ADDITIVE FOR A DRILLING FLUID

Abstract: An additive for a drilling fluid, comprising a composition which is at least dispersible in said drilling fluid at ambient temperatures, and has a solubility in said drilling fluid at drilling temperatures which is lower than the solubility at said ambient temperatures. The composition, when dispersed in water, has a higher affinity than water for the surface of a rock being drilled.

Description: ___-DIT___E KJ A DEΠTT.TIJK TTUID This invention relates to additives for drilling fluids. Drilling fluids are circulated down the wellbore being drilled during well drilling operations. The fluid is usually pumped down the inside of the drillpipe and then passes tiirouefli the drill bit into the wellbore. The fluid returns to the surface through the annulus defined between the exterior of the drillpipe and the wellbore. The fluid is then recovered, processed and reused. Drilling fluids are desirable in order to lubricate the drill bit, to cool the drill bit, and to remove rock cuttings generated. The physical properties of the drilling fluid (such as viscosity, density, salinity and filtrate loss) can be modified as necessary. One problem that occurs with water based drilling fluids is hydration of the rock being drilled. This problem is particularly acute when the rock includes clays and shales. Clays and shales display a great affinity for water. Absorption of water leads to swelling of the clay car shale and the resulting stresses can lead to failure. Thus, when the rock has been hydrated it can be subject to structural failure, and can collapse into the wellbore. This causes wellbore expansion, and can cause the drillpipe to become stuck. One solution to this problem involves the use of oil based drilling fluids, which are usually fσππulated with mineral oils. Although these fluids generally contain water, it is tightly emulsified using pc_werful surfactants. Such fluids are substantially inert to hydratable rocks. Such oil based drilling fluids are expensive but have been used extensively, particularly in the North Sea area. However, due to environmental problems they are being used less extensively. Another solution to the problem involves the use of water based drilling fluids which incorporate additives having surface active properties; such additives are sorbed onto the rock, reducing the access of water, and thereby reducing the rate of hydration. Anionic, cationic and nαnionic materials are used, because difference parts of a rock st_πaσture shew different affinities to different ionic charges. Examples of known additives for reducing hydration include: potassium chloride, partially hydrolysed polyaσrylamides (EHPA) ; gilsonite; polyglycerols; ca____G_y_____yl<-ellulose derivatives, such as polyanionic cellulose; calcium chloride; and sodium silicate. Potassium chloride cαnbined with PHPA has been widely used, despite the high costs and the difficulty in using the materials. Cationic Ifeterials can be hiφly effective but tend to be ilnccπpatible with other additives (mostly anionic) used in water based drilling fluids. There exists a need for an effective additive for use with water based drilling fluids, additive is cxaipatible with the other cxxnpαneπts of drilling fluids. According to one aspect of the present invention there is provided an additive for a drilling fluid, comprising a σcπpositica which is at least dispersible in said drilling fluid at ambient te_p_a_atures, and has a solubility in said drilling fluid at drilling temperatures vfaich is lcwer than the solubility at said ambient t__πperatures, and said exposition, when dispersed in water, has a higher affinity than water for the surface of a rock being drilled. Advantageously said cxmpositiαn is substantially insoluble in said drilling fluid at said drilling temperatures; and is soluble at ambient t__ιpe__at_u__-__. Ambient te_ιperatures would generally be frcm 0° to 50°C typically 0 to 30 . The upper end of the range is only likely to be reached in desert areas. Drilling t___peratures may be anything frcm ambient up to 200°C or more. A typical "bottom hole" ternperature in the North Sea would be about 120°C. Thus, the additive according to the invention will readily mix with the fluid at ambient t__ι eratures, but will ccme out of solution at drilling t___peratures and attach itself to the rock, thereby protecting the rock from hydration by water in the drilling fluid. Preferably the composition <__xπprises a polymer containing aUylene oxide repeating units. It is particularly preferred that the alkylene oxide includes ethylene oxide and/or propylene oxide. The polymer may advantageously be selected frcm the group consisting of: An ethylene oxide/propylene oxide block ccpolymer; a polyalkylene oxide-modified polymethyl siloxane; a polyalkylene oxide modified polyamine, alcohol or glycerine; and polypropylene glyσol. It is preferable that the polymer -___t__ins 5 to 75 wt% ethylene oxide and 25 to 95 wt% propylene oxide, more preferably 5 to 50 wt% ethylene oxide and 50 to 95 wt% propylene oxide. Desirably the molecular weight of the polymer is greater than 1000. A particularly effective polyalkylene oxide modified diamine is available under the registered trade mark SEPABASE 31. This material is available frcm BASF. A particularly effective polyalkylene oxide modified alcohol is available under the registered trade mark BP 75W 18000. This material is available frcm BP and is disclosed in a BP publication relating to BRE0X polyalkylene glyucol fluids and lubricants. Some of the additives acccx-Iing to the invention also provide excellent lubricity c__a_ac_teristics, particularly the ethylene oxide/prcpylene oxide block σopolymers. According to another aspect of the invention there is provided the use in the wellbore drilling of a stratum, of a saturated or unsaturated aqueous solution of a substance which exhibits a critical solution tercperature between ambient teπ erature and the service temperature at the stratum. The substance may comprise an additive having any combination of the features of the additive described above. The invention will now be further described with reference to the following examples. Comparative Example 1 A test was devised to establish a relative measure of shale stabilisation performance for various drilling fluids. The test involved static storage of 250 - 1000 micron sieved raw bentonite ore in the fluid under test for 16 hours at 80°C. A__terwards the bentonite was washed with a solution of 10% potassium chloride in tap water, and then resieved. The test was carried out twice for each fluid (test A and test B) , and the results are summarised in Table 1. Example 1 The test of comparative example 1 was carried cut with a drilling fluid consisting of seawater mixed with 1% by weight of a fluid according to the present invention. The test was carried out twice for each fluid (test C and test D) and the results are summarised in Table 2. Example 2 The test of example 1 was repeated using 5% by weight of additive, instead of 1%. The test was carried cut twice for each fluid (test E and test F) and the results are summarised in Table 3. In ccπparative example l, and examples 1 and 2, swelling leads to particle d_Lsintegration and weiφt loss, although in seme cases absαrbanσe can lead to a weight increase. The results from examples 1 and 2 shew that use of 5 wt% of the additive aexording to the invention, rather than 1 wt%, may lead to a significant iπp_oveπ_ant in inhibition only with cx_rtain additions and that performance may be optimized by suitable material selection. Comparative Example 2 Ccmparative example 3 was repeated for three prior art additives, using a different bentonite ore particle size of 1.0 to 2.0 mm. The tests were repeated twice for each fluid (tests G and H) , and the results are summarised in Table 4. Example 3 Ccmparative example 2 was repeated, each drilling fluid also being mixed with an additive ac_coι_ding to the invention, as well as the prior art additive. The tests were repeated twice for each fluid (tests I and J) , and the results are summarised in Table 5. A comparison of Table 5 with Table 4 shews that the addition of additives according to the invention to the drilling fluid can cause a significant reduction in weight loss, even when the drilling fluid already contains other additives. TART.T- ι : Results of Ccmparative Example 1 TEST A TEST B FLUID % Weight Chancre % Weight Chancre Seawater. -25.4 -26.2 Beπtonite/PAC (polyanionic cellulose)/Seawater. -23.5 -24.4 Oil base mud (lew aromatic mineral oil) . +5.9 +4.3 Xaπthan gum salt saturated (Sodium chloride) . -6.5 -6.8 PAC/35ppb Potassium chloride/ Seawater. -8.4 -9.7 20% HF100 (Hdrafluids polyglycerols)/Seawater. 5% HF100 1% HF100 TAT-T.TC _: Results of Example 1 TABLE 5: Results of Example 3 TEST I TEST J FLUID % Weight Change % Weicftit Change As Bentonite/PAC/seawater In coBoparative example 2 + 5% PE 310 -29.0 -11.0 As Bentonite/PAC/seawater In ccsiparative example 2 + 5% Silwet 720. -2.0 0.0 As Xaπthan gum/seawater In αaπparative example 2 + 2.8% Silwet 720. -12.0 -6.0 As Xanthan gum/seawater In cxxπparat ive exaπple 2 + 2.2% PE 3100. -16.0 -16.0

