are often added to the cement slurry to reduce the fluid loss (filtration rate) and to minimize settlement (free water).

[0015] Fluid loss additives (FLAs) are widely used in drilling fluids and cementing slurries to control the loss of fluid to the formation through filtration. Drilling fluids or cementing slurries that lose water can also lose or degrade their design properties. Fluid loss additives help operators retain the key characteristics of their drilling fluids or cementing slurries, including viscosity, thickening time, rheology and comprehensive strength-development. Moreover, FLA's minimize the high risk of permeability damage.

[0016] Natural biopolymers such as cellulosic polymers, starches, modified starches, CMC/polysaccharide have been used as FLAs. However the thermal stability of the starch and cellulose derivatives is in the range of 120-150°C, which is not suitable for challenging wellbore drilling operations such as high pressure, high temperature (HPHT). Therefore, synthetic polymers are typically used as FLAs in the severe drilling and cementing conditions. [0017] Polyacrylamide and its copolymers with other monomers (e.g., 2-acrylamido-2- methyl-propanesulfonic acid (AMPS), vinylpyrrolidone (NVP), N-vinylacetamide, alkylacrylamide, etc) have been reported to have fluid loss control effects. Solution polymerization and other polymerization techniques are typically used to manufacture synthetic fluid loss additives.

[0018] As more and more challenging conditions are encountered in wellbore operations, there has arisen a need for improved performance water-based wellbore fluids comprising synthetic polymers that exhibit improved tolerance to high temperatures and to electrolytes.

[0019] More specifically, there is a need for high-performance rheology modifiers and fluid loss additives used in water-based drilling fluids. The enhanced performance of the drilling fluids especially the High Pressure/High Temperature (HPHT) compatibility will allow faster and safer drilling. A rheology modifier is a critical component in water-based drilling fluids to ensure a proper rheology profile which performs specific functions such as suspending weighting agents and hole cleaning. Xanthan Gum is one of the most commonly used rheology modifiers in water-based drilling fluids, but Xanthan gum starts losing rheological properties at above 250 F so it is not suitable for HPHT drilling operations. A desired rheology modifier should possess similar rheological properties (e.g., highly shear thinning) with enhanced salt tolerance and thermal stability. These enhanced properties will allow successful drilling operations under HPHT conditions. Development of such a salt-tolerant, thermally-stable rheology modifier is critically important to the drilling industry. HPHT compatible water-based drilling fluid will allow more environmentally friendly drilling operations in a safe and efficient manner. Without a high performance rheology modifier, such drilling operations under HPHT conditions are extremely challenging.

[0020] As more and more challenging conditions are encountered in oilfield drilling operations, there is a need for improved high-performance fluid loss additives and rheology modifiers. The enhanced performance of the drilling fluids especially the High Pressure, High Temperature (HPHT) compatibility will allow faster and safer drilling.

SUMMARY

[0021] The present application is directed to compositions containing a synergistic combination of polymers with high fluid loss control and excellent retention of rheological properties. In accordance with certain aspects, the disclosed compositions provide these benefits even under HPHT conditions. In accordance with certain aspects, the present application is directed to a dispersion polymerization process and chemistry modification to make fluid loss control and rheology modifier polymers. Moreover, the fluid loss additives and rheology modifiers disclosed herein can be delivered to the field as water dispersions or dry powders, either separately or blended together, to facilitate handling and processing.

[0022] The synergistic polymer combination described herein exhibits rheological and thermal stability properties that are particularly useful in high-pressure/high temperature drilling operations. In accordance with one aspect, the fluid loss additive comprises an amphoteric terpolymer of poly(NaAMPS/AM/MAPTAC) or poly(NaAMPS/AM/APTAC) and the rheology modifier comprises a terpolymer comprising Acrylamide (AM), AMPS and a hydrophobe. The polymers may be made via water dispersion polymerization and other conventional polymerization techniques (such as solution polymerization). This combination of polymers not only provides excellent fluid loss control, but imparts excellent retention of drilling mud rheological properties.

[0023] In accordance with other aspects, the fluid loss additive described herein exhibits synergy with other rheology modifiers, in particular with other types of synthetic polymers, such as copolymers of acrylamide and AMPS, AMPS and N- Vinyl pyrrolidone, acrylamide and acrylates etc.

DETAILED DESCRIPTION

[0024] The present application is directed to compositions containing a synergistic combination of a thermally-stable fluid loss additive and a rheology modifier. In particular, the present application is directed to a synergistic combination of polymers for oil-field drilling applications. More particularly, the present application is directed to a combination of polymers suitable for use under HPHT conditions. HPHT refers generally to wells that are hotter or at higher pressure than most wells. In accordance with some aspects, HPHT may refer to a well having an undisturbed bottomhole temperature of greater than 300°F (149°C) and a pore pressure of at least 0.8 psi/ft (-15.3 lbm/gal). The present application describes an HPHT filtration test (i.e., HPHT fluid loss test) wherein the test is conducted at conditions that provide an indication as to how the composition would perform under HPHT conditions. In accordance with this test, static filtration behavior of water mud or oil mud is measured at elevated temperature, up to about 380°F (193°C) maximum (450°F (227°C) maximum if a special cell is used), usually according to the specifications of API with the exception of temperature and pressure. The standard API test is conducted at room temperature and a differential pressure of lOOpsi. Although the HPHT test method described herein can simulate downhole temperature conditions, it does not simulate downhole pressure. Total pressure in a cell should not exceed 700 psi (4900 kPa), and the differential pressure across the filter medium is specified as 500 psi (3500 kPa). Therefore, in the examples described herein, the HPHT fluid loss test is conducted at temperatures of at least 200°F or more and at differential pressures of about 500psi.

[0025] In accordance with one aspect of the present application, the synergistic combination includes a thermally- stable fluid loss additive comprising a polymer of an acrylamide, a sulfonic acid or salt thereof and a cationic monomer, such as APTAC and/or MAPTAC. In accordance with one aspect, the polymer composition comprises from about 20 -80 wt. % of an acrylamide, 20 -80 wt. % of a sulfonic acid or salt thereof, and 5 -30 wt. % of a cationic monomer such as APTAC and/or MAPTAC. In accordance with one aspect of the invention, the fluid loss additive comprises a terpolymer of acrylamide (AM), 2- acrylamido-2-methyl-propanesulfonic acid (AMPS) and acrylamidopropyl-trimethyl ammonium chloride (APTAC) and/or methacrylamidopropyltrimethyl ammonium chloride (MAPTAC).

[0026] In accordance with one aspect of the present invention, the FLA comprises a terpolymer comprising about 20 -80 wt. % acrylamide, 20 -80 wt. % AMPS, and 2 -50 wt. % of a cationic monomer such as APTAC and/or MAPTAC. In accordance with particularly useful aspects of the present invention, the FLA comprises a terpolymer comprising about 40-60wt. % acrylamide, 40 -60 wt. % AMPS, and 5 -30 wt. % cationic monomer. In accordance with another embodiment of the present invention, the polymer comprises approximately equal parts by weight acrylamide and AMPS. In certain embodiments, the cationic monomer such as APTAC and/or MAPTAC may be present in an amount of about 8 - 25 wt . The weight percentages provided herein are based on the total weight (100%) of AM and AMPS and/or salts monomers. In some of the working examples provided below weight percentages are provided based on total weight of all monomers. The context in each case clearly indicates which weight percentage calculation is being used. [0027] The synergistic combination also includes a rheology modifier. In accordance with particularly useful embodiments, the rheology modifier comprises a polymer of about 30-70 wt. % acrylamide (AM), about 70-30 wt. % AMPS and about 0.01 - 5 wt. % of a hydrophobe. In accordance with particularly useful aspects, the rheology modifier comprises a terpolymer comprising about 40-60 wt. % acrylamide, about 60-40 wt. % AMPS and about 0.01 - 5 wt. % of a hydrophobe. In accordance with another embodiment, the polymer comprises approximately equal parts by weight acrylamide and AMPS. In certain embodiments, the hydrophobe may be present in an amount of about 0.05 - 3 wt . The hydrophobe percentage is based on the total dry weight of the other two monomers (e.g., ACM and AMPS) of the terpolymer.

[0028] In accordance with certain aspects of the present invention, the hydrophobe may be an alkyl acrylate having a chain length for the alkyl group of from about 12 - 25, more particularly from about 16 - 20. The long chain alkyl group can be linear, branched or cycloalkyl. Examples of useful long alkyl acrylates include, but are not limited to, n-lauryl acrylate, n-hexadecyl acrylate and n-stearyl acrylate.

[0029] In accordance with other aspects, the synergistic combination of polymers comprises the FLA described herein and a rheology polymer such as copolymers of acrylamide and AMPS, acrylamide and acrylates, AMPS and N- vinyl pyrrolidone etc.

[0030] In accordance with certain aspects of the present invention, the synergistic combination of polymers disclosed herein exhibits improved salt-tolerance, temperature- stability, and fluid loss properties as compared to conventional additives. The polymer combination may be provided in water dispersion or powder form to facilitate processing and the use of the composition in water-based drilling muds as a temperature-stable fluid loss additive and rheology modifier. In accordance with one aspect, the present application provides a method of preventing fluid loss and rheology instability during oilfield drilling operations, wherein the method includes drilling a wellbore and circulating a fluid containing an effective amount of the synergistic combination of polymers described herein.

