WO1992014798A1

WO1992014798A1 - Drilling fluid

Info

Publication number: WO1992014798A1
Application number: PCT/GB1992/000251
Authority: WO
Inventors: David Brankling
Original assignee: David Brankling
Priority date: 1991-02-23
Filing date: 1992-02-12
Publication date: 1992-09-03
Also published as: GB2252993B; GB2252993A; GB9103834D0; GB9221226D0

Abstract

Drilling fluids are used to remove rock cuttings generated during the boring process in the drilling of wells. The drilling fluid has the composition of a water-soluble organic fluid maintained as the continuous phase of an emulsion by the action of a salt-containing aqueous solution material emulsified into the water-soluble organic fluid by suitable surfactants. The water-soluble organic fluid is insoluble in the emulsion with the salt-containing aqueous material. The water-soluble organic fluid is soluble in sea water or fresh water and upon disposal of the drilling fluid or coated drilled cuttings the emulsion organic fluid phase is resolubilised and removed from the immediate environment.

Description

"D-rϋXin- g 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 water-based 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 water- based 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 drill 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 an organic fluid which is at least 40% soluble in water and a salt-containing aqueous solution wherein the water-soluble organic fluid is insolubilised in the salt-containing aqueous solution.

Preferably, the water-soluble organic fluid is at least 50% soluble in water. More preferably, the water- soluble organic fluid is at least 80%, most preferably 100%, so that on exposure to water the organic phase of the emulsion resolubilises.

Preferably, the ratio of water-soluble organic fluid to water is in the region of 30/70 to 95/5 by volume.

Preferably, a gelling aid, for example a clay or modified organoclay, is present in the drilling fluid.

Preferably, a weighting agent, for example barite or haematite, is present in the drilling fluid.

Preferably, an emulsifier surfactant is used to produce the emulsion between the water-soluble organic fluid and the salt-containing aqueous solution.

Preferably, the water-soluble organic fluid comprises an ester, glycol, alcohol, glycol ether, alkanolamine or a combination thereof. Most preferably the organic fluid comprises a glycol ether.

Preferably, at least 5% of the content of the salt-containing aqueous solution is in the form of a soluble salt. Preferably the soluble salt comprises calcium chloride, sodium chloride or potassium chloride, most preferably calcuim chloride.

The presence of the salt in the salt-containing aqueous material results in the insolubility of the water- soluble ether in the salt-containing aqueous material.

Preferably the structure of the water soluble ether is: R[(-0C₂H₄)_n(-OC₃H₆)_m(-OC₄H₉)_p-0H]_r where R may be alkyl, alkylaro atic or alkoxide, substituted alkyl, alkylaromatic or substituted aromatic function, n and m being from 0 to 10 and r being from 1 to 3.

Minor proportions of treatment additives may be included in the drilling fluid to control fluid loss and viscosity.

Embodiments of the invention will now be described by way of example.

EXAMPLE 1

A drilling mud emulsion in which a sea water-soluble glycol ether forms an external continuous phase of the mud formulation is produced by mixing the following in order on a Silverson blender:

166.0 ml EDP (glycol ether ex BP Chemicals) 0.8 g Klucel H (hydroxypropyl cellulose ex Aqualon) 3.5 g Calcium oxide 15.0 ml PB82 (polyacid ex Union Camp) 3.0 ml Bitran H (imidazoline ex ABM Chemicals) 166.0 ml 25% calcium chloride solution

The Klucel H is provided as a viscosifier. The Bitran H is used as a surfactant emulsifier and the PB82 is also used as an emulsifier. The calcium oxide is present as an aid to emulsification which neutralise∑, the PB82 acid emulsifier. Mud properties measured on a Fann 6 speed rheometer at 38^°C (100°F) give the mud values:

Apparent viscosity 101 cP Yield point 40.3 Pa (84 lb/100 sq ft) Plastic viscosity 59 cP Gel strength 9.6/13.9 Pa (20/29 lb/100 sq ft)

The results demonstrate the formation of a drilling fluid emulsion of rheology suitable for drilling purposes.

5 g of sodium bentonite sieved to 1-2 mm particle size is placed in 50 g of the resultant drilling fluid and stored at 90-95°C in an oven for 16 hours. The sample is then removed, sieved through a 1 mm mesh screen and the extracted bentonite particles washed clean in a 10% solution of potassium chloride. The recovered solids are then dried at 90-95°C and reweighed.

Allowing for moisture content of the original material the percentage recovery of unchanged particle size bentonite is then calculated and is a measure of the inhibitive property of the fluid in preventing hydration of the bentonite clay.

The same procedure is used to compare the performance of sea water, a 50/50 oil mud emulsion, a bentonite/PAC gel mud and KCl/polyacrylamide blend which are all typical drilling fluid systems.

The results obtained are: Drilling Fluid %. Bentonite Recovery

Sea Water 12 Bentonite/PAC 28 KCl/Polymer 60 50/50 Oil mud 95 Fluid of Example 1 100

The results show that the drilling fluid obtained from the water-soluble glycol ether is highly inhibitive to clay materials giving better performance than the standard oil mud system and conventional water-based fluids.

EXAMPLE 2

A drilling mud emulsion in which a sea water-soluble glycol ether comprises the external phase of the mud formulation is produced by mixing the following in order on a Silverson blender:

100 ml P400 (400 mol wt polypropylene glycol ex Dow Chemicals) 15 g Perchem DMB (organoclay gellant ex Akzo Chemicals) 8 ml Bitran H (imidazoline ex ABM Chemicals) 100 ml 25% calcium chloride solution 2 ml ϋnitol AC (tall oil fatty acid ex Union Camp)

The Perchem DMB is used as a gelling aid. The Bitran H is again used as a surfactant emulsifier and the Unitol AC also being used as a emulsifier. Mixing is continued until a stable emulsion is produced with the polypropylene glycol as the external continuous phase. The mud at 38°C measured on a Fann 6 speed rheometer gives the mud values:

Apparent Viscosity 80.5 cP Yield point 11 pa (231b/100 sq ft) Plastic viscosity 69 cP Gel strengths 2.4/2.9 (5/6 lb/100 sq ft)

The above values demonstrate the formation of a fluid suitable for drilling purposes.

An advantage of the drilling fluid is that the water- soluble ether compound prevents hydration of clay minerals. The ether compound does not dissolve in the salt-containing aqueous solution but does so in low salinity fluids such as sea water.

The drilling fluid may be used to drill through water- sensitive rocks but upon disposal either as whole mud, or as a coating on removed cuttings the ether compound may be removed by washing or will redissolve upon disposal to the marine environment and degrade by natural processes.

This embodiment of the invention therefore provides a water-soluble fluid of improved environmental acceptance which acts in the manner of an oil mud as the formulated system but is readily removed from drilled materials.

Modifications and improvements may be incorporated without departing from the scope of the invention.

Claims

1. A drilling fluid comprising an emulsion of an organic fluid which is at least 40% soluble in water and a salt-containing aqueous solution wherein the water-soluble organic fluid is insolubilised in the salt-containing aqueous solution.

2. A drilling fluid as claimed in Claim l, wherein the organic fluid comprises at least one of an ether, an ester, an alkanolamine, a glycol, a glycol ether and an alcohol.

3. A drilling fluid as claimed in Claim 2, wherein the organic fluid comprises a glycol ether.

4. A drilling fluid as claimed in Claim l, 2 or 3, wherein the salt-containing aqueous solution comprises at least one of calcium chloride, sodium chloride and potassium chloride.

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

6. A drilling fluid as claimed in any one of the preceding Claims, included a gelling aid.

7. A drilling fuid as claimed in any one of the preceding Claims, included an emulsifier surfactant.

8. A drilling fluid as claimed in any one of the preceding Claims, wherein the salt-containing aqueous solution has a concentration of at least 5% by weight.

A drilling fluid as claimed in any one of the preceding Claims, wherein the organic fluid has at least 80% solubility in water.