GB2297103A - Base oil for well-bore fluids - Google Patents

Abstract

A base oil for well-bore fluids, such as drilling fluids, comprising a high proportion of normal alkanes having from 10 to 20 carbon atoms has low toxicity and good aerobic and anaerobic biodegradability.

Description

BASE OIL FOR WELL-BORE FLUIDS The present invention relates to base oils for use in well-bore fluids, such as drilling fluids, to well-bore fluids incorporating the base oil and to the use of such fluids.

Well-bore fluids, for example drilling fluids, are used in oil and gas recovery and in geothermal energy and mineral exploration and extraction operations. The fluid serves a number a functions including removal of drilled cuttings from the bore-hole and sealing of the well-bore surfaces so that fluid loss into the formation being drilled is minimized. The fluid also lubricates and cools the drill pipe during the drilling operation.

Oil based drilling fluids have been used for around 50 years for drilling underground formations to recover oil and gas. Oil based fluids are preferred systems compared with water-based formulations, especially where highly water-sensitive underground formations are being drilled.

In water-based fluids water tends to migrate from the fluid to the formation being drilled. This destabilizes the formation and can lead to disintegration and breakdown of the bore-hole. Furthermore, water-based fluids tend to be unsuitable for use at high temperatures and where highly deviated bore-holes may be required to reach the targeted formations.

In view of the disadvantages of water-based fluids the recent trend has been to use drilling fluids which are oilbased. However, oil-based fluids previously used containing base oils such as diesel and crude oil, are toxic and only slightly biodegradable. This is obviously unsatisfactory with respect to environmental health and safety considerations, especially as drill cuttings coated with or containing the fluid are usually discharged to the sea floor when drilling off-shore. The use of these kinds of oil-based fluids can have a detrimental effect on marine organisms.

It has been proposed to use mineral oils as the base oil in drilling fluids instead of other petroleum derived oils. However, while less toxic than these other petroleum based drilling fluids mineral oil based fluids are not very biodegradable. Surveys on drill cuttings on the seabed of the North Sea have confirmed the persistence of mineral oils.

More recently attention has focused on the use of esters, ethers and white oils as base oils. These have improved toxicity characteristics, approaching that of water, and have reasonably good aerobic biodegradability.

However, with the exception of the esters they typically exhibit poor anaerobic biodegradation. Anaerobic biodegradation is required, for example, at the sea floor.

Also, these base oils tend to have high kinematic viscosities and temperature limitations. Furthermore, with the exception of the white oils they are very expensive when compared to the kinds of mineral oils described above.

The present invention seeks to provide a base oil for use in oil-based well-bore fluids, such as drilling fluids, which overcomes the disadvantages discussed above. In particular, the present invention seeks to provide a base oil which has low toxicity, good aerobic and anaerobic biodegradability and a combination of physical characteristics which are especially well suited to its use as a well-bore fluid. It is also an object of the invention to provide a base oil which is inexpensive.

Accordingly, the present invention resides in the use as base oil in a well-bore fluid of a mixture comprising at least 70% by weight of one or more normal alkanes having from 10 to 20 carbon atoms, wherein the mixture has a pour point of less than 0oC.

The mixture typically comprises 30% by weight, or less, of one or more branched or cyclic alkanes having from 10 to 20 carbon atoms.

As it is extremely important that the base oil biodegrade as quickly as possible it is preferred that the mixture comprises a very high proportion of normal (straight chain) alkanes. Branched-chain and cyclic hydrocarbons are not broken down by bacteria as rapidly as normal hydrocarbons. According to a preferred embodiment of the invention the mixture comprises at least 90% by weight of the one or more normal alkanes. More preferably the mixture comprises at least about 98% by weight of the one or more normal alkanes. Mixtures of normal alkanes having from 11 to 16 carbon atoms are most preferably used.

The toxicity to flora and fauna of base oils used in drilling fluids is believed to be directly linked to aromatic content. To minimise toxicity therefore the aromatic content in the mixture must be kept as low as possible, 0.1t by weight being the typical maximum allowed.

The pour point of drilling fluids is a particularly important characteristic as it is essential that the fluid is capable of being pumped at the lowest temperature encountered during mixing, storage, transportation and use.

The pour point of the fluid is primarily influenced by the pour point of the base oil which is used. In accordance with the present invention the base oil mixture has a pour point of less than 0 C, preferably -6 C or less, and more preferably still, -9 C or less. These represent the pour point of the mixture without the addition of pour point depressants. Pour point depressants could be used but they tend to be of little effect in pure normal-alkane mixtures.

The addition of pour point depressants may in fact lead to undesirable side-effects such as increased toxicity and low flash point. Pour point depressants may also cause decreased stability of invert emulsion well-bore fluids.

To minimise or prevent the danger of fire or explosion of oil-based drilling fluids the base oil used should have a flash point which is higher than the surface circulating temperature of the fluid during drilling of the well-bore.

Preferably, the base oil used in the present invention has a flash point of at least 65"C, more preferably at least 80 C.