[0031] The fluid loss additives described herein typically have a weight average molecular weight (Mw) over 3,000 daltons, more particularly over 10,000 daltons, more particularly from about 100,000 to 10,000,000 daltons, and in certain cases from about 1 ,000,000 to 5,000,000 daltons as determined by GPC.

[0032] The Rheology Modifiers described herein typically have a molecular weight (Mw) over 500,000 daltons, more particularly from about 2,000,000 to 20,000,000 daltons, and in certain cases from about 3,000,000 to 15,000,000 daltons as determined by GPC. One method for determining molecular weight is as follows: Samples are prepared as -0.15% (w/v) solution of polymer in 50/50 water/methanol mobile phase. The sample is mixed on a rotating wheel until dissolved and then filtered and injected into the GPC system at

0.5mL/min flow rate for analysis. Molecular weight values are determined relative to PEO/PEG standards injected in the beginning and end of the sample analysis. A Shodex degasser and Waters Empower 2 software interphased with Waters pump and auto-sampler can be used.

[0033] Weight average molecular weight (Mw) is defined in the following equation

[0034] Moreover, each of the polymers described herein can be produced at relatively high polymer solids (e.g. 20 - 30% in water dispersion form) while still providing acceptable bulk viscosity for processing the water dispersion (e.g., spray drying).

[0035] The synergistic combination described herein is particularly useful in oil-field drilling applications. The combination described herein may also find use in other oil well applications. For example, it may be used in applications including, but not limited to, rheology modifier/ thickener for drilling fluids and cementing, friction reducer (lime, freshwater, salt water muds), shale swell inhibitor/clay stabilizer, viscosifier (fresh water, seawater, saline muds), filtration control, cementing retarder, oil well fracturing (e.g., friction reducer), oil well stimulation (viscosifier for acidizing), drilling aids (oil, water, geological drillings), completion fluids and workover fluids, and polymer flooding (enhanced oil recovery).

[0036] The combination can also find use in HI&I (household, industrial and institutional products) applications including, but not limited to, thickener of bleach (e.g., disinfectants, bleaching material, sterilization, washing concentrates, etc), alkaline environments (>KOH) gels (e.g., battery applications), thickener for hydrogen peroxide (e.g., antiseptics, disinfectants, sterilization agents, cleaners), thickener for acidic hard surface cleaners, air fresheners gel applications (thickener, fragrance delivery), controlled release of actives (antiseptics, biocides, fragrances), formation of clear gels for handwash and hair styling products.

[0037] The polymers described herein can also be used in adhesives, coatings and textiles. Examples of particular applications include, but are not limited to, latex adhesives and paints, water based resins (thickener), adhesive hardeners and catalysts (thickener, where extreme pH conditions are common). Additional applications include lubricants for the batch dyeing of textiles and thickeners for adhesives and defoamers.

[0038] The described polymers can also be used in applications relating to solid-liquid separation (flocculation). Specific applications include, but are not limited to, flocculation of municipal and industrial effluents, particularly at low or high pH, clarification of acidic and alkaline mining and mineral slurries, separation of oily waters, dewatering of paper slurries, and thickener for clay slurries and tailings.

[0039] Although the present application is primarily described with respect to polymer compositions comprising acrylamide and 2-acrylamido-2-methyl-propanesulfonic acid (AMPS), it is believed that suitable polymers can also be prepared utilizing other

ethylenically unsaturated monomeric acrylamides and other monomeric acrylic acids, monomelic sulfonic acids, phosphonic acids and/or salts thereof as well as other cationic monomers. In accordance with certain embodiments, these other monomers can be used to produce a polymer capable of providing the attributes discussed in more detail below.

[0040] Examples of ethylenically unsaturated acrylamides include acrylamide,

alkylacrylamide such as methacrylamide, and furmaramide. Other monomers believed to be useful include without limitation N-alkyl Substituted or Ν,Ν' -alkyl disubstituted amides such as: N-methyacrylamide, Ν,Ν'-dimethylacrylamide, N- isopropylacrylamide, Ν,Ν'- diethylacrylamide. N-(dimethylaminomethyl)acrylamide, N-

(trimethylammoniummethyl)acrylamide, N-(trimethylammoniumpropyl)methacrylamide chloride, p-acrylamidomethylbenzyl bromide, p-acrylamidomethylbenzyl chloride, p- acrylamidomethylphenethyl chloride, p-acrylamidomethylphenethyl bromide, 3- acrylamidomethyl-p-xylyl chloride, 3-acrylamidomethyl-p-xylyl bromide, 3- methacrylamidomethyl-p-xylyl chloride, 3-methacrylamidomethyl-p-xylyl bromide, p- acrylamidomethyl-0-(2-bromethyl)-phenol, p-acrylamidomethyl-0-(2-chloroethyl)-phenol may also be useful.

[0041] Examples of sulfonic acids or salts can be summarized as shown in the following structure:

S0₃ ^"M⁺ where Rl and R2 are independently hydrogen or methyl group (CH₃), n is a number from 0 to 18. M is H or metal salt Na, K, NH₄ etc. For example, when n=0, Ri is H, R₂ is methyl, the structure represents acrylamidopropanesulfonic acid (AMPS) and when n=2, Ri is H, R₂ is methyl, the structure represents acrylamidobutanesulfonic acid, etc.

[0042] Examples of other sulfonic acids that may be used include those represented by the following structure:

where R is hydrogen or lower alkyl (C1-C5) group; X is a direct bond or a functionalized or unfunctionalized, branched or linear, alkylene, cycloalkylene, alkenylene, or arylene group, typically having from 1 to 6 carbon atoms, wherein any of the before mentioned groups may be with or without heteroatoms groups. Salts of the foregoing acids may also be used. Specific examples of these monomers include, but are not limited to, sodium vinyl sulfonic acid/salts (SVS), sodium sulfonic acid/salts (SSS), styrenesulfonic acid/salts (SSA) and 1- Allyloxy 2-Hydroxy Propyl Sulfonic Acid and salts thereof (AHPS). AHPS is a particularly useful monomer that can be used instead of AMPS in any of the compositions disclosed herein. APHS is thermally and hydrolytically stable at high pH, saturated salt and elevated temperature conditions.

[0043] Examples of sulfonate monomers also include: 2-chloroethylene sulfonic acid, ethylenesulfonic acid, ethylenedisulfonic acid, 1 -nitriloethylenesulfonic acid, 2- formylethylenesulfonic acid, 1 -carboxyethylenesulfonic acid, 1 -propene- 1 -sulfonic acid, 1- propene-2-sulfonic acid, 2-formyl-l -methylethylene sulfonic acid, l-carboxy-2- methylethylene sulfonic acid, 2-methyl-l,3-propenedisulfonic acid, 1-butene-l -sulfonic acid, 1 -carboxy-2,2-dimethyl-ethylene sulfonic acid, 1-pentene-l- sulfonic acid, 1-hexene-l- sulfonic acid, 2-(p-nitrophenyl)ethylene sulfonic acid, 2-phenylethylene sulfonic acid, 2-(p- hydroxyphenyl)ethylene sulfonic acid, 2-(2-aminophenyl)ethylene sulfonic acid, l-methyl-2- phenylethylene sulfonic acid, 2-(p-methoxyphenyl)ethylene sulfonic acid, 4-phenyl-l ,3- butadiene sulfonic acid, 2-(p-acetamidophenyl)ethylene sulfonic acid, 3-chloroallyl sulfonic acid, allyl sulfonic acid, 1-hydroxyallyl sulfonic acid, 2-cynoallyl sulfonic acid, 3- chloromethallyl sulfonic acid, 1-carboxyallyl sulfonic acid, 3-carboxyallyl sulfonic acid, methallyl sulfonic acid, 2-methylene-4,4-dimethyl-l ,3-disulfo-pentene, 4-methylene-4,4- dimethyl pentene sulfonic acid, l-hydroxy-3-phenylallyl sulfonic acid, 3-phenylallyl sulfonic acid, 2-benzylallyl sulfonic acid, 2-(p-methylphenoxy)allyl-sulfonic acid, 3- phenoxymethallyl sulfonic acid, 2-sulfoethyl acrylate, 2-sulfoethyl maleate, 3-sulfopropyl acrylate, 2-sulfonyl methacrylate, 3-sulfopropyl acrylate, 2-sulfo-l-(sulfomethyl)ethyl methacrylate, 3-sulfopropyl maleate, 4-sulfobutyl methacrylate, 2-(acyloxymethyl)-c- sulfuran, bis-2-sulfoethyl fumarate, 3-sulfopropyl itaconate, p-sulfophenyl acrylate, 2-(2- methylacryloxymethyl)-sulfofuran, bis(2-sulfoethyl)itaconate, p-sulfophenyl methacrylate, bis(3-sulfopropyl)maleate, bis(3-sulfopropyl)fumarate, bis(2-sulfopropyl)maleate, bis(2- sulfopropyl)fumarate, 5-methyl-2-(methallyloxy)benzene sulfonic acid, bis(2- sulfopropyl)itaconate, ar-(2-acryloyloxyethoxy)-2-naphthalene sulfonic acid, ar-(2- methacryloyloxyethoxy)-naphthalene sulfonic acid, dodecyl-4-sulfopropyl itaconate, dodecyl-4-sulfobutyl itaconate, n-acryloyl taurine, allylthioethyl sulfonic acid, alloxy propene sulfonic acid, n-allyl-n-methylaminoethane-sulfonic acid, n-(methacrylamidomethyl)- sulfoacetamide, vinyloxybenzene sulfonic acid, n-(p-sulfophenyl)methacrylamide, p-[(2- vinylsulfonyl)ethoxy] -benzene sulfonic acid, n-methyl-n-(2-vinylsulfonyl-ethyl)-p- (sodiumsulfo) benzyl amine, dichlorostyrene sulfonic acid, 2-chlorostyrene sulfonic acid, p- styrene sulfonic acid, p-sulfonic acid, vinyltoluene sulfonic acid, 2-methyl styrene sulfonic acid, the potassium, sodium and ammonium salts of each of the foregoing compounds, 4- methylene-2,2,6,6-tetramethyl-3,5-disulfoheptene, allyloxyethyl sulfonic acid, allyl oxybenzene sulfonic acid, and styrene sulfonic acid.