Claims: CLAIMS: 1. An additive for a drilling fluid, comprising a ccsrposition vSiich is at least dispersible in said drilling fluid at ambient temperatures, and has a solubility in said drilling fluid at drilling teπperatures which is lcwer than the solubility at said ambient teπperatures, and said composition, when dispersed in water, has a higher affinity than water for the surface of a rock being drilled. 2. An additive according to claim 1, wherein said composition is substantially insoluble in said drilling fluid at said drilling teπperatures. 3. An additive according to claim 2, wherein said composition is soluble in said drilling fluid at ambient t__speratures. 4. An additive according to claim 1, 2 or 3 wherein said cxsrposi iαn comprises a polymer ∞ntaining alkylene oxide repeating units. 5. An additive acxxarding to claim 4, wherein said polymer has a molecular weight of at least 1000. 6. An additive according to claim 4 or 5 wherein said alkyl oxide includes ethylene oxide. 7. An additive according to claim 4, 5 or 6, wherein said alkyl oxide includes propylene oxide. 8. An additive acx__rding to claim 3 or 4, -Λierein said polymer is selected frcm at least one of the group consisting of: an ethylene oxide/propylene oxide block ccpolymer; a polyalkylene oxide-modified polvjnethyl-siloxane; a polyalkylene oxide modified polyamine, alcohol or glycerine; and polypropylene glycol. 9. An additive according to claim 1, 2, 3 or 4, wherein said polymer contains 5 to 75 wt% ethylene oxide and 25 to 95 wt% propylene oxide. 10. An additive according to claim 1, 2, 3 or 4, wherein said polymer contains 5 to 50 wt% ethylene oxide and 50 to 95 wt% propylene oxide. 11. A drilling fluid comprising a mixture of water and substantially 0.1 to substantially 10 wt% of an additive according to any preceding claim. 12. A drilling fluid according to claim 11, comprising between substantially 0.1% and substantially 5% of said additive 13. A drilling fluid according to claim 11, comprising substantially 1 wt% of said additive. 14. A drilling fluid ac___α_ding to claim 11, 12 or 13, further cαaπprising Beπtor te/polyaniαnic cellulose. 15. A drilling fluid according to claim 11, 12 or 13, further apprising xanthan gum. 16. A drilling fluid according to claim 15, wherein said xanthan gum is saturated with sodium chloride. 17. A drilling fluid acx-ording to any of claims 11 to 15, wherein said water is seawater. 18. The use of an additive a<_x»rding to any one of claims 1 to 10 in a drilling fluid. 19. The use in the wellbore drilling of a stratum, of a saturated or xinsaturated aqueous solution of a substance which €s hibits a critical sol tion temperature between ambient temperature and the service tempera ture at the stratum.

Inventors: BRANKLING DAVID, BRANKLING, DAVID
Applicants: MOBIL NORTH SEA LTD, MOBIL NORTH SEA LIMITED
Application year: 1991
Main classification: E21B33
IPC codes: C09K8