GB2245292A - Drilling fluid - Google Patents

Abstract

A water based drilling fluid comprises: (a) an aqueous medium, (b) a cationic polysaccharide in amount 2 - 15 pounds per barrel (5.72 - 42.9 g/litre), (c) a water soluble polyacrylamide in amount 0.25 - 3 pounds per barrel (0.72 - 8.58 g/litre), (d) a water soluble salt of an alkali or alkaline earth metal in amount 5-50 pounds per barrel (14.3 - 143 g/litre), and (e) a monomeric or polymeric di- or poly-hydric alcohol or an ester of such a compound in amount 5 - 20% by volume of the total volume of the fluid. The combination of the cationic polysaccharide, the polyacrylamide and the alcohol or ester is particularly effective in providing fluid loss control and enhanced shale inhibition.

Description

DRILLING FLUID This invention relates to a water-based drilling fluid which is suitable for use in shale formations.

A drilling fluid is used in conjunction with the rotary system of drilling. The drilling fluid is pumped from the surface down the inside of the rotating drill string, discharged through ports in the bit and returned to the surface via the annular space between the drill string and the hole.

The drilling fluid serves to cool and lubricate the bit and drill string, bring drill cuttings to the surface, consolidate the side of the drilled hole, prevent squeezing-in or caving af the formation, control subsurface pressures, suspend drill cuttings when the column is static and minimise damage to any potential pay zone that might be encountered.

Drilling fluids generally contain a carrier, a weighting agent and chemical additives. They are commonly divided into two categories, water based muds (WBM) and oil based muds (OBM). In the former the carrier is an aqueous medium and in the latter it is an oil. Diesel oil was used in the past as the oil, but recently low toxicity drilling oils have been developed for this purpose.

While WBM are more environmentally acceptable than OBM (since the latter still give rise to the problem of disposing of large quantities of oil contaminated drill cuttings, even if the oil is of low toxicity), they are recognised as being technically inferior in a number of important areas, such as thermal stability, lubricity, and shale inhibition.

Although shale is soft and therefore relatively easy to drill through, it still causes many problems for the drilling engineer.

It disperses easily into the fluid, large lumps break off and fall into the hole, pores in the shale can contain fluids trapped under pressure, and in extreme cases, the borehole wall may collapse.

Since shale makes up a high proportion of the rocks drilled in exploratory and production wells for oil and gas, particularly in important producing areas such as the North Sea, it is important that drilling times and problems be kept to a minimum when drilling through such formations.

Many WBM formulations incorporating additives have been suggested in an attempt to control reactive shales. Such additives include: (a) salts such as potassium chloride to limit water uptake, reduce the swelling of the shale, and reduce leaching of any salt deposits encoubtered, (b) sodium carboxymethyl cellulose (CMC) or starch which is used to reduce fluid loss, (c) water soluble polyacrylamides which adsorb on the surface of shale to bind it with a coating of polymer, thereby reducing dispersion of the shale, (d) lime or gypsum which, although sparingly soluble, acts in a similar manner to (a), (e) gilsonite which assists in fluid loss control by acting as a blocking agent for cracks and microfractures.

To date, however, none of these formulations has been able to provide shale inhibition to the levels achieved with OBM.

This is because an OBM does not react with shale. A conventional WBM will, however, react to a greater or lesser extent with many shales causing them to swell and can give rise to problems such as stuck pipe, tight hole, overgauge hole, poor directional control, poor cementing and poor mud condition (leading to extensive dumping and diluting and therefore high mud costs).

It is an object of the present invention to develop a WBM which approaches the technical performance of an OBM without sacrificing its environmental advantages.

Our copending British patent application No 9013758.9 (BP Case No 7532) discloses a water based drilling fluid comprising: (a) an aqueous medium, (b) a cationic polysaccharide in amount 2 - 15 pounds per barrel (5.72 - 42.9 g/litre), preferably 4 - 8 pounds per barrel (11.44 - 22.88 g/litre), (c) a water soluble polyacrylamide in amount 0.25 - 3 pounds per barrel (0.72 - 8.58 g/litre), preferably 0.5 - 1.5 pounds per barrel (1.43 - 4.29 g/litre) and (d) a water soluble salt of an alkali or alkaline earth metal in amount 5 - 50 pounds per barrel (14.3 - 143 g/litre), preferably 10-40 pounds per barrel (28.6 - 114.4 g/litre).

The combination of a cationic polysaccharide and a polyacrylamide is particularly effective in the context of that application in providing both fluid loss control and enhanced shale inhibition.

We have now discovered that the addition of a monomeric or polymeric di-or poly-hydric alcohol or an ester of such a compound results in a markedly improved composition.