[0044] Phosphonic acid and salts may also be useful in accordance with certain aspects. Examples of phosphonic acids or salts can be represented by following structure:

H ₂)nCH ₃

C H

OM OM where Ri and R₂ are independently hydrogen or methyl group (CH₃), n is a number from 0 to 18. M is H or a metal salt such as Na, K, etc.

[0045] Examples of phosphonic acid and phosphonate monomers include without limitation: vinylidenediphosphonic acid, vinylphosphonic acid (VP A), styrenephosphonic acid (SPA), 4-vinylbenzylphoshonic acid (VBPA), or a-phenylvinylphosphonic acid (PVPA).

[0046] Cationic monomers other than APTAC and/or M APT AC that may be used in the fluid loss additive include diallydimethyl ammonium chloride (DADMAC) and N-methyl 2- vinyl pyridinium methyl sulfate.

[0047] Also suitable as the third monomer in the FLA polymer are quaternized

(meth)acrylate or (meth)acrylamide monomers including those described by the following structure:

wherein:Ri is hydrogen or methyl, X is O or NH,

Q is selected from a functionalized and unfunctionalizedalkylene, cycloalkylene, alkenylene, or arylene group, wherein any of the before mentioned groups may be with or without heteroatoms (more particularly, C1-C6 alkylene or cycloalkylene groups),

R2, R3, and R4 are independently selected from the group consisting of functionalized or unfunctionalized alkyl groups (more particularly, independently selected C1-C8 alkyl groups), and

M is independently selected from the group consisting of alkali metal ions, alkaline earth metal ions, and the ammonium ion, and combinations thereof.

[0048] To maintain the polymer's water solubility, the groups Q, R2, R3, and R4 can be selected to modulate the polymer' s overall hydrophilic/hydrophobic balance.

[0049] Examples of quaternized (meth)acrylates and (meth)acrylamides include, but are not limited to: (meth)acrylamidopropyltrimethyl ammonium chloride , dimethylaminoethyl methacrylate (DMAEMA), 3-methacryloyloxy-2-hydroxypropyl trimethyl ammonium chloride, 3-acrylamido-3-methylbutyl trimethyl ammonium chloride, N-propyl acrylamido trimethyl ammonium chloride, and 2-methacryloyloxy-ethyl trimethyl ammonium methosulfate.

[0050] With respect to the rheology modifier polymers, suitable hydrophobic monomers, with the alkyl chain in linear, branched, or cyclic form, include without limitation the higher alkyl esters of α,β-ethylenically unsaturated carboxylic acids such as dodecyl acrylate, dodecyl methacrylate, tridecyl acrylate, tridecyl methacrylate, tetradecyl acrylate, tetradecyl methacrylate, octadecyl acrylate, octadecyl methacrylate, the ethyl half ester of maleic anhydride, diethyl maleate, and other alkyl esters derived from the reactions of alkanols having from 8 to 25 carbon atoms with ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, itaconic acid and aconitic acid, alkylaryl esters of ethylenically unsaturated carboxylic acids such as nonyl-o-phenyl acrylate, nonyl-a-phenyl methacrylate, dodecyl- a-phenyl acrylate and dodecyl- a-phenyl methacrylate; N-alkyl, ethylenically unsaturated amides such as N-octadecyl acrylamide, N-octadecyl methacrylamide, Ν,Ν-dioctyl acrylamide and similar derivatives thereof; a-olefins such as 1- octene, 1-decene, 1-dodecene and 1-hexadecene; vinyl alkylates wherein alkyl has at least 8 carbons such as vinyl laurate and vinyl stearate; vinyl alkyl ethers such as dodecyl vinyl ether and hexadecyl vinyl ether; N- vinyl amides such as N-vinyl lauramide and N- vinyl stearamide; and ar-alkylstyrenes such as t-butyl styrene. Of the foregoing hydrophobic monomers, the alkyl esters of acrylic acid and methacrylic acid wherein the alkyl has from 8 to 25 carbon atoms are preferred. The alkyl acrylates wherein the alkyl has from 16 to 20 carbon atoms are particularly useful. Octadecyl acrylate is the most preferred. Such long chain acrylate could be in the linear or branched form for alkyl chains.

[0051] The polymers according to the invention may be readily synthesized by procedures known by those skilled in the art, and include free radical polymerization, solution polymerization, emulsion polymerization, and inverse emulsion polymerization (including Liquid dispersion polymerization (LDP)), and "dispersion polymerization" (water-in water). Dispersion polymerization is a particularly useful method for producing the polymers described herein. "Dispersion polymer" means a water-soluble polymer dispersed in an aqueous continuous phase containing one or more inorganic salts. In the process of dispersion polymerization, the monomer and the initiator are both soluble in the polymerization medium, but the medium is a poor solvent for the resulting polymer. Accordingly, the reaction mixture is homogeneous at the onset, and polymerization is initiated in a

homogeneous solution. Depending on the solvency of the medium for the resulting oligomers or macroradicals and macromolecules, phase separation occurs at an early stage. This leads to nucleation and the formation of primary particles called "precursors" and the precursors are colloidally stabilized by adsorption of stabilizers. The particles are believed to be swollen by the polymerization medium and/or the monomer, leading to the formation of spherical particles. Typically, the particles range from about 0.1 - 500 microns, more particularly from about 1 - 200 microns.

[0052] In dispersion polymerization, the variables that are usually controlled are the concentrations of the stabilizer, the monomer and the initiator, solvency of the dispersion medium, and the reaction temperature and choice of initiator. It has been found that these variables can have a significant effect on the particle size, the molecular weight of the final polymer particles, and the kinetics of the polymerization process.

[0053] Particles produced by dispersion polymerization in the absence of any stabilizer are not sufficiently stable and may coagulate after their formation. Addition of a small percentage of a suitable stabilizer to the polymerization mixture produces stable dispersion particles. Particle stabilization in dispersion polymerization is usually referred to as "steric

stabilization". Good stabilizers for dispersion polymerization are polymer or oligomer compounds with low solubility in the polymerization medium and moderate affinity for the polymer particles.

[0054] Typically, as the stabilizer concentration is increased, the particle size decreases, which implies that the number of nuclei formed increases with increasing stabilizer concentration.

[0055] As the solvency of the dispersion medium increases, (a) the oligomers will grow to a larger MW before they become a precursor nuclei, (b) the anchoring of the stabilizer moiety will probably be reduced and (c) the particle size increases. As the initiator concentration is increased, it has been observed that the final particle size increases. As for the kinetics, it is reported that when the dispersion medium is a non-solvent for the polymer being formed, then the locus of polymerization is largely within the growing particles and the system follows the bulk polymerization kinetics, n (the kinetic chain length) = Rp/Rt, where Rp is the propagation rate and Rt is the termination rate. As the solvency of the dispersion medium for the growing polymer particle is increased, polymer growth proceeds in solution. The polymeric radicals that are formed in solution are then captured by growing particles.

Consequently, the locus of the particle polymerization process changes and there is a concomitant change in the kinetics of polymerization.

[0056] Stabilizers as used herein include anionically charged water soluble polymers having a molecular weight of from about 100,000 to about 5,000,000 and preferably from about 1,000,000 to about 3,000,000. The stabilizer polymer should be soluble or slightly soluble in the salt solution, and should be soluble in water.

[0057] Particularly useful stabilizers include polyacrylic acid, poly (meth) acrylic acid, poly (2- acrylamido-2-methyl-l-propanesulfonic acid) and copolymers of 2-acrylamido-2- methyl- 1-propanesulfonic acid and an anionic comonomer selected from acrylic acid and methacrylic acid.

[0058] The stabilizer polymers may be prepared using conventional solution

polymerization techniques, are prepared in water-in-oil emulsion form or are prepared in accordance with the dispersion polymerization techniques described herein. The choice of a particular stabilizer polymer will be based upon the particular polymer being produced, the particular salts contains in the salt solution, and the other reaction conditions to which the dispersion is subjected during the formation of the polymer.

[0059] Preferably from about 0.1 to about 20 percent by weight, more preferably from about 0.5 to about 15 percent and still more preferably, from about 2 to about 10 percent by weight of stabilizer, based on the weight of the total dispersion polymer solids is utilized.

