GB2264131A - Drilling fluid - Google Patents

Abstract

Drilling fluid comprises an emulsion of which the continuous phase is a water-soluble ester and the non-continuous phase is a salt-containing aqueous solution in which the ester is substantially insoluble. The fluid provides an environmentally-acceptable water-based system which resists shale hydration and salt dissolution.

Description

DRILLING FLUID This invention relates to a drilling fluid for use in the drilling of wells.

Drilling fluids are required to remove rock cuttings generated during a boring process, to lubricate and cool the drill bit and maintain the integrity of the hole.

Drilling fluids are circulated down a wellbore during well drilling operations. The fluid is usually pumped down a central drillstring, passes through the drill bit into the wellbore and then returns to the surface.

The fluid is then recovered, solid materials are extracted, and the fluid is processed and reused.

Physical properties of the drilling fluid such as viscosity, density, salinity and filtrate loss may be modified by chemical addition as necessary.

One major problem which occurs in the use of waterbased fluids as drilling muds is the hydration of rock being drilled; this is particularly acute when the interval contains clays and shales. These materials exhibit a great affinity for water and adsorption leads to swelling of the rock with resultant stresses leading to collapse of the borehole or loss of structure. Such failures lead to wellbore expansion, stuck pipe, excessive rheology, and general drilling problems.

A second problem with water-based drilling fluids which is particularly prevalent in the North Sea is the drilling of so called "salt stringers". These intervals comprise regions of high concentrations of water-soluble salts such as sodium, magnesium and potassium chloride which will dissolve in the drilling fluid and lead to hole enlargement, wash out and general failure of the wellbore.

One solution to the above problems has been the use of so called "salt saturated" solutions in which a soluble salt, usually sodium chloride, is dissolved at maximum concentration in the aqueous medium and used as the drilling fluid base. Such solutions limit shale hydration and prevent further dissolution of drilled salts into the fluid. However, salt saturated solutions are expensive, have limitations on the density range which may be used and limit the number of additives which may be used to control the properties of the drilling fluid.

A second and more widely applied solution involves the use of oil-based drilling fluids which are usually formulated with mineral oils. These fluids comprise a brine phase which is tightly emulsified into an external oil phase by the use of suitable surfactants.

Oil-based drilling fluids therefore present to the surface of drilled rocks an inert oil phase which will not hydrate shale nor dissolve salt. Oil-based fluids have a much wider range of density, rheology, thermal stability and application than salt saturated or waterbased fluids, and are greatly used. Cuttings recovered from oil-based fluids are covered with a thin film of oil which prevents hydration and breakage. However, disposal of rock cuttings which contain a significant proportion of water-insoluble oil, especially by disposal through marine dumping at the drilling site, is becoming environmentally unacceptable.

Other additives have been used to attempt to control the shale hydration of water-based fluids, for example potassium chloride, polyacrylamide, polyglycerols, carboxymethyl derivatives, gilsonite, calcium chloride and sodium silicate. However, none of these systems has proved to match the performance of oil-based fluid and importantly have minimal effect in preventing solution of salt sections.

There exists a need for an environmentally acceptable alternative to oil-based fluid which exhibits control of both shale hydration and salt dissolution and which may be used over the density range covered by oil-based fluid systems.

According to the present invention there is provided a drilling fluid comprising an emulsion of which the continuous phase is a water-soluble ester and the noncontinuous phase is a salt-containing aqueous solution in which the ester is substantially insoluble.

Preferably, the ester is a derivative of a polyalkylene glycol, for example a glycol ester acetate. The ester may have the general formula: R(OC2H4) (-0C H ) OCOR1 where R and R1 may be the same or different and each is selected from alkyl, alkylaromatic, substituted alkyl, substituted alkylaromatic and substituted aromatic, and n and m may be the same or different and each is from 0 to 10. The ethylene oxide/propylene oxide ratio may be selected to ensure adequate solubility of the ester group in water.

Typical esters for use in this invention are butyl diglycol actate, ethyl diglycol acetate, methoxypropyl acetate and ethyl glycol acetate.

The ester preferably has a solubility of at least 10% by volume in water, most preferably at least 80%, at a temperature between 0 and 300C.

The ratio of ester/water in the emulsion is typically from 30/70 to 95/5 by volume.

The water referred to herein may be sea water.

An emulsifier is effectively included in the emulsion and may be, for example, an organic acid, amide, ethoxylate, amine or phosphate More than one such emulsifier may be used.

The dissolved salt content should preferably be sufficient to provide insolubility of the ester. The salt can comprise for example calcium chloride, sodium chloride or potassium chloride, preferably calcium chloride.

Embodiments of the invention will now be described in the following Examples.

Example 1 A drilling mud emulsion in which a sea water soluble ester comprised the external phase of a mud formulation was produced by mixing the following in order on a Silverson blender: 197 ml Ethyl diglycol acetate 12 g Perchem 97 organoclay (Akzo Chemicals) 2 g PB82 (acid emulsifier ex Union Camp) 7 g Z-940 (amine emulsifier ex Union Camp) 3 g Calcium oxide 35 g Calcium chloride 82 ml Water 180 g Barite Mixing was continued until a stable emulsion was produced with an ester/water ratio of 70/30 and density of 1.44.

