NO812494L - PROCEDURE FOR THE PREPARATION OF A HOMOGENE, VISCED BROWN TREATMENT LIQUID IN A DRILL, AND COMPOSITIONS OF BROWN TREATMENT LIQUID - Google Patents

Description

The present invention relates to a method for increasing the viscosity and reducing fluid loss in aqueous salt solutions and aqueous well treatment fluids produced from the salt solutions.

Thickened aqueous media, especially those containing soluble salts, are usually used as well treatment fluids, e.g. drilling fluids, revision fluids, packing fluids, completion fluids, for treatment of underground formations, when leaving boreholes, for distance holding and for other aqueous fluids where high viscosity is desired. It is known to use hydrophilic polymer materials, such as hydroxyethyl cellulose (HEC) and xanthan gum, as thickeners for aqueous media used in such well treatment fluids. Such polymers, however, cannot be easily hydrated, solvated or dispersed in certain aqueous systems without the use of extremely high temperatures and/or mixing with high shear for longer periods of time. Hydroxyethyl cellulose polymer can e.g. do not hydrate, solvate or disperse well at ambient temperature in aqueous solutions containing one or more polyvalent water-soluble cation salts, such as strong solutions with a density greater than about 1.40 g/cm , which are used in well treatment fluids. Other polymers, such as xanthan gum, require elevated temperatures in solutions of even lower density. In many cases, e.g. in revision operations, the available equipment for the preparation of well treatment fluids cannot be readily used for mixing at high temperatures. In well treatment operations where a solids-free well treatment fluid is desired, e.g. in some completion operations, the fluid cannot be circulated in the hot wellbore to increase its temperature. It is therefore common to define ("spot") the fluid at the desired location in the borehole, where completion is to take place. The temperature in the borehole is sufficient for the polymer to hydrate when the fluid contains one or more hydrophilic polymers. However, it has been shown that in many cases the polymer has formed a lump or mass of hydrated polymer which has a high polymer content. This lump of hydrated polymer not only prevents the polymer from increasing the viscosity of the entire salt solution and/or reducing the fluid loss in the entire salt solution, it can also stop the perforations in the wells when the fluid is circulated out of the well.

The present invention therefore aims to provide a new method for producing a homogeneous, aqueous medium in a borehole, which contains one or more hydrophilic polymers which do not hydrate in the aqueous medium at ambient temperature.

The invention further seeks to provide an improved, aqueous, polymer-containing well treatment fluid which will form a homogeneous dispersion of hydrated polymer when aged statically in a borehole.

Another purpose of the invention is to provide a method for utilizing hydrophilic polymers in the well treatment fluid.

These and other purposes of the present invention will be apparent from the following description,

In one embodiment of the invention, a method is provided for producing a viscous, homogeneous, aqueous solution in a borehole. According to this procedure is added

the solution (1) a hydrophilic polymer which will increase the viscosity of the solution at ambient temperature and a hydrophilic polymer which will not increase the viscosity of the solution appreciably at ambient temperature, but which will hydrate in the solution at elevated temperature to increase the viscosity of the solution. The polymer-containing solution is mixed to increase the viscosity of the solution, the solution is pumped to the desired location in the borehole and aged so that both polymers are completely hydrated in the solution as a result of the elevated borehole temperature.

In another embodiment of the invention, a well treatment fluid is provided, which is distinguished by a much higher viscosity when it is aged at an elevated temperature than it had for the ageing, and which comprises an aqueous solution, a hydrophilic polymer which will not increase the viscosity of the solution appreciably at ambient temperature, but which will hydrate in the solution and make it viscous at elevated temperature, and a hydrophilic polymer which will hydrate in the solution and make it more viscous at ambient temperature in a sufficient amount to suspend the other polymer which does not hydrate at ambient temperature.

The well treatment fluid according to the invention requires an aqueous solution, a first hydrophilic polymer (hereinafter referred to as FHP), which does not hydrate appreciably in the solution at ambient temperature to make it more viscous, but which will hydrate in the solution and make it more viscous at an elevated temperature, and a second hydrophilic polymer (hereafter referred to as AHP) which will hydrate in and make the solution more viscous at ambient temperature. Thus, the AHP gives the solution an initial viscosity which makes it possible to keep the FHP suspended, so that the FHP is not hydrated as a lump of particles separated from the solution when it is statically aged at an elevated temperature.