[0060] The remainder of the dispersion comprises an aqueous solution containing from about 10 to about 40 weight percent based on the total weight of the dispersion of a water soluble salt selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates.

[0061] The salt is important in that the polymer produced in such aqueous media will be rendered insoluble on formation, and polymerization will accordingly produce particles of water soluble polymer when suitable agitation is provided. The selection of the particular salt to be utilized is dependent upon the particular polymer to be produced, and the stabilizer to be employed. The selection of salt, and the amount of salt present should be made such that the polymer being produced will be insoluble in the salt solution.

[0062] Particularly useful salts include a mixture of ammonium sulfate and sodium sulfate in such quantity to saturate the aqueous solution. Salts containing di-or trivalent anions are particularly useful because of their reduced solubility in water as compared to for example alkali, alkaline earth, or ammonium halide salts, although monovalent anion salts may be employed in certain circumstances. The use of salts containing di-or trivalent anions generally results in polymer dispersions having lower percentages of salt materials as compared to salts containing monovalent anions. [0063] The particular salt to be utilized is determined by preparing a saturated solution of the salt or salts, and determining the solubility of the desired stabilizer and the desired polymer. Preferably from about 5 to about 30, more preferably from about 5 to about 25 and still more preferably from about 8 to about 20 weight percent based on the weight of the dispersion of the salt is utilized. When using higher quantities of monomer less salt will be required.

[0064] In addition to the above, other ingredients may be employed in making the polymer dispersions of the present invention. These additional ingredients may include chelating agents designed to remove metallic impurities from interfering with the activity of the free radical catalyst employed, chain transfer agents to regulate molecular weight, nucleating agents, and co-dispersant materials. Nucleating agents when utilized generally encompass a small amount of the same polymer to be produced. Thus if a polymer containing 70 mole percent AMPS acid (or its water soluble salts) and 30 percent acrylamide were to be produced, a nucleating agent or "seed" of the same or similar polymer composition may be utilized. Generally up to about 10 weight percent, preferably about 0.1 to about 5, more preferably from about 0.5 to about 4 and still more preferably from about 0.75 to about 2 weight percent of a nucleating agent is used based on the polymer contains in the dispersion is utilized.

[0065] Co-dispersant materials that may be utilized include dispersants from the classes consisting of water soluble sugars, polyethylene glycols having a molecular weight of from about 2000 to about 50,000, and other polyhydric alcohol type materials. Amines and polyamines having from 2-12 carbon atoms are often times also useful as co-dispersant materials, but, must be used with caution because they may also act as chain transfer agents during polymerization. The function of a co-dispersant is to act as a colloidal stabilizer during the early stages of polymerization. The use of co-dispersant materials is optional, and not required to obtain the polymer dispersions of the invention. When utilized, the co-dispersant may be present at a level of up to about 10, preferably from about 0.1- 4 and more preferably from about 0.2-2 weight percent based on the dispersion.

[0066] The total amount of water soluble polymer prepared from the anionic and the nonionic water soluble monomers in the dispersion may vary from about 5 to about 50 percent by weight of the total weight of the dispersion, and preferably from about 10 to about 40 percent by weight of the dispersion. Most preferably the dispersion contains from about 15 to about 30 percent by weight of the polymer prepared from the nonionic and anionic water soluble monomers.

[0067] Polymerization reactions described herein may be initiated by any means which results in generation of a suitable free -radical. Thermally derived radicals, in which the radical species results from thermal, homolytic dissociation of an azo, peroxide,

hydroperoxide and perester compound are preferred. Especially preferred initiators are azo compounds including 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- (2- imidazolin-2- yl) propane] dihydrochloride, 2,2'-azobis (isobutyronitrile) (AIBN), 2,2'-azobis (2,4- dimethylvaleronitrile) (AIVN), and the like.

[0068] The monomers may be mixed together with the water, salt and stabilizer prior to polymerization, or alternatively, one or both monomers may be added stepwise during polymerization in order to obtain proper incorporation of the monomers into the resultant dispersion polymer. Polymerizations of this invention may be run at temperatures ranging from -10°C to as high as the boiling point of the monomers employed. Preferably, the dispersion polymerization is conducted at from -10°C to about 80°C. More preferably, polymerization is conducted at from about 20°C to about 60°C.

[0069] The dispersion polymers of this invention typically are prepared at a pH greater than 5, preferably at a pH of about 6 - 8. After polymerization the pH of the dispersion may be adjusted to any desired value as long as the polymer remains insoluble to maintain the dispersed nature. Preferably, polymerization is conducted under inert atmosphere with sufficient agitation to maintain the dispersion.

[0070] The polymer dispersions made through the "dispersion polymerization" process described herein typically have apparent bulk viscosities of less than about 50,000 cps at 25 °C (Brookfield), more preferably less than 30,000 cps and still more preferably less than about 20,000 cps and in certain embodiments from about 300-3000 cps. At these viscosities, the polymer dispersions are easily handled in conventional polymerization equipment and are suitable for subsequent manufacturing processing (e.g., drum and spray drying).

[0071] A powdered polymer product can be made through drying (e.g., vacuum drying, spray drying, belt drying, drum drying, etc.) the above mentioned polymer dispersions. Powdered polymers can also be manufactured through drying the solutions, emulsions, inverse emulsions or suspensions described in the following text.

[0072] In another aspect, the present application describes a method of preparing a moderate molecular weight dispersion FLA polymer having a bulk Brookfield viscosity of from about 200 to about 8000 cps at 25 °C comprising a) adding an initiator to an aqueous mixture comprising: i) from about 20 -30 weight percent of a mixture comprising 5-55, more particularly 10-43, mole percent of Na AMPS, 30-95, more particularly 43 -88, mole percent of acrylamide and 0.5-30, more particularly 1 -25, mole percent of MAPTAC and/or APTAC; ii) from about 2 -20 of weight percent based on the total weight of the dispersion of a stabilizer; and iii) from about 10 -40 of weight percent based on the weight of the dispersion of a water soluble salt selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates; and b) polymerizing the monomers.

[0073] In another aspect, the present application also describes a method of preparing a high molecular weight dispersion rheology modifier polymer having a bulk Brookfield viscosity of from about 100 to about 25,000 cps at 25°C comprising a) adding an initiator to an aqueous mixture comprising: i) from about 10 to about 40 weight percent of a mixture comprising 10 - 45 mole percent of Na AMPS and 55 - 90 mole percent of acrylamide and about 0.01 - 5 wt based on the weight of Na AMPS and acrylamide of a long-chain alkyl acrylate having a chain length for the alkyl group of from 12 - 25; ii) from about 0.1 to about 10 weight percent based on the total weight of the dispersion of a stabilizer; and iii) from about 10 to about 40 weight percent based on the weight of the dispersion of a water soluble salt selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates; and b) polymerizing the monomers.

[0074] Free radical polymerization is another useful polymerization method, especially when using water-dispersible and/or water-soluble reaction solvent(s), and is described in "Decomposition Rate of Organic Free Radical Polymerization" by K.W. Dixon (section II in Polymer Handbook, volume 1, 4th edition, Wiley-Interscience, 1999), which is incorporated herein by reference.

[0075] Compounds capable of initiating the free-radical polymerization include those materials known to function in the prescribed manner, and include the peroxo and azo classes of materials. Exemplary peroxo and azo compounds include, but are not limited to: acetyl peroxide; azobis-(2-amidinopropane) dihydrochloride; azobis-isobutyronitrile; 2,2'-azobis-(2- methylbutyronitrile); benzoyl peroxide; di-tert-amyl peroxide; di-tert-butyl diperphthalate; butyl peroctoate; tert-butyl dicumyl peroxide; tert-butyl hydroperoxide; tert-butyl perbenzoate; tert-butyl permaleate; tert-butyl perisobuty Irate; tert-butyl peracetate; tert-butyl perpivalate; para-chlorobenzoyl peroxide; cumenehydroperoxide; diacetyl peroxide;

dibenzoyl peroxide; dicumyl peroxide; didecanoyl peroxide; dilauroyl peroxide; diisopropyl peroxodicarbamate; dioctanoyl peroxide; lauroyl peroxide; octanoyl peroxide; succinyl peroxide; and bis-(ortho-toluoyl) peroxide.

[0076] Also suitable to initiate the free-radical polymerization are initiator mixtures or redox initiator systems, including: ascorbic acid/iron (II) sulfate/sodium peroxodisulfate, tert-butyl hydroperoxide/sodium disulfite, and tert-butyl hydroperoxide/sodium

hydroxymethanesulfinate.

[0077] Alternatively, monomer units may be polymerized concurrently together, using an appropriate initiator and optional solvent(s). Alternatively, the polymerization reaction may be initiated with one or more of the monomers, the reaction temporarily slowed or stopped, and then reinitiated upon the addition of more or different monomers and initiator.

[0078] The initiators are often called "free radical initiators." Various decomposition methods of the initiators are discussed first, followed by a description of the emulsion, solution, and suspension polymerization methods. The initiator can be decomposed homolytically to form free radicals. Homolytic decomposition of the initiator can be induced by using heat energy (thermolysis), using light energy (photolysis), or using appropriate catalysts. Light energy can be supplied by means of visible or ultraviolet sources, including low intensity fluorescent black light lamps, medium pressure mercury arc lamps, and germicidal mercury lamps.