A further important characteristic of the base oil used is its kinematic viscosity. This is crucial to the ability of the drilling fluid to tclerate solids and water, whether added as integral components or accumulated during the mechanism of drilling into formations or by formation water intrusion. Generally, the lower the kinematic viscosity of the base oil used the higher the tolerance of the fluid is. Through practical experience it has been found that the base oil should preferably have a kinematic viscosity of from 1 to 10 cSt, more preferably from 1 to 6 cSt, at 40or, and this is a feature of the base oil of the present invention. Base oils having a viscosity of about 1 cSt at 40"C are believed to be the most tolerant to solids and water.Difficulty in pumping tends to be encountered when using base oils having a viscosity at 40"C of greater than 6 cSt.

Base oils which may be used in the present invention are commercially available, and tend to be produced through cracking, reaction and distillation processes.

The present invention also provides a well-bore fluid comprising as base oil the mixture as described above in combination with an emulsifier, oil-wetting agent, viscosifier, filtration control additive, rheology modifier, thinner and/or weighting agent. These are additives conventionally used in well-bore fluids and the combination which is used depends upon the desired characteristics of the fluid being formulated.

Emulsifiers which may be used include fatty acids, soaps of fatty acids and fatty acid derivatives and amidoamines, polyamides, polyamines, esters such as sorbitan monoleate polyethoxylate and sorbitan dioleate polyethoxylate, imidaxolines and alcohols.

Typical oil wetting agents which may be used include lecithin, fatty acids, crude tall oil, oxidized crude tall oil, organic phosphate esters, imidazolines, amidoamines, alkyl aromatic sulphates, alkyl aromatic sulphonates, and organic esters of polyhydric alcohols.

Typical viscosifiers include organophilic clays (eg.

hectorite, bentonite and attapulgite), oil soluble polymers and resins, and polymers such as sulphonated ethylene propylene diene (EPDM) terpolymers and sulphonated butadiene styrene copolymers.

As filtration control additives which may be used there may be mentioned asphalt and derivatives thereof, gilsonite, amine-treated lignite and polymers such as EPDM terpolymers, styrene butadiene copolymers and acrylate styrene copolymers.

Typical rheology modifiers include fatty acids and polymeric fatty acids.

Thinners which may be used include petroleum sulphonates, amidoamines, alkaryl sulphonates and polyamines.

Examples of weighting agents include barite, iron oxide, iron carbonate, calcium carbonate and galena.

It is possible to use the base oils alone, i.e.

without the use of the conventional additives described.

Typically, however additives are required to tailor the properties of the fluid to meet specific requirements.

The well-bore fluid may be an all oil-based fluid or an invert emulsion (i.e. a water-in-oil emulsion) formed using water, brine or a polar organic liquid which is insoluble in the base oil. Preferably, the polar organic liquid is glycerol, methanol or propylene carbonate. When the well-bore fluid is an invert emulsion, the emulsified phase typically represents from 1 to 70% by volume of the fluid.

The use of fluid which is an invert emulsion enables the overall cost to be reduced by reducing the volume of base oil needed. The water activity of the invert emulsion can be adjusted by the addition of inorganic salts to balance the water activity of the formulation being drilled into. Examples of inorganic salts which may be used include the sodium, potassium, calcium, magnesium, caesium and zinc chlorides, sodium, calcium and zinc bromides, sodium, potassium and caesium formates, sodium and potassium acetates and calcium and ammonium nitrates.

The typical proportions of these additives in the well-bore fluids of the present invention are shown in the following table.

Typical Xore Topical Base oil, Volume % 20-100 50-90 Emulsifier, lb/bbl 1-20 4-16 Oil wetting agent, lb/bbl 0-10 0.5-4 Viscosifier, lb/bbl 0-15 1-6 Filtration control additive, lb/bbl 0.5-25 1-10 Rheology modifier, lb/bbl 0-4 0.5-2 Thinner, lb/bbl 0-10 0.5-4 Weighting agent, lb/bbl 0-700 0-500 Water, Volume % 0-60 20-50 Calcium Chloride, lb/bbl 0-150 2-100 (to adjust water activity) In this table lb/bbl represents pounds per US barrel.

The well-bore fluids of the present invention are prepared by conventional techniques by mixing of the constituents.

Preparation of the oil-based drilling fluid may take place at a land-based mud-mixing facility, or at the well-site.

Mixing typically takes place in tanks equipped with circulating centrifugal pumps and agitation/shear equipment.

Although primarily described as a drilling fluid, the base oil described is suitable for use generally as a wellbore fluid. The base oil can be used for example as a pay zone drill in fluid, a completion fluid, a "kill" fluid, a packer fluid or casing pack, a "spotting" fluid or a "spacer".

The invention further provides a method of drilling using a well-bore fluid of the invention as described above.

The following Examples illustrate the present invention. Unless otherwise states US gallons and barrels are referred to herein.

Example 1 A laboratory barrel (350ml) of drilling fluid was prepared by mixing the various ingredients shown in the table below using a Silverson mixer at a speed of 6,000 rpm using a square hole disintegrator head. The total mixing time was 1 hour and the ingredients were added in the order listed, a period of five minutes being allowed between each ingredient addition. A water bath was used to maintain the temperature below 65"C (150"F).

TABLE 1

Ingredient Amount Function Base oil A 136 ml Base oil EZ MUL 2F1 10 g Emulsifier GELTONE 111 2.5 g Viscosifier Lime 4 g Alkalinity Control DURATONE HT1 4 g Filtration Control Freshwater 136 ml Emulsified