The aqueous solution used can be an optional solution with the desired characteristics of a well treatment fluid, such as density, minimal influence on the formation, etc., for which an FHP and an AHP can be found.

In one embodiment of the invention, the aqueous solution contains a salt from the group comprising calcium chloride, calcium bromide, zinc bromide and mixtures thereof, and has a density in the range from approx. 1.29 g/cm 3 to approx. 2.30 g/cm 3 • Representative solutions which do not limit the invention are listed in Table I. Solutions in the density range from approx. 1.29 to approx. 1.81 g/cm contains a salt from the group comprising calcium chloride, calcium bromide and mixtures thereof. Solutions in the density range from approx. 1.81 to about 2.30 g/cm contains zinc bromide and a salt from the group comprising calcium chloride, calcium bromide and mixtures thereof. Solutions in the density range from about 1.70 to about 1.81 g/cm can contain zinc bromide and either calcium chloride, calcium bromide or mixtures thereof. In all these solutions, FHP can be XC polymer.

XC polymer is the heteropolysaccharide produced by Xantomonas campestris bacteria from a suitable carbohydrate containing nutrient solution. This is a well-known xanthan gum polymer that has found many uses as a viscosity finder in aqueous media, cf. e.g. US-PS 3,988,246. XC polymer hydrates in and increases the viscosity of strong salt solutions, as indicated in Table I to very little or no degree at ambient temperatures. At a temperature above 54.4°C, XC polymer is hydrated in these solutions. If the solution is stirred at the elevated temperatures, the XC polymer will hydrate to form a homogeneous, viscous solution. However, on static aging the XC polymer will hydrate as a mass of particles forming a "clump" and a low viscosity solution.

In solutions that have a density greater than about 1.74 g/cm, FHP can be hydroxyethyl cellulose (HEC)• In solutions that contain a salt from the group that essentially consists of calcium chloride, calcium bromide and mixtures thereof, and have a density in the range from approx. 1.74 to approx. 1.81 g/cm , HEC will not hydrate at ambient temperatures, but hydrates easily at elevated temperatures above approx. 54.4°C. Hydroxyethyl cellulose is also a suitable FHP in solutions containing zinc bromide and a salt from the group consisting of calcium chloride, calcium bromide and mixtures thereof and has a density in the range from approx. 1.71 g/cm 3 to about 2.30 g/cm 3„

Hydroxyethyl cellulose is a well-known viscosity-increasing agent for aqueous media, see e.g. US-PS 3 852 201. The HEC polymers are solid, particulate substances which are water-soluble or dispersible in water and which, after dissolution or dispersion in an aqueous medium, increase the viscosity of the system. HEC polymers are generally water-soluble, high-yield, non-ionic materials produced by treating cellulose with sodium hydroxide, followed by reaction with ethylene oxide. Each anhydroglycose unit in the cellulose molecule has three reactive hydroxy groups. the degree of substitution (SG) indicates the average number of hydroxy positions on the anhydroglycose unit that has been brought into reaction with ethylene oxide. The maximum SG

is 30 The molar substitution (MS) is defined as the average number of ethylene oxide molecules that have reacted with each anhydroglycose unit. When a hydroxyethyl group is attached to each unit, it can further react with additional ethylene oxide. Good water solubility is achieved with SG values in the range from approx

0.75 to approx. 1.75 and an MS value in the range from approx. 1.75 to about 3.0.

In solutions with a lower density than about 1.74 g/cm , which contain a salt from the group comprising calcium chloride, calcium bromide and mixtures thereof, AHP can be hydroxyethyl cellulose" HEC will easily hydrate in and make such solutions more viscous at ambient temperatures.

Solutions with a density in the range from approx. 1.74 g/cm to approx. 1.81 g/cm , which contains a salt from the group which essentially consists of calcium chloride, calcium bromide and mixtures thereof, and solutions which have a density in the range from approx. 1.74 to approx. 2.30 g/cm and contains zinc bromide and a salt from the group consisting of calcium chloride, calcium bromide and mixtures thereof which are difficult to make viscous at ambient temperatures. However, it is shown in Norwegian patent application no. 810352 and Norwegian patent application no. 81145R

that HEC can be activated, so that it will readily "hydrate in and make such salt solutions viscous at ambient temperatures. Thus, for these solutions, HEC is an effective AHP, provided it is activated.