[0079] Catalyst induced homolytic decomposition of the initiator typically involves an electron transfer mechanism resulting in a reduction-oxidation (redox) reaction. This redox method of initiation is described in Elias, Chapter 20 (detailed below). Initiators such as persulfates, peroxides, and hydroperoxides are more susceptible to this type of

decomposition. Useful catalysts include, but are not limited to, (1) amines, (2) metal ions used in combination with peroxide or hydroperoxide initiators, and (3) bisulfite or mercapto- based compounds used in combination with persulfate initiators.

[0080] Presently, particularly useful methods of initiation comprise thermolysis or catalysis. Thermolysis has an additional advantage in that it provides ease of control of the reaction rate and exotherm.

[0081] Useful initiators are described in Chapters 20 & 21 Macromolecules, Vol. 2, 2nd Ed., H. G. Elias, Plenum Press, 1984, New York. Useful thermal initiators include, but are not limited to, the following: (1) azo compounds such as 2,2-azo-bis-(isobutyronitrile), dimethyl 2,2'-azo-bis-isobutyrate, azo-bis-(diphenyl methane), 4-4'-azo-bis-(4- cyanopentanoic acid); (2) peroxides such as benzoyl peroxide, cumyl peroxide, tert-butyl peroxide, cyclohexanone peroxide, glutaric acid peroxide, lauroyl peroxide, methyl ethyl ketone peroxide; (3) hydrogen peroxide and hydroperoxides such as tert-butyl hydroperoxide and cumenehydroperoxide; (4) peracids such as peracetic acid and perbenzoic acid;

potassium persulfate; ammonium persulfate; and (5) peresters such as diisopropyl percarbonate.

[0082] Useful photochemical initiators include but are not limited to benzoin ethers such as diethoxyacetophenone, oximino-ketones, acylphosphine oxides, diaryl ketones such as benzophenone and 2-isopropyl thioxanthone, benzyl and quinone derivatives, and 3- ketocoumarins as described by S. P. Pappas, J. Rad. Cur., July 1987, p.6.

[0083] Various other polymerization methods may be employed to synthesize the polymers described herein. Examples of other methods that may be used include, but are not limited to, solution polymerization and optional inversion, suspension polymerization, emulsion polymerization and inverse emulsion polymerization. These methods are described in more detail in U.S. Provisional App. No. 61/610203, filed March 13, 2012 entitled "Synthesis and Application of High Pressure High Temperature Fluid Loss Additive and Rheology

Stabilizer" and International Application No. PCT/US2011/057718 filed October 25, 2011 , which claims the benefit of U.S. Provisional Application No. 61/406,402 filed October 25, 2010, entitled "Synthesis And Application Of Salt-Tolerant, Thermally-Stable Rheology Modifiers," the contents these three applications are hereby incorporated by reference. [0084] In oilfield applications any convenient concentration of fluid loss additive and/or rheology modifier can be used, so long as it is effective in its purpose. Generally, the fluid loss additive or rheology modifier is used in an amount of about 0.05% to about 5%, more particularly about 0.1% to about 3% and in certain cases about 0.5% to about 2% by weight based on total formulation.

[0085] It is contemplated that higher concentrations may be preferred in some applications. Nonetheless, it is understood that the actual concentration will vary, depending upon many parameters. A suitable concentration for a particular application, however, can be determined by those skilled in the art by taking into account the polymer's performance under such application.

[0086] Unless otherwise specified, the concentrations of the polymers detailed herein refer to the concentration of active polymer per unit volume. In practice the polymers may be utilized in the form of an aqueous dispersion polymer wherein the polymer concentration in the liquid product may be less than 50% by weight or as a dilute solution in water which could be diluted from a more concentrated aqueous solution or from a powdered form. In accordance with some aspects, each of the polymers may be present in the wellbore fluids at a dose of between 0.5g/l and 50 g/1 (between 0.175 ppb and 17.5 ppb where ppb means pounds per barrel) unless the application of the polymer is as a friction reducer where lower doses such as between 0.001 -0.5 g/1 (0.0035 - 0.175ppb) are effective at reducing turbulence and pressure losses. More particularly, each of the polymers may be present in the wellbore fluid at a dose of between 1.0 g/1 and 36 g/1 (0.35 ppb and 12.6 ppb).

[0087] Particular features of the wellbore fluids in accordance with certain aspects of the present invention will now be described below.

[0088] The base fluid of the wellbore fluid may be water, seawater or a solution of a salt or a solution of a combination of salts. Generally, the base fluid is present in the wellbore fluid in an amount in the range of from about 30 to 99% by weight of the fluid, preferably about 50 to 97% by weight.

[0089] The base fluid may be an aqueous solution of one or more density-increasing water- soluble salt. The density increasing water-soluble salt may be selected from the group consisting of alkali metal halides (for example, sodium chloride, sodium bromide, potassium chloride and potassium bromide) alkali metal carboxylates (for example, sodium formate, potassium formate, caesium formate, sodium acetate, potassium acetate or caesium acetate), sodium carbonate, potassium carbonate, alkaline earth metal halides (for example, calcium chloride and calcium bromide), and zinc halide salts.

[0090] Alternatively, density control may be provided to the water-based wellbore fluid using insoluble weighting agents. Suitable weighting agents include suspended dense mineral particles such as ground barite, iron oxides (for example, haematite), ilmenite, calcite, magnesite (MgCC^), dolomite, olivine, siderite, hausmannite (M^C ) or suspended metal particles.

[0091] Examples of additives that may be added to the aqueous based wellbore fluids include the clays bentonite, attapulgite, hectorite, sepiolite, and the synthetic minerals Laponite™ (a synthetic hectorite ex Rockwood Additives Limited) and mixed metal hydroxides. Other clays which may be present in the fluids include kaolinite and illite which can be contaminants arising from drilled shale formations. Some of the shale cuttings inevitably become dispersed in a wellbore fluid as fine illite and kaolinite clay particles.

[0092] The wellbore fluids may additionally comprise a particulate bridging agent, for example acid-soluble materials such as calcium carbonate, water-soluble particles such as alkali metal halides; and sparingly water-soluble materials such as Ulexite (a sodium calcium borate salt) and magnesium salts of carboxylic acids. Where the particulate bridging agent is comprised of a water-soluble or sparingly water-soluble material, it is employed in an aqueous based fluid in amounts above the saturation concentration of the water-soluble or sparingly water-soluble material in water at the conditions encountered downhole so as to provide suspended particles of the conventional particulate bridging agent. Other suspended solids can include graphite particles, cellulose fibers, and mica flakes.

[0093] The aqueous based wellbore fluid may comprise additional additives for improving the performance of the wellbore fluid with respect to one or more properties. Examples include: pH control agents such as calcium hydroxide, magnesium hydroxide, magnesium oxide, potassium hydroxide, sodium hydroxide and citric acid; clay or shale hydration inhibitors (such as polyalkylene glycols), bactericides, detergents and emulsifiers, solid and liquid lubricants, gas-hydrate inhibitors, corrosion inhibitors, oxygen scavengers, defoamers, scale inhibitors, enzymes, oxidising polymer-breakers, emulsified hydrophobic liquids such as oils, acid gas-scavengers (such as hydrogen sulfide scavengers), thinners (such as lignosulfonates), demulsifiers and surfactants designed to assist the clean-up of invaded fluid from producing formations.

[0094] Besides the synergistic combination of polymers, other water-soluble polymers may be added to the water-based wellbore fluid to impart viscous properties, solids-dispersion and/or filtration control to the fluid. A wide range of water-soluble polymers may be used including cellulose derivatives such as carboxymethyl cellulose, hydroxyethylcellulose, carboxymethylhydroxyethyl cellulose, sulphoethylcellulose; starch derivatives (which may be cross-linked) including carboxymethyl starch, hydroxyethylstarch, hydroxypropyl starch; bacterial gums including xanthan, welan, diutan, succinoglycan, scleroglucan, dextran, pullulan; plant derived gums such as guar gum, locust-bean gum, tara gum and their derivatives; synthetic polymers and copolymers.

[0095] Particularly useful compositions contain a synergistic combination of a fluid loss additive (FLA) and a rheology modifier (RM) with one or more polysaccharides. Examples of particularly useful polysaccharides include, but are not limited to, polyanionic cellulose (PAC), hydroxyethyl cellose (HEC), Hydroxypropyl cellose (HPC), carboxymethyl hydroxyethyl cellulose (CMHEC), carboxymethyl cellulose (CMC), xanthan gum, guar gum and mixtures thereof. Although some of these additives are typically prone to degradation at high temperatures, they unexpectedly provide an improvement in properties even when used at relatively high temperatures when used in combination with the synergistic compositions described herein. Typically, these addtives may be used in amounts of about 50% or less by weight, more particularly from about 5 - 40%, from about 10-35% and in some cases from about 30-35%, based on the rheology modifier active solids.

