GB2265924A - Well completion and workover fluids - Google Patents

Abstract

Hydrocarbon, particularly oil and gas, well completion and workover fluids which comprise inorganic caesium compounds, particularly salts, can be made with high densities and can incorporate viscosity improvers such as polysaccharides. Typically the fluid is based on caesium chloride and can include rubidium chloride and/or potassium chloride. Viscosity improvers such as polysaccharide e.g. starch, modified starch and xanthan gum can be included. The fluids have particularly good compatibility properties with polysaccharide viscosity improvers, especially at elevated temperatures. Completion and workover of hydrocarbon, especially oil and gas, wells using such fluids is particularly convenient because of the good control of properties of the fluid over a wide range of conditions, particularly temperature.

Description

Oil and Gas Well Completion and Workover and Fluids for Oil and Gas Well Completion and Workover This invention relates to completion and workover operations on hydrocarbon bearing formations, particularly oil or gas wells and to fluids for use in such operations.

In hydrocarbon recovery, particularly oil and gas well completion or workover operations, it is common practice to fill a bore hole leading into an oil bearing formation with a dense liquid which often desirably has a high viscosity. The density of the liquid is chosen so that the hydrostatic pressure at the base of the bore hole equals, or exceeds the pressure in the oil bearing formation at that point.

Preferably the viscosity of the liquid is sufficient to prevent undue loss of the liquid into the formation or entry of oil from the formation into the bore hole. When this has been achieved, mechanical devices and objects may be introduced down the well without the danger which would be attendant on doing so under conditions such that oil or gas might escape up the well. Such objects commonly include tubing for oil production. The tubing may be provided with packers at the bottom and sometimes also at the top to isolate the surrounding dense fluid when the tubing is installed. This arrangement enables a central tube to be used for the production of oil without the oil contacting the main bore hole and has the advantage that corrosion due to hydrogen sulphide andlor carbon dioxide passing from the oil bearing formation is limited to the inner tube which may be readily replaced.

Dense fluids of this type commonly include sodium, potassium, calcium and/or zinc in the form of their chlorides or preferably bromides. It is, however, difficult to produce liquids using these materials having a specific gravity greater than 1.7 without the inclusion of zinc bromide. Zinc bromide is toxic and the high concentrations of dissolved solids makes it difficult to disperse viscosity improving agents in the fluid. Typical viscosity improving agents such as polysaccharides e.g. starches, modified starches or xanthan gums, are difficult to hydrate in the fluid particularly in the presence of zinc bromide.

Previous proposals for this general type of fluid include those described in GB 2038907 A of Great Lakes Chemical Corporation which is directed to the use of a combination of aqueous zinc and calcium bromides and WO 92(09719 A of Mobil North Sea Limited which describes drilling fluids including additives such as polyalkylene glycols to reduce rock hydration during drilling as replacements for compounds such as potassium and calcium chlorides that had been used previously.

The disclosure of these documents is incorporated by reference herein.

This invention accordingly provides a method for completing or working over a well in a hydrocarbon bearing formation, particularly an oil or gas well, comprising a bore leading to a hydrocarbon, particularly an oil and/or gas, bearing formation in which the hydrocarbon, particularly oil and/or gas, is under pressure, which method includes the step of introducing into the bore a fluid comprising water and an inorganic compound of caesium, the fluid having a density such that the hydrostatic pressure exerted by the fluid at the opening of the bore into the formation is substantially the same as that exerted by the oil in the formation whereby little movement of fluid in the bore occurs.

Further steps for completing or working over the well by introducing items for example tubing and optionally packers into the bore or passing items through it may then be accomplished without substantial release of oil from the structure.

The invention also provides a hydrocarbon recovery, particularly an oil or gas well, completion or workover fluid which comprises water and an inorganic compound of caesium, the fluid having a density of 1.5 to 2 g.cm-3, and a viscosity of 10 to 1000 Poise (1 to 100 Pa.s) measured at 200C and a shear rate of 0.066 sec-l.