Thus according to the present invention there is provided a water based drilling fluid comprising: (a) an aqueous medium, (b) a cat ionic polysaccharide in amount 2 - 15 pounds per barrel (5.72 - 42.9 g/litre), preferably 4 - 8 pounds per barrel (11.44 - 22.88 g/litre), (c) a water soluble polyacrylamide in amount 0.25 - 3 pounds per barrel (0.72 - 8.58 g/itre), preferably 0.5 - 1.5 pounds per barrel, (d) a water soluble salt of an alkali or alkaline earth metal in amount 5 - 50 pounds per barrel (14.3 - 143 g/litre), preferably 10-40 pounds per barrel (28.6 - 114.4 g/litre), and (e) a monomeric or polymeric di-or poly-hydric alcohol or an ester of such a compound in amount 5 - 20Z by volume of the total volume of the fluid, preferably 5 - 10% by volume.

The fluid may also contain additional conventional ingredients such as water-insoluble weighting agents, eg barite, haematite or galena; viscosifiers, eg xanthan gum; and pH control agents, eg sodium or potassium hydroxide. The pH is suitably controlled to a value in the range 7 to 13.

The aqueous medium may be fresh or saline water.

The cat ionic polysaccharide is preferably a cationic starch.

The cat ionic starch may be a tertiary aminoalkyl ether of starch which is manufactured by reacting an alkaline starch slurry with 2-dimethylaminoethyl chloride. Another suitable cationic starch is a quaternary ammonium ether of starch formed by reacting an alkaline starch slurry with N-(2,3-epoxypropyl)-trimethyl ammonium chloride.

Suitable cat ionic starches include those sold under the Trade Name Catogel by National Starch and Adhesives.

The polyacrylamide may be a cationic polyacrylamide. Suitable cat ionic polyacrylamides include those sold under the Trade Names Alcomer 230 and 630 by Allied Colloids and Floc Aid 303, 304 and 307 by National Starch and Adhesives.

However it is prefered that the polyacrylamide is an anionic polyacrylamide.

Anionic polyacrylamides contain a proportion of acrylate or acrylic acid groups. These can be made by polymerising acrylamide and hydrolysing some of the amide groups which can then be converted to a salt. Alternatively they can be made by copolymerising acrylamide and an acrylate or acrylic acid and converting to a salt.

Suitable anionic polyacrylamides are those sold under the Trade Names Alcomer 120L by Allied Colloids, Floc Aid 201, 203 and 204 by National Starch and Adhesives and Drillam EL by Lamberti.

Suitable water-soluble salts of alkali or alkaline earth metals include the chlorides of sodium, potassium, calcium and magnesium.

The preferred salt is potassium chloride.

Suitable alcohols and esters include glycols, (eg triethylene glycol), polyglycols, glycerols, polyglycerols, mixtures thereof, and esters of such compounds. A suitable polyglycerol is that sold under the Trade Name HF100 by Hydrafluids.

The invention is illustrated with reference to the following Examples, bar charts presenting the results obtained in the examples.

Examples The following procedure was used for Example 1 and Examples 3-8.

Muds were prepared by dissolving the polymers in sea water and allowing the mixture to age for a minimum of 30 minutes. All mixing was carried out on a Hamilton Beach mixer. Potassium chloride was added to the solution of polymers and, finally, the pH adjusted with potassium hydroxide to pH 9.

The compositions were then subjected to a dispersion test and a swelling test.

These tests were carried out on London Clay, a Tertiary clay rich in swelling minerals and representative of a North Sea "gumbo".

a. Dispersion Test It has been found that the hot roll dispersion test commonly used by the drilling industry (and described in our copending British Patent application no 9013785.9, BP Case 7532) is not sufficiently severe to distinguish the inhibiting properties of water based muds once a certain level of inhibition has been attained. Consequently, a second and more aggressive dispersion test has been adopted which enables highly inhibitive WBM to be ranked in order of effectiveness. The test uses approximately 100 grams of shale chips in the size range 4 - 8mm. The chips are placed inside a cylindrical drum whose curved surface comprises a 0.5 mm brass or steel mesh. The drum containing the shale is partly immersed (to half the diameter of the drum) in the mud under test and the drum rotated for 4 hours at ambient temperature.After this time the shale is recovered, washed, dried and the amount of shale lost by dispersion into the mud is calculated.

b. Swelling Test It is often vital to know the extent by which a shale can swell when in contact with WBM since this property can have a major effect on the stability of the wellbore. Swelling of shale can lead to problems such as sloughing, caving, tight hole and stuck pipe.

An unconfined swelling test is carried out to assess the inhibiting power of WBM. The test uses core plug of shale with a diameter of about t inch and a length of between t and 1 inch. The shale is placed in a glass cell and a linearly variable differential transducer (LVDT) placed in contact with the top of the plug. The drilling fluid under test is then circulated past the plug and the extent and rate of swelling determined. Low amounts of swelling, and, in some circumstances, reduced rates of swelling, are equated with the most inhibitive muds.

Example 1 is a basic WBM 2 is illustrative of a conventional invert OBM.

3 is a modified WBM 4 is a WBM according to the invention disclosed in our copending applIcation no 9013758.9 5 is the mud according to Example 1 with the addition of 10% of a polygycerol 6 is the mud according to Example 1 with the addition of 20% of a polyglycerol 7 is the mud according to Example 3 with the addition of 10Z of a polyglycerol 8 is the mud according to Example 4 with the addition of 10% of a polyglycerol Examples 1-7 are provided for purposes of comparison and are not in accordance with the present invention. Example 8 is an illustration of the present invention.