[0096] Synthetic water-soluble homopolymers and copolymers can be used in conjunction with the fluid loss additive polymers to obtain synergistic results. These polymers may be included to viscosify a fluid and/or as a fluid loss reducer, or as an encapsulating polymer to reduce the rate of shale hydration and cuttings dispersion. Such synthetic (co)polymers are typically based on one or more monomers selected from the group consisting of acrylic acid, methacrylic acid, hydroxyethylmethacrylic acid, hydroxypropylmethacrylic acid; acrylamide, Ν,Ν-dimethylacrylamide, maleic acid or maleic anhydride, fumaric acid, itaconic acid, vinyl acetate, 2-acrylamido-2-methyl propane sulfonic acid (AMPS), 2-acrylamidoethane sulfonic acid, 2-acrylamidopropane sulfonic acid, 3-methacrylamidopropane sulfonic acid, styrene sulphonic acid, vinylsulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, vinylbenzyl sulfonic acid, N- vinyl formamide, N- vinyl acetamide, N- vinyl pyrrolidone, N- vinyl caprolactone and Ν,Ν-dimethylacrylamide, N-vinylpyridine and other cationic vinylic monomers (for example, diallyldimethylammonium chloride, DADMAC). Typically, alkali metal, ammonium or amine salts of the acidic functional groups in the (co)polymer are used in the wellbore fluids.

[0097] Frequently these synthetic (co)polymers have a number average molecular weight in the range 100,000 to 20,000,000 daltons, although molecular weights lower than 100,000 are of value where it is desired that the (co)polymer exert a useful solids-dispersing effect.

[0098] Another class of viscosifying agents that may be used is that of viscoelastic surfactants (VESs). VESs are used in wellbore fluids where easy clean-up of the fluid after use is needed. Examples include a gravel -pack carrier fluid and a fracturing fluid.. The polymers described herein may extend VES - based fluids and allow their use at higher temperatures than heretofore obtainable.

[0099] Other examples of Fluid loss Reducers that can be used include asphalts (for example sulfonated asphalts); gilsonite; lignite (humic acid) and its derivatives such as sulfomethylated lignite; lignin derivatives such as lignin sulfonates and condensed polymeric lignin sulfonates.

[00100] Dispersants for solids suspended in the fluid (also known as thinners) can be used in conjunction with the synergistic combination of polymers. Such dispersants include lignosulfonates, polyphosphates, tannins and/or quebracho extract, sulfomethylated tannins. Other useful dispersants include synthetic water-soluble polyanionic polymers such as sodium polyacrylate and other largely anionic (co)polymers produced from the monomers listed above and having a number average molecular weight, Mn, in the range 1 ,000 to 100,000, more typically 3,000 to 30,000.

[00101] Shale hydration inhibitors may also be used. These include partially hydrolyzed polyacrylamide or copolymers of acrylamide and acrylic acid, polyalkyleneglycols, polvinylpyrrolidone or N-vinylpyrrolidone copolymers, polyamines, amine end-capped poly alky leneglycols, at least partially cationic polymers derived from cationic monomers such as diallyl dimethyl ammonium chloride, and organic cations such as

tetramethylammonium and choline cations.

[00102] Substantially water-insoluble polyalkylene glycols, such as polypropylene glycols (PPGs) having a molecular weight above about 1,000, have utility as defoamers, and also as lubricants for the bit and drill-string. PPGs may be incorporated in a fluid together with the polymers disclosed herein.

[00103] Aqueous-based fluids comprising the synergistic combination of polymers may contain some added oil such as a refined petroleum fraction, a mineral oil, a synthetic hydrocarbon, or any suitable non-hydrocarbon liquid that is substantially water-insoluble. Usually the added oil is biodegradable and is of low toxicity. Examples of such oils include the biodegradable ester base fluids that are used in environmentally-friendly ester-oil-based drilling fluids, triglycerides such as rapeseed oil, linear alphaolefins, internal olefins and n- alkanes. The added oil phase may improve lubricity, fluid loss, and will reduce the density of the fluid.

[00104] Dispersed gas bubbles such as nitrogen and air can also be used to reduce the density of a wellbore fluid comprising the synergistic combination of polymers, and foam or aphrons can aid in diverting treatment fluids or reducing leak-off.

[00105] Where the wellbore fluid comprising the synergistic combination of polymers is an acidic treatment fluid, the acid may be a mineral acid such as hydrochloric acid, hydrofluoric acid, and sulfamic acid; or an organic carboxylic acid such as formic acid, acetic acid, and citric acid; or partially neutralized polycarboxylic acid sequestrants such as EDTA di- potassium salt or a biodegradable sequestrant alternative such as L-glutamic acid N,N- diacetic acid di-potassium salt; or readily hydrolysable acid precursors such as formate esters, acetate esters, orthoformate esters, and particles of polyesters such as poly(lactic acid).

[00106] Besides the wide variety of wellbore fluids that can usefully comprise the synergistic combination of polymers, the combination of improved thermal stability and electrolyte tolerance exhibited by the composition enables improved methods of performing wellbore operations. [00107] Water-based drilling fluids generally comprise an aqueous base fluid such as fresh water or sea water, a weight material capable of increasing the density of the fluid such as a particulate dense mineral suspended in the fluid or a salt dissolved in the fluid, material(s) capable of increasing the viscosity and gel strength, and material(s) that reduce the filtration rate (Fluid Loss) of the drilling fluid. The synergistic combination of polymers has been found to effectively increase the viscosity/gels and reduce the Fluid Loss of aqueous drilling fluids over a wide range of conditions.

[00108] Accordingly, in a particularly useful embodiment of the present invention there is provided a method of drilling a wellbore through a subterranean rock formation comprising: a) mixing a drilling fluid comprising an aqueous base fluid, a weight material, and between 0.5g/l and 36 g/1 (0.175 ppb and 12.6 ppb) of each of the polymers in the synergistic combination of polymers; b) pumping said drilling fluid into tubing placed in the wellbore and through nozzles in a drilling bit attached to the bottom end of the tubing while rotating the bit to detach cuttings from the rock formation; c) transporting the cuttings up the annulus between the tubing and the wellbore wall, the cuttings transport being facilitated by the flow of the modified drilling fluid; d) using apparatus to remove the cuttings from the drilling fluid after it exits the wellbore on surface; e) followed by pumping the cleaned drilling fluid again down the tubing thereby repeating the process until the desired interval of rock formation has been drilled.

[00109] An advantage of this embodiment is that the filtration control and rheological properties of the drilling fluid can be maintained at much higher temperatures in the wellbore than have been obtainable using viscosifiers and fluid loss reducers based on polysaccharide polymers. [00110] Water-based completion fluids and water-based workover fluids often comprise an aqueous base fluid such as fresh water or sea water, a weight material capable of increasing the density of the fluid such as a particulate mineral suspended in the fluid (such as calcium carbonate that may ultimately be dissolved in acids) or a salt dissolved in the fluid, material(s) capable of increasing the viscosity and gel strength, and material(s) that reduce the filtration rate (Fluid Loss) of the completion fluid. The synergistic combination of polymers has been found to effectively increase the viscosity/gels and reduce the Fluid Loss of aqueous completion fluids over a wide range of conditions and brine (electrolyte) types.

[00111] Accordingly, in a particular embodiment of the present application there is provided a method of completing a wellbore that penetrates through a porous and permeable subterranean rock formation comprising: a) mixing a completion fluid comprising an aqueous base fluid, weight material (a salt and / or insoluble particulate weight material) , and between 0.5g/l and 36 g/1 (0.175 ppb and 12.6 ppb) of each of the polymers in the synergistic combination of polymers; b) pumping said completion fluid into the wellbore so that the formation fluid pressure and/or losses of completion fluid to the formation are controlled; c) performing the operations required to complete the well (such as perforating the casing, under-reaming (widening the wellbore diameter), fracturing the formation, placing gravel packs for the purpose of sand-control, installing sand screens, installing production tubing and packers); d) displacing completion fluid from the production tubing; and e) allowing production to commence or injection to begin.

[00112] Similarly, in another embodiment, there is provided a method of working over a wellbore that penetrates through a porous and permeable subterranean rock formation comprising: a) mixing a workover fluid comprising an aqueous base fluid, a weight material, and between 0.5g/l and 36 g/1 (0.175 ppb and 12.6 ppb) of each of the polymers in the synergistic combination of polymers; b) pumping said workover fluid into the wellbore so that the completed interval and the production tubing is at least partly filled with said workover fluid and the formation fluid pressure is controlled; c) performing the operations required to repair the well (such as removing the production tubing, milling out the packer, sealing intervals that delivered unwanted high water-cut, re -perforating, fracturing the formation, placing gravel packs and or sand screens for the purpose of sand-control, installing new production tubing and packers); d) displacing workover fluid from the production tubing; and e) allowing production to commence or injection to begin.

[00113] Fracturing fluids generally comprise a proppant (for example, sand particles or ceramic beads) suspended in an aqueous base fluid that is normally viscosified by a polymer or a viscoelastic surfactant such that the proppant that is used to prop open the fractures is efficiently transported into the fractures that are created when the fracturing fluid is pumped at high pressure into a rock formation. However, if the fracturing fluid leaks off too quickly into the formation the high pressure dissipates and the fractures cease to grow. Leak-off control is normally achieved by dispersing ground particles such as silica flour in the fracturing fluid to block / bridge the exposed pores in the fracture that are accepting the "leak-off, combined with the filtration-rate reducing effects of the dissolved polymer or viscoelastic surfactant. Solutions of the synergistic combination of polymers exhibit excellent particle suspension and filtration reduction characteristics that make them well suited for application in fracturing fluids.