The inorganic compound of caesium is typically a caesium salt of an inorganic acid. Suitable examples include caesium sulphate, hydrogen sulphate, carbonate, bicarbonate, nitrite, nitrate, hydroxide, fluoride, chloride, bromide and iodide and mixtures of these. The chloride and bromide are particularly useful and the salt is especially caesium chloride. Optionally a sodium or preferably a rubidium and/or potassium halide, for example a chloride may also be present. If desired other materials for example alkaline earth metal for example calcium or zinc halides for example chlorides may be present. However, zinc is preferably absent for toxicity reasons.

Bromides may be used instead of chlorides to increase the density further. However for toxicity reasons it is preferred that the only halide ions are chloride ions. Preferably, the fluid is substantially free of carboxylate ions.

The fluids of this invention can include rubidium and/or potassium ions as well as caesium ions. In such fluids, we generally prefer that the concentration of rubidium ions is higher than that of potassium ions as this allows the density of the brine to be maximised.

We have found that compositions in which at least 50X and particularly 60 to 95Z of the cations by weight are caesium ions and in which any remaining cations include rubidium and/or potassium ions with the weight concentration of rubidium ions preferably exceeding that of any potassium ions by at least 50% and desirably at least 75% and in which any other cations present are at most 5% by weight of the total cations are very suitable.

In general it is desired that high densities of the fluid should be achieved, for example in the range 1.5 to 2 g.cm-3.

The ore from which caesium is derived commonly also contains rubidium and potassium. This makes it particularly convenient as a source of caesium for use in this invention especially as it minimtses or avoids purification of the caesium to remove rubidium and/or potassium. As the primary material recovered by caesium ore is a chloride brine, this can be used with the minimum of purification and this is economically advantageous.

A viscosity improving agent (thickening agent) is preferably also present. The viscosity improving agent is typically a polysaccharide such as starch, modified starch or xanthan gum. Other materials that can be used as viscosity improving agents include polymers of bacterial origin such as succinoglycan, cellulose derivatives such as carbomethoxycellulose or hydroxyethylcellulose, or other water soluble polymers such as guar gum, locust bean gum and similar polymers, which increase the viscosity of aqueous solutions. Synthetic polymers can also be used such as polyacrylamides, polyacrylates, polyamides and similar polymers. Such polymers work well with caesium compound based, particularly caesium chloride based, fluids.Our experience is that the compatibility of water soluble polymers and caesium compound based, particularly caesium chloride based, fluids is so good that water soluble polymers which act as viscosity improving agents in water or sodium chloride brines would be expected to give similar properties in caesium compound based fluids, particularly in caesium chloride brines.

By way of more particular application, if the cations in the fluid are substantially only alkali metal ions and the anions are substantially only halide, sulphate and/or nitrate ions, a polysaccharide thickening agent, for example starch, a modified starch or a xanthan gum may be used very effectively as the fluid favours hydration of such agents. Even when working with saturated solutions of such salts (e.g. about 5 molar), hydration of polysaccharides is easy and a very wide range of viscosities, even very high viscosities, may be obtained conveniently Incorporated into fluids the viscosity improving agent desirably behave as pseudoplastic polymers. The use of caesium compounds, especially salts such as the chloride enables the pseudoplastic properties of these polymers to be exhibited over a wide range of conditions.

Other additives can be included in the fluids of the invention in accordance with common practice. Examples of such additives include fluid loss agents particularly such as synthetic polymers such as polyacrylamides, polyacrylates, polyamides and similar polymers (some of which can also function as viscosity improving agents); corrosion inhibitors; scale inhibitors oxygen scavengers; and other similar additive materials.

The behaviour of the fluids of this invention towards viscosity improving agents, such as polysaccharides like starch, modified starch or, and particularly, xanthan gum contrasts with that of the currently used dense brines including calcium chloride, calcium bromide and zinc bromide, or their mixtures. In such conventional brines, it is extremely difficult to disperse typically used pseudoplastic polymers.

Dispersion of these polymers becomes easier if the brines are diluted with water; but this necessitates reducing the brine density. In the case of calcium chloride brine, if the density is greater than 1.28 g.cm-3, xanthan will not readily hydrate. Similarly, calcium bromide brine of density 1.7 g.cm-3 is not compatible with xanthan gum, and even if the water content is increased to reduce the density, for example to 1.57 g.cm-3 (3.6 molar), such that xanthan can be induced to hydrate a highly pseudoplastic fluid cannot be achieved.