Examples of appropriate ASP compositions in connection with the present invention, as indicated in Norwegian patent application no. 810352 include: hydroxyethyl cellulose, a solvating agent containing a water-miscible, polar, organic liquid, which in uniform mixture with HEC in a weight ratio HEC: solvating agent of 1:2 forms a mixture in which no free liquid solvating agent is present after standing for one week at ambient temperature in a sealed container, and a diluent comprising an organic liquid that is not a solvating agent. Preferred solvating agents are ethylene glycol, glycerol, the propane glycols, the butane glycols and mixtures thereof. The preferred diluents are isopropanol and the lower alkyl ethylene glycol ethers, such as ethylene glycol, n-propyl ether, ethylene glycol n-butyl ether, ethylene glycol isobutyl ether and the like.

This ASP composition preferably contains from about 15% to about 25% HEC, from about 15% to about 50% solvating agent and from about 25% to about 70% diluent. Other AHP compositions that are, for example, appropriate in connection with the invention, as indicated in Norwegian patent application no. 811456 include: HEC, an aqueous liquid and a water-soluble, organic liquid, which when mixed evenly with HEC in a weight ratio HEC: organic liquid of 1 :2 forms a mixture with the presence of free liquid after standing for one week at ambient temperature in a sealed container. Representative organic liquids are isopropanol and the lower alkyl ethers of ethylene glycol, such as 2-ethoxyethanol, 2-propoxyethanol, 2-soethoxyethanol and the like. The aqueous liquid can be water or an acidic solution, but is preferably a basic solution with a dissolved hydroxide content of less than about 3N. This AHP composition contains steps from approx. 15% to approx. 25% HEC, from about 15% to about 30% aqueous liquid and from about 45% to about 70% of the water-soluble, organic liquid.

The AHP concentration in the well treatment fluid according to the invention must only be sufficient to create a viscosity that keeps the FHP suspended and dispersed, while the well treatment fluid is aged statically at an elevated temperature in a borehole. The concentration will preferably be sufficient to give an API apparent viscosity of at least 100 centipoise of the fluid. The maximum concentration will only be limited by the maximum viscosity of the fluid that can be pumped down the hole with the equipment available at the well.

The FHP concentration in the well treatment fluid depends on the desired viscosity after the fluid is statically aged at an elevated temperature. The more FHP there is in the fluid, the higher the viscosity will be after heating. The FHP concentration will preferably lie in the range from about 1.4 g/l to about 28.5 g/l, most preferably 2.85 g/l to about 14.3 g/l.

Another feature of the invention is that highly viscous, homogeneous solutions can be obtained down in the borehole at the desired location and the prevailing temperature in a borehole, while previously known solutions would be too viscous to be prepared and pumped on the surface.

The method according to the present invention can be practiced by producing the well treatment fluid containing the aqueous fluid, FHP and AHP, by mixing the fluid, so that the AHP can be hydrated in and make the fluid sufficiently viscous for the FHP to be suspended, pumping the fluid to a desired location in a borehole and aging of the fluid at the prevailing temperature in the borehole.

The term "homogeneous" in the description and claims applies to the entire burn treatment fluid volume, as the FHP is hydrated in the fluid in the limited area where it is suspended, and the vis-

kose well treatment fluid can thus, after aging in the borehole, be considered a viscous suspension of hydrated polymer and on a microscopic scale may not appear to be homogeneous.

A simple test to determine whether a hydrophilic polymer will function as an FHP or AHP in a specific aqueous medium is as follows: 1. the aqueous medium and the hydrophilic polymer are mixed together at room temperature for one hour and 2. the polymer-containing, aqueous medium is stirred overnight at a temperature in the range from about 54.4°C to about 121°C, preferably at the expected borehole temperature.