[00114] Accordingly, in another embodiment, there is provided a method of fracturing a rock formation comprising: injecting a fracturing fluid into an interval of a wellbore across the rock formation that is to be fractured wherein the fracturing fluid comprises an aqueous base fluid, proppant, the combination of polymers at a dose of between about 0.48g/l (about 4 lbs per 1 ,000 US gallons) and about 6.00 g/1 (about 50 lbs per 1,000 US gallons) for each polymer; and maintaining the pressure of the fracturing fluid at above the fracture pressure of the formation whereby the fractures grow and the synergistic combination of polymers solution assists the transport of the proppant particles along the fractures and reduces the rate of leak- off of the fluid into the rock formation. Optionally the synergistic combination of polymers solution contains a cross-linking agent to boost the viscosity and gel strength.

[00115] Advantages of this embodiment of the present invention are that the pressure of the fracturing fluid in the growing fracture is maintained for as long as possible at above the fracturing pressure of the rock formation by reducing leak-off of fluid to the formation and hence reducing pressure dissipation to the formation, and that the transport of proppant evenly and deeply into the fractures is facilitated.

[00116] Even low concentrations of the synergistic combination of polymers effectively delay the onset of turbulent flow at high shear rates. Accordingly, in another embodiment of the present invention there is provided a method of fracturing ("slickwater fracturing") a rock formation comprising: injecting a fracturing fluid into an interval of a wellbore across the rock formation that is to be fractured wherein the fracturing fluid comprises an aqueous base fluid, optionally a proppant, and the combination of polymers at a dose of between about 0.01 g/1 (about 0.08 lbs per 1,000 US gallons) and about 0.48g/l (about 4 lbs per 1,000 US gallons) for each of the polymers ; and maintaining the pressure of the fracturing fluid at above the fracture pressure of the formation by pumping at a sufficiently high rate.

[00117] An advantage of this embodiment is that under the high rate pumping conditions the synergistic combination of polymers solution reduces the pressure losses and turbulence in the injection tubing and in the fractures, thereby promoting the extent of the growth of the fractures.

[00118] In both methods of fracturing as detailed above it is especially preferred to utilize the synergistic combination of polymers in the form of either an Aqueous Dispersion Polymer or an Emulsion polymer suspended in an oleaginous liquid. This greatly facilitates the dosing and rapid, smooth dissolution of the polymer to give an essentially lump-free solution when employing in-line "on-the fly" dosing methods.

[00119] Aspects of the present application will be described in more detail by reference to the following non-limiting examples.

[00120] Example 1 (Comparative): To make anionic poly(AMPS) as dispersant.

[00121] To a 1 L 4-neck round bottom flask, equipped with water bath, thermal couple, mechanic stirrer and nitrogen inlet and outlet, was added 80 g 2-acrylamido-2-methyl propanesulfonic acid (AMPS acid), and 700 g deionized water, the mixture as purged with N2 at 40°C for 60 min, 0.10-0.20 g ammonium persulfate dissolved in 20 g deionized water was added in batches over 1 hour. The reaction was maintained for 10 hrs at 40°C to obtain viscous poly (AMPS). A 10% by weight actives solution of polyacrylamidomethylpropane sulfonic acid [poly(AMPS)]was recovered with a bulk viscosity of 2,000 - 10,000 cps. The polymer had a Mw of about 1,000,000 as measured by GPC.

[00122] Example 2: To make AM/AMPS/MAPTAC (47.6/47.6/4.8) fluid loss additive terpolymer (hereinafter FLA) by dispersion polymerization:

To a 1L resin reactor, equipped with water bath, thermocouple, mechanical stirrer and nitrogen inlet and outlet, was added 58g of 10% poly(AMPS) made in a manner similar to example 1 with a bulk viscosity of 2,000-10,000 cps. 54.27 g AMPS acid and 290g water were added into the reactor and the temperature was maintained below 30~40°C. After AMPS acid and poly(AMPS) were completely dissolved, approximately 24g of 50% NaOH was dropwise added to the above solution to adjust the pH value to 7. 130 g ammonium sulfate, 60g acrylamide, and 12g of 50% 3-trimethylammonium propyl methacrylamide chloride (MAPTAC) were added in the mixture. The above mixture was heated to 60°C and purged with nitrogen for 30 minutes. 7.5 g 2% ammonium persulfate and 7.5g 2% potassium metabisulfite were added into the reactor over 30 minutes. Polymerization began after 20-30 minutes after initiators and the solution become a milky dispersion after another 30 min. The reaction was kept for an additional 1 hour at 60°C, then the temperature was raised to 72°C and 0.05g VA-044 dissolved in 2g DIW was added into the reactor. The reaction was kept for an additional 2 hours. The resulting polymer dispersion with a bulky viscosity in the range of 200-1500cps was poured into a glass jar. 32

The polymerization reaction can generally be summarized as set forth in Scheme 1

Where: AM mole% range is 30-95%

NaAMPS mole% range is 5-55%

APTAC mole% range is 0.5-30%

Scheme 1. Synthesis of terpolymer of AM, NaAMPS, and APTAC

[00124] Example 3: To prepare AM/AMPS/alkyl acrylate (39.2/58.8/2.0) rheology modifier (hereinafter - RM) by dispersion polymerization.

[00125] To a 1L resin reactor, equipped with water bath, thermal couple, mechanic stirrer and nitrogen inlet and outlet, was added 10.75 g NaOH dissolved into 56.12 g deionized water. 54.27 g AMPS acid was slowly added into the reactor and control the temperature below 30~40°C. After AMPS acid was completely dissolved, 40.66 acrylamide (+98%), 300g deionized water, 61.61 g of 13.7% solution of polyacrylamidomethylpropane sulfonic acid having been prepared in a manner similar to example 1 with a Mn of 522,000, Mw of 3,500,000 were added into the reactor, 50% NaOH was dropwise added to the above solution to adjust the pH value to 9 (in the range of 7-9). 135.63 g ammonium sulfate and 2.03 g Ageflex FA68 (mixture of hexadecyl acrylate and stearyl acrylate) was added in the mixture. The above mixture was heated to 40°C and purged with nitrogen for 30 minutes. 0.1975 g ammonium persulfate was dissolved in 20 g DIW and dropwise added into reactor within 90 minutes. Polymerization begins after 20 minutes and the solution become viscous. After 30 minutes the mixture became a milky dispersion. The reaction was continued for a total of 21 hours, during which time the temperature was maintained at 39-42°C. The resulting polymer dispersion was poured into an aluminum pan and dried in the oven at 110°C for 3-4 hours to obtain the dry polymer cake that is further grinded into powder by a blender.

[00126] A ~12.5ppg drilling fluid formulation as described in Table 1 was made on a 600g scale containing weighting agents, rheology modifier (RM), and fluid loss control additives (PAC ) as shown in following table. Sufficient mixing was required to facilitate dissolving of the polymer and avoid local viscosified agglomerates (fish eyes). The drilling fluids were allowed to agitate for 5-15 minutes between the addition of each component and with 60 minutes total for complete and homogenous mixing. Rheological properties were then measured on Fann 35 before and after hot rolling (BHR and AHR) aging tests.

Table 1. For comparison: mud formulation using RM alone without PAC FLA

RM - Example 3

PAC -Poly anionic Cellulose

[00127] The drilling fluid muds were prepared from the formulation provided in Table 1 and sealed in OFITE stainless cells under N₂ pressure of 150psi for 350°F for 16 hours aging. HPHT fluid loss tests on drilling fluid formulations were conducted in accordance with the procedures detailed in API RP 13B-1. BHR and AHR rheology results and HPHT Fluid loss control properties are provided in Table 2. The Table 2 results show the good retention of rheological profiles of 1 % AA350 without FLA. However, HPHT Fluid loss control properties of the muds are poor after 350°F/16hr aging in the absence of the FLA polymer described herein. Table 2. For comparison: Mud Rheology before and after hot rolling at 350F containing 1% or 0.5% RM +PAC,

Fluid Loss tests were conducted at high pressure (ΔΡ=500 psi) and high temperature (260°F)

[00128] The measured specifications of the fluid are outlined herein. Plastic viscosity (PV), Yield Point (YP) and Gel Strength are measured on an oilfield type rotational viscometer Fann 35. PV is a measure of the high-shear-rate viscosity of the fluid and is calculated from the measurements at 600 and 300 rpm rotational speeds and is equal to

Θ300 cps. YP is a measure of the yield stress of the fluid and is calculated from ΥΡ=2θ₃₀ο- e₆₀olb/100 ft². The unit lb/100ft² is an oilfield unit, which is equivalent to 0.48 Pa.

[00129] Gel strength is the ability of fluid to suspend mud while mud is in static condition. Before testing gel strength, mud must be agitated for a while in order to prevent precipitation and then let mud is in static condition for a certain limited time (10 seconds, 10 minutes) and then slowly turn the gel knob counter wise and read the maximum reading value. The measured 10-second or 10-min gel strength of a fluid is the maximum reading (deflection) taken from a direct-reading viscometer after the fluid has been quiescent for 10 seconds or 10 minutes. The reading is reported in lb/ 100 ft².