Mud properties were measured on a Fann 6 speed rheometer of 380C (1000F) giving the mud values: Apparent viscosity 39 cP Yield point 15.4 Pa (32 lb/100 sq. ft.) Plastic viscosity 23 cP Gel strength 7.2/9.6 Pa (15/20 lb/100 sq. ft.) After hot rolling the above system for 16 hours at 1210C (2500F) mud properties were remeasured at: Apparent viscosity 62.5 cP Yield point 28.3 Pa (59 lb/100 sq. ft.) Plastic viscosity 33 cP Gel strength 15./17.3 Pa (32/36 lb/100 sq. ft.) The results demonstrate the suitability of ethyl diglycol acetate to produce stable invert emulsion drilling fluids.

The solubility of the ester can be shown to be complete at all ratios in fresh water at ambient temperature and hence disposal of the fluid upon dilution has significant advantages over conventional insoluble bases.

ExamPle 2 The ethyl ester of propylene glycol monomethyl ether was produced by reaction of dipropylene glycol monomethyl ether with acetic acid in the presence of sulphuric acid at a temperature of 105-1150C.

Reaction was continued until conversion to the ester was almost complete as evidenced by acid value and saponification values.

The resultant ester was water soluble but insolubilised by the addition of calcium chloride.

An invert emulsion mud was produced as the following composition using a Silverson blender and mixing in the following order: 197 ml Dipropylene glycol monomethyl ether acetate 10 g Bentone 27 organoclay (Steetley) 2 g PB 82 (acid emulsifier ex Union Camp) 9 g Crodazoline 0 (amide emulsifier ex Croda) 3 g Calcium oxide 35 g Calcium chloride 82 ml Water 180 g Barite Mixing resulted in a dense drilling fluid emulsion of SG 1.44 in which the ester/water ratio was 70/30 and in which the ester formed an insoluble continuous phase in the emulsion.

Initial properties at 380C (1200F) were: Apparent viscosity 54.5 cP Yield point 20.6 Pa (43 lb/100 sq. ft.) Plastic viscosity 33 cP Gel strength 10.6/14.4 Pa (22/30 lb/100 sq. ft.) Again this fluid demonstrates the ready production of an emulsion with oil mud characteristics but in which the insoluble phase will redissolve in fresh water on dilution.

Example 3 Ethyl diglycol acetate was produced by the reaction of ethyl diglycol in the presence of a sulphonic acid with acetic acid.

At completion of the reaction the product was divided into two portions - one half remaining as the impure product containing catalyst and unreacted products, the second half distilled to obtain the pure ester.

Both products were found to be soluble to approximately 30% in sea water but essentially insoluble in 25% calcium chloride.

Emulsion mud were produced to the following formulation: 195 ml EDGA (ethyl diglycol acetate) 8 g Phosphate emulsifier SHA 5 g PB82 (acid emulsifier) 4 g Lime (calcium oxide) 35 g Calcium chloride 6 g Perchem 97 gellant 180 g Barite Muds properties were measured on a Fann 6 rheometer at 380C (1000F) with filtration properties measured at 930C (2000F) and 500 psi differential.

Muds were subjected to 16 hours hot rolling at 200"F before remeasurement of properties.

MUD PURE EDGA IMPURE EDGA BHR AHR BHR AHR Apparent viscosity 33.5 40.0 36.0 27.5 /cp Yield Point 4.3 12.5 4.7 6.3 /Pa Plastic Viscosity 29.0 27.0 29.0 21.0 /cP Gel Strengths 4.8/13.5 13.0/34.2 4.8/14.4 5.8/18.8 /Pa Fluid Loss 6.0 9.0 6.0 8.0 /ml The results demonstrate the formation of a good quality drilling fluid and show that it is not necessary to conduct the ester distillation and purification step to obtain a suitable mud system.

Modifications and variations of the above described embodiment can be adopted without departing from the scope of the invention.

Claims (11)

1. Drilling fluid comprising an emulsion of which the continuous phase is a water-soluble ester and the non-continuous phase is a salt-containing aqueous solution in which the ester is substantially insoluble.

2. Drilling fluid as claimed in Claim 1, wherein the ester is a derivative of a polyalkylene glycol.

3. Drilling fluid as claimed in Claim 2, wherein the ester is a glycol ether acetate.

4. Drilling fluid as claimed in Claim 1, 2 or 3 wherein the ester has the general formula: R(-0C H4) (OC3H6)rn OCOR1 where R and R1 may be the same or different and each is selected from alkyl, alkylaromatic, substituted alkyl, substituted alkylaromatic and substituted aromatic, and n and m may be the same or different and each is from 0 to 10.

5. Drilling fluid as claimed in any one of the preceding Claims, wherein the ester is selected from butyl diglycol acetate, ethyl diglycol acetate, methoxypropyl acetate and ethyl glycol acetate.

6. Drilling fluid as claimed in any one of the preceding Claims, wherein the ester has a solubility of at least 10% by volume in water at a temperature between 0 and 30"C.

7. Drilling fluid as claimed in any one of the preceding Claims, wherein the ester/water ratio in the emulsion is from 30/70 to 95/5 by volume.

8. Drilling fluid as claimed in any one of the preceding Claims, wherein the emulsion contains an emulsifier surfactant.

9. Drilling fluid as claimed in Claim 8, wherein the surfactant is selected from organic acids, amides, ethoxylates, amines and phosphates.

10. Drilling fluid as claimed in any one of the preceding Claims, wherein the salt is at least one of calcium chloride, sodium chloride and potassium chloride.

11. Drilling fluid substantially as hereinbefore described with reference to any one of the Examples.