We think than the reason for this behaviour may be that xanthan in dispersion, like other similar polymers, will undergo, at a certain temperature, a transition from an ordered to a disordered state. In the higher temperature, disordered, state the viscosity of the dispersion is markedly reduced. The transition temperature for xanthan is highly dependent upon the composition of the base brine.

Calcium bromide in particular has a very marked depressing effect on the transition temperature - at a calcium bromide concentration equivalent to a fluid density 1.5 g.cm-3, the transition temperature is less than 200C and the transition temperature decreases as the bromide concentration increases. Thus, in calcium bromide brines, although it is possible to achieve hydration of xanthan by dilution, to give a fluid with high viscosity at low shear rate, the viscosity produced is not stable at temperatures much above 200C.

In contrast, we have found that brine comprising water and caesium chloride - even up to the saturation concentration of the solute presents no difficulty in respect of dispersion of the commonly used biopolymers. A highly pseudoplastic dispersion is obtained with gzod temperature stability.

The following Example illustrates the invention.

Examole 1 Hydrocarbon completion or workover fluids, particularly suited to use in oil or gas wells, based on metal salt brines and containing xanthan as a viscosity improving agent were made up and tested. Example 1 is a fluid based on caesium chloride and was compared with fluids based on calcium bromide (Example 1C) and sodium chloride (Example 2C). The results are set out in Table 1 below which includes the brine concentrations, densities, xanthan concentrations and low shear viscosity measurements of the dispersions made using a model LVT Brookfield viscometer at a shear rate of 0.067 sec-1 at 200C and at 500C. From the results in Table 1, it is clear that the fluid of this invention can be made with a high density and a viscosity that can be controlled stably across a range of temperatures using xanthan as a viscosity improving agent.As temperatures of hydrocarbon e.g. oil and/or gas, bearing formations of 50 C and higher are frequently encountered the fluid of Example 1 has very attractive properties as a completion or workover fluid.

Table S

Salt Fluid Xanthan Viscosity Ex Salt Concn. density Concn. (cps) at No used | | | (molar) I(g.cm-3) | (g.1-1) I 200C I 500C I 1 CsCl 7.5 1.95 2.86 6900 5100 1C CaBr 3.13 1.5 2.86 740 461 4.28 1440 541 I I I 3.5 1 1.56 1 5.70 l 10900 1 2400 I I I I 1 4.28 1 3710 1 1000 1 I I I I I I I 2C NaCl 5.27 1.2 2.86 10000 7100 4.28 26000 27000

Claims (10)

1. CLAIMS 1 An oil or gas well completion or workover fluid comprising water and an inorganic compound of caesium, the fluid having a density of 1.5 to 2 g.cm-3 and a viscosity of 10 to 1000 Poise measured at 200C and a shear rate of 0.066 sec-1.
2. 2 A fluid as claimed in claim 1 which also includes a viscosity improving agent.
3. 3 A fluid as claimed in claim 2 wherein the viscosity improving agent is a polysaccharide.
4. 4 A fluid as claimed in claim 3 wherein the polysaccharide viscosity improving agent is starch, modified starch or xanthan gum.
5. 5 A fluid as claimed any one of claims 1 to 4 in which the inorganic compound of caesium is caesium chloride.
6. 6 A fluid as claimed in claim 5 which also includes rubidium and/or potassium chloride.
7. 7 A fluid as claimed in either claim 5 or claim 6 wherein substantially the only halide ions in the fluid are chloride.
8. 8 A method of completing or working over a hydrocarbon well comprising a bore leading to a hydrocarbon bearing formation in which the hydrocarbon is under pressure, which method includesjthe step of introducing into the bore a fluid comprising water and an inorganic compound of caesium, the fluid having a density such that the hydrostatic pressure exerted by the fluid at the opening of the bore into the formation is substantially the same as that exerted by the hydrocarbon in the formation whereby little movement of fluid in the bore occurs.
9. 9 A method as claimed in claim 8 wherein the fluid used is a fluid as claimed in any one of claims 1 to 7.
10. 10 A method as claimed in either claim 8 or claim 9 wherein the hydrocarbon well is an oil or a gas well.