[00130] Gel strength should be just high enough to suspend weighing agents and drilling cuttings when circulation is stopped. Higher gel strengths are undesirable because they retard the separation of cuttings and of entrained gas at the surface, and also because they raise the pressure required to re-establish circulation after changing bits.

[00131] OFITE Aging Cells are patented pressure vessels that enable samples to be subjected to temperatures higher than the boiling point of water and still be maintained in a liquid state. The cells may be used for static temperature exposure or in a dynamic mode in a roller oven with a normal minimum aging time of 16 hours. The mud formulations described herein were aged in 500 ml OFITE 303 grade stainless cells sealed with Teflon liner and O- rings in a OFITE roller oven.

[00132] After mud formulations were aged at certain temperature, and their PV, YP, gel strength were still maintained > 40% of their original values, they are considered as passing the aging tests. In accordance with certain embodiments, these values may be maintained at values that are at least 60%, 80% or even 90% of the original values for one or more of the specified tests. HPHT fluid losses are measured in accordance with the procedures detailed in API RP 13B-1. HPHT fluid loss value should be less than 50ml/30min, more particularly less than 30ml/30 min and in some cases less than 20ml/30min.

[00133] The effects of FLA polymers described herein were examined in a water based drilling fluid formulation as shown in Table 3. The FLA polymers set forth in the following examples were prepared in accordance with Example 2 with some polymers being formed from different ratios of monomers as set forth in the following tables. The drilling fluid muds were prepared and sealed in OFITE stainless cells under N2 pressure of 150psi for 350°F, 200psi for 375°F, 250psi for 400°F, for 16 hours aging. HPHT fluid loss tests on drilling fluid formulations were conducted in accordance with the procedures detailed in API RP 13 B-1. Their BHR and AHR rheology results and HPHT Fluid loss measurements are provided in Tables 4-6.

[00134] Table 3 Mud formulation containing 0.3 % RM and 0.7- 1 % FLA for fluid loss measurements

Components Weight (g) Dosage in lb/barrel (ppb) Fresh water 310 271.25

KC1 55 48.13

RM HPHT viscosifier, -20% 8 1.4

PAC 0.5 0.44

FLA , -20% 20g (-0.7%) or 3.5

25g (0.8%), or 30g or 4.38

(1%) or 5.25

NaOH, 50% (pH value 10-10.5) Variable —

Barite 190 166.25

Hymod Prima Clay 15 13.13

Total volume (mL) 400ml

[00135] The following tables illustrate the synergism between the FLA described herein and other synthetic rheology modifier polymers.

[00136] Table 4 Synergy between RM and FLA at 350F

[00137] Table 5. Synergy between Rheology Modifier and FLA, between RM+FLA+PAC after 400F aging

Fann data 1% RM alone 0.3% RM+ 0.3% RM+ 0.3% RM+ 0.3%RM+

1 % FLA 0.83% FLA 1 % FLA 1 % FLA

500psi)

[00138] Table 5 clearly indicates the unexpected improvement in properties under high temperature conditions obtained from the inclusion of a polysaccharide such as PAC.

[00139] Table 6. Synergism between RM+FLA+PAC after 350F

[00140] Table 7. Synergism between RM+FLA+PAC after 400F

[00141] Table 8, FLA +PAC alone at 350 and 400F aging

500psi)

[00142] Table 9, solution study on synergy between RM+PAC and FLA

* - AM/NaAMPS/SA: 49/49/2

** - AM/NaAMPS/APTAC: 43.5/43.5/13

[00143] Although not wishing to be bound by theory, applicants propose the following explanation for the synergism observed in accordance with the present application: the cationic groups (e.g., trimethyl ammonium chloride from APTAC) on the polymer chain interact with inter or intra anionic sulfonic group from either RM or FLA to form a strong ionic bonds, which enhance the polymer thermal stability and prevent water from penetrating through the filter cake, thereby providing the desired fluid loss control property. Various methods can be utilized to measure the synergy associated with the FLA and RM polymers disclosed herein. One method involves aging the mud containing both the RM and FLA polymers described herein and then measuring the AHR mud rheology, and comparing the results with those obtained from muds containing only RM, or RM with other competitive FLA polymers. The synergistic combination of polymers described herein results in an improved AHR rheology. The synergistic combination will retain AHR mud rheology and exhibit acceptable fluid loss control properties. Those compositions without synergy will result in relatively poor AHR and FL.

[00144] The inclusion of some monomers may affect the basic characteristics of the polymer and typically should be avoided. For example the following composition resulted in an unacceptable reaction mixture:

NaAMPS/ACM/MAPTAC/DMACM DMACM is insoluble in the reaction mixture. Gel (60/40/6/16) particles formed.

Claims

What is claimed is:

1. A method of preventing fluid loss during oilfield drilling operations, said method comprising: drilling a wellbore; and circulating a fluid containing a synergistic combination of a fluid loss additive (FLA) and a rheology modifier (RM) wherein the fluid loss additive comprises a polymer polymerized from monomers consisting essentially of (A) 20-80 wt. % acrylamide, (B) 20- 80 wt. % 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and/or salt(s) thereof, and (C) 2-50 wt. % of a cationic monomer selected from the group consisting of quaternized

(meth)acrylate monomers, quaternized (meth)acrylamide monomers, diallydimethyl ammonium chloride (DADMAC), dimethylaminoethyl methacrylate (DMAEMA), 3- methacryloyloxy-2-hydroxypropyl trimethyl ammonium chloride, 3-acrylamido-3- methylbutyl trimethyl ammonium chloride, N-methyl 2-vinyl pyridinium methyl sulfate, N- propyl acrylamido trimethyl ammonium chloride, 2-methacryloyloxy-ethyl trimethyl ammonium methosulfate and combinations thereof, wherein said weight percentages are based on the total weight of (A) and (B) and the rheology modifier comprises a synthetic polymer polymerized from monomers comprising at least one of acrylamide and AMPS.

2. The method of claim 1 wherein monomer (C) comprises a quaternized (meth)acrylate or (meth)acrylamide monomer represented by the following structure:

wherein: Ri is hydrogen or methyl, X is O or NH,

Q is selected from a functionalized and unfunctionalizedalkylene, cycloalkylene, alkenylene, or arylene group, wherein any of the before mentioned groups may be with or without heteroatoms, R2, R3, and R4 are independently selected from the group consisting of functionalized or unfunctionalized alkyl groups, and

M is independently selected from the group consisting of alkali metal ions, alkaline earth metal ions, and the ammonium ion, and combinations thereof.

3. The method of claim 2 wherein monomer (C) comprises acrylamidopropyl trimethyl ammonium chloride (APT AC) and/or methacrylamidopropyltrimethylammonium chloride (MAPTAC).

4. The method of claim 1 wherein said FLA polymer comprises from about 40-60 wt. % acrylamide, 60-40 wt. % AMPS, and 5 - 30 wt. % APTAC and/or MAPTAC.

5. The method of claim 1 wherein said FLA polymer has a weight- average molecular weight of at least 3,000 Da.

6. The method of claim 1 wherein said FLA polymer has a weight- average molecular weight from 100,000 Da to 10,000,000 Da.

7. The method of claim 1 wherein said monomer (C) is MAPTAC.

8. The method of claim 1 wherein said monomer (C) is APTAC.

9. The method of claim 1 wherein the RM polymer comprises a terpolymer of acrylamide, 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and a long-chain alkyl acrylate having a chain length for the alkyl group of from 12 - 25.

10. The method of claim 9 wherein the RM polymer comprises from about 30-70 wt. % acrylamide, 70-30 wt. % AMPS, and 0.01 - 5 wt. % long-chain alkyl acrylate.

11. The method of claim 10 wherein the molecular weight (Mw) of the RM polymer is over 1,000,000 daltons.

12. The method of claim 11 wherein the molecular weight (Mw) of the RM polymer is from about 2,000,000 to 20,000,000 daltons.

13. The method of claim 1 wherein the fluid containing a synergistic combination of a fluid loss additive (FLA) and a rheology modifier (RM) is suitable for High Pressure/High Temperature (HP/HT) drilling operations at >300°F temperature.

14. The method of claim 9 wherein the RM polymer comprises from about 40-60 wt. % acrylamide, 60-40 wt. % AMPS, and 0.01 - 3 wt. % long-chain alkyl acrylate.

15. The method of claim 14 wherein the long-chain alkyl acrylate is selected from the group consisting of n-lauryl acrylate, n-hexadecyl acrylate, n-stearyl acrylate and combinations thereof.

16. The method of claim 15 wherein the long-chain alkyl acrylate comprises n-stearyl acrylate.

17. The method of claim 1 wherein the fluid containing a synergistic combination of a fluid loss additive (FLA) and a rheology modifier (RM) further comprises a polysaccharide.

18. The method of claim 17 wherein the polysaccharide is selected from the group consisting of polyanionic cellulose (PAC), hydroxyethyl cellose (HEC), Hydroxypropyl cellose (HPC), carboxymethyl hydroxyethyl cellulose (CMHEC), carboxymethyl cellulose (CMC), xanthan gum, guar gum and mixtures thereof.

19. The method of claim 18 wherein the polysaccharide is polyanionic cellulose (PAC).