If the polymer easily makes the aqueous medium viscous at room temperature, it can be used as AHP. If the polymer does not noticeably increase the viscosity at room temperature, but increases it at elevated temperature, it can be used as FHP. If the polymer does not increase the viscosity at elevated temperature, it cannot be used in well treatment fluids and in the method according to the invention.

Other hydrophilic polymers useful in connection with

the invention can be selected from the group consisting of other cellulose derivatives, water-dispersible starch derivatives, other polysaccharide gums, synthetic acrylic polymers and copolymers, etc. Examples of cellulose derivatives are the carboxyalkyl cellulose ethers, such as carboxymethyl cellulose and carboxyethyl cellulose; alkylhydroxyalkylcellulose, ie methylhydroxyethylcellulose, methylhydroxypropylcellulose; alkylcarboxyalkylcellulose, ie ethylcarboxymethylcellulose. Cf. US-PS 4,110,230. Examples of starch derivatives are the carboxyalkyl starch ethers, such as carboxymethyl starch and carboxyethyl starch; hydroxyalkyl starch ethers, such as hydroxyethyl starch and hydroxypropyl starch soap and mixed ethers, such as carboxyalkyl ;hydroxyalkyl starch, i.e. carboxymethyl-hydroxyethyl starch* alkyl-hydroxyalkyl starch, i.e. methyl-hydroxyethyl starch; alkyl-carboxyalkyl starch, i.e. ethylcarboxy-methyl starch. Examples of polysaccharide gums include other biopolymers, such as other xanthomonas (xanthan) gums, galactomannan gums, such as guar gum, locust bean gum, tara gum, glucomannan gum and derivatives thereof, especially the hydroxyalkyl derivatives. Cf. US-PS 4,021,355 and 4,105,461.

In particular, guar gum can be used as AHP in solutions with a density of up to approx. 1.70 g/cm , which contains calcium chloride, calcium bromide or mixtures thereof.

As a more complete illustration of the present invention, the following non-limiting examples are presented. All physical property measurements were performed in accordance with API test methods specified in Standard Procedure for Testing Drilling Fluid, API RP 133, 7th ed. April 1978.

Example I

An aqueous solution with a density of 1.80 g/cm 2 , containing 16.3% calcium chloride, 43.2% calcium bromide and 40.5% water, was mixed with the polymers listed in Table II for 5 minutes in a Multimixer at ambient temperature (23°C). After the rheological API values were obtained, the media were rolled at ambient temperature for 55 minutes and the API rheology was assessed. The media were then statically aged at 74°C for 18 hours, cooled to room temperature and assessed. The data obtained are shown in Table II.

Data indicate that the AHP composition easily made this solution viscous and formed a highly viscous fluid at ambient temperature. The FHP did not make this solution viscous at ambient temperature and, due to "a large lump of hydrated polymer formed by hot, static aging, produced poor viscosity after heating. The fluid containing both the FHP and AHP compositions illustrating the present invention exhibited low viscosity at ambient temperature and a

very high viscosity after static ageing.

The FHP used in this example was hydroxyethyl cellulose (Natrosol 250 HHR, a product of Hercules Inc.). The AHP used in this example was a composition containing 20% hydroxyethyl cellulose (Natrosol 250 HHR), 25% glycerol, 54.6% isopropanol and 0.4% CAB-0-SIL M5. The concentration indicated in Table II is based on 100% active HEC.

Example 2

An aqueous solution with a density of 1.92 g/cm and containing 38.7% calcium bromide, 24.6% zinc bromide and 36.7% water was mixed with 8.56 g/l HEC (Natrosol 250 HHR) as in Example 1. After hot, static aging, a large amount of HEC was present on the surface of the solution. By mixing this solution with 2.85 g/l HEC (Natrosol 250 HHR) and 5.7 g/l (on a 100% active HEC basis) of the AHP composition used in Example 1, a well treatment fluid was provided which after hot static aging produced a homogeneous, viscous fluid.

Example 3

An aqueous solution having a density of 1.39 g/cm and containing 37.6% calcium chloride and 62.4% water was mixed with the polymers listed in Table III for 15 minutes on a Multimixer and rolled at ambient temperature for 45 minutes. After achieving the rheological API values, the media were statically aged for 16 hours at 65.6°C, cooled to room temperature and evaluated. Data are reproduced in Table III.

Data indicate that HEC readily made this solution viscous at ambient temperature and was an effective AHP. The XC polymer

this solution did not become viscous at ambient temperature and hydrated after hot, static aging as a polymer mass at the bottom of the container. Well treatment fluids containing HEC and XC polymer showed an initial viscosity resulting from the hydration of HEC and a very high viscosity after

hot, static aging, as a result of the combined hydration of the HEC and XC polymers.

The invention can be used in other special embodiments without abandoning the idea and framework of the invention. The presented examples are therefore in all respects to be regarded as illustrative, but not limiting, as the scope of the invention is indicated in the following claims and not in the above description. All changes that fall within the meaning and scope of the claims are therefore considered to be within the scope of the invention.

Claims (10)

1. Method for producing a viscous, homogeneous, aqueous solution in a borehole, which contains one or more hydrophilic polymers, which will not hydrate in the aqueous solution at ambient temperature, characterized by (a) a hydrophilic polymer is added to the solution in sufficient quantity to increase the viscosity of the solution at ambient temperature, (b) a hydrophilic polymer is added to the solution which will not appreciably increase the viscosity of the solution at ambient temperature, but which will hydrate in the solution at elevated temperature to increase the viscosity of the solution, (c) mixing the polymer-containing solution so that the polymer in step (a) is allowed to increase the viscosity of the solution, (d) the polymer-containing solution is pumped to the desired location in a borehole and that (e) the polymer-containing solution is aged so that the polymers hydrate completely as a result of the borehole temperature to form the viscous, homogeneous solution. 2. Method as stated in claim 1, characterized in that the hydrophilic polymer in step (a) is hydroxyl cellulose and that the hydrophilic polymer in step (b) is XC polymer. 3. Method as stated in claim 1 or 2, characterized in that the aqueous solution contains a salt, selected from the group consisting of calcium chloride, calcium bromide and mixtures thereof and having a density in the range from about 1.29 to about 1.81 g/cm <3> , preferably from 1.74 to 1.81 g/cm <3> or a salt selected from the group consisting of calcium chloride, calcium bromide, zinc bromide and mixtures thereof and having a density in the range of 1, 29 to 2.30, preferably from 1.71 to 2.30 g/cm <3> . 4. Method as stated in claims 1-3, characterized in that the aqueous solution has such a composition that hydroxyethyl cellulose will not hydrate in the solution at ambient temperature. 5. Method as stated in claims 1-4, characterized in that the hydrophilic polymer in step (a) is hydroxyethyl cellulose, which has been activated to hydrate in the solution at ambient temperature and that the hydrophilic polymer in step (b) essentially consists of hydroxyethyl cellulose. 6. Well treatment fluid which is distinguished by having a much higher viscosity when aged at an elevated temperature than it had before ageing, characterized in that it comprises (a) an aqueous solution, (b) a hydrophilic polymer in an amount sufficient to increase the viscosity of the solution at ambient temperature and (c) a hydrophilic polymer which will not increase the viscosity appreciably at ambient temperature, but which will hydrate in the solution at an elevated temperature to increase the viscosity of the solution. 7. Well treatment fluid as stated in claim 6, characterized in that the hydrophilic polymer in (b) is hydroxyethyl cellulose and that the hydrophilic polymer in (c) is XC polymer. 8. Well treatment fluid as stated in claim 6 or 7, characterized in that the aqueous solution contains a salt selected from the group consisting of calcium chloride, calcium bromide and mixtures thereof, and in that the aqueous solution has a density in the range from 1, 29 to 1.81 g/cm <3> . 9. Well treatment fluid as specified in claim 6 or 7, characterized in that the aqueous solution contains a salt selected from the group consisting of calcium chloride, calcium bromide, zinc bromide and mixtures thereof, and in that the solution has a density in the range from 1, 29 to 2.30 g/cm 3 , preferably from 1.74 to 2.30 g/cm 3 . 10. Well treatment fluid as stated in claims 6-9, characterized in that the hydrophilic polymer in (c) is hydroxyethyl cellulose and that the hydrophilic polymer in (b) is hydroxyethyl cellulose, which has been activated so that it will hydrate in solution at ambient temperature.