The compositions of the formulations of Examples 1-8 are set out in the following Table 1.

Table 1

Component Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Ex 7 Ex 8 Sea Water Base - Base Base Base Base Base Base CaCl2 Brine - 20Z - - - - - KCl 25 25 25 25 25 25 25 Oil (1) - 80Z - - - - - Xanthan gum (2) 1 - 1 1 1 1 1 1 CMC (3) 4 - 4 - 4 4 4 - Cationic starch (4) - - - 4 - - - 4 Anionic polyacrylamide (5) - - 1 1 - - 1 1 Polyglycerol (6) - - - - 10% 20% 10% 10% All concentrations are in ppb (pounds per barrel) unless otherwise stated. 1 ppb = 2.86 g/l g's are percentages by volume (1) Oil - Low toxicity drilling oil BP 83HF ex BPCL.

(2) Xanthan gum - XC Polymer, ex Kelco (3) CMC - Na Carboxymethyl cellulose - CMC Lovis, ex Baroid (4) Cationic starch - Catogel, ex National Starch and Adhesives (5) Anionic polyacrylamide - Alcomer 120L, ex Allied Colloids (6) Polyglycerol - HF100, ex Hydrafluids The results of the dispersion and swelling tests for the compositions of Examples 1-8 are set out in the following Table 2.

Table 2

Property Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Ex 7 Ex 8 Recovery from Cuttings 16 98 41 65 70 84 78 84 Dispersion Test X Swelling Test Z 2.6 0.05 2.5 2.5 1.7 1.4 0.8 0.4 Example 1 shows poor results.

Example 2, the OBM, is clearly the best formulation. The formulation of Example 3 is an improvement over that of Example 1 but does not approach the inhibiting properties of the OBM.

The mud of Example 4 shows a significant improvement in dispersion over that of Example 3 but shows little improvement in terms of control of swelling. Example 5 shows some improvement in both dispersion and swelling.

Example 6 shows a further improvement in both dispersion and swelling, but the concentration of the polyglycerol is relatively high.

Example 7 shows a further improvement in swelling, but a deterioration in dispersion when compared with Example 6.

The combination of swelling and dispersion results makes the mud of Example 8 the most inhibitive of those WBM tested.

Claims (14)

Claims

1. A water based drilling fluid comprising: (a) an aqueous medium, (b) a cat ionic polysaccharide in amount 2 - 15 pounds per barrel (5.72 - 42.9 g/litre), (c) a water soluble polyacrylamide in amount 0.25 - 3 pounds per barrel (0.72 - 8.58 g/litre), preferably 0.5 - 1.5 pounds per barrel (1.43 -4.29 g/litre), (d) a water soluble salt of an alkali or alkaline earth metal in amount 5 - 50 pounds per barrel (14.3 - 143 g/litre), (e) a monomeric or polymeric di-or poly-hydric alcohol, or an ester of such a compound, in amount 5 - 20% by volume of the total volume of the fluid.

2. A water based drilling fluid according to claim 1 comprising: (a) an aqueous medium, (b) a cationic polysaccharide in amount 4 - 8 pounds per barrel (11.44 - 22.88 g/litre), (c) a water soluble polyacrylamide in amount 0.5 - 1.5 pounds per barrel (1.43 - 4.29 g/litre), (d) a water soluble salt of an alkali or alkaline earth metal in amount 10-40 pounds per barrel (28.6 - 114.4 g/litre), and (e) a monomeric or polymeric di-or poly-hydric alcohol, or an ester of such a compound, in amount 5 - 10Z by volume of the total volume of the fluid.

3. A water based drilling fluid according to either of the preceeding claims containing in addition one or more components selected from the group consisting of weighting agents, viscosifiers and pH control agents.

4. A water based drilling fluid according to claim 3 wherein the weighting agent is barite.

5. A water based drilling fluid according to claim 3 or 4 wherein the viscosifier is xanthan gum.

6. A water based drilling fluid according to any of claims 3 to 5 wherein the pH is controlled to a value in the range 7-13.

7. A water based drilling fluid according to any of the preceding claims wherein the cationic polysaccharide is a cationic starch.

8. A water based drilling fluid according to claim 7 wherein the cationic starch is a tertiary aminoalkyl ether of starch or a quaternary ammonium ether of starch.

9. A water based drilling fluid according to any of the preceding claims wherein the polyacrylamide is a cationic polyacrylamide.

10. A water based drilling fluid according to any of claims 1 to 8 wherein the pobyacrylamide is an anionic polyacrylamide.

11. A water based drilling fluid according to any of the preceding claims wherein the water soluble salt of the alkali or alkaline earth metal is a chloride.

12. A water based drilling fluid according to claim 11 wherein the chloride is potassium chloride.

13. A water based drilling fluid according to any of the preceding claims wherein the monomeric or polymeric di- or poly-hydric alcohol is a glycol, a polyglycol, a glycerol, a polyglycerol, or a mixture thereof.

14. A water based drilling fluid according to claim 1 as hereinbefore described with reference to the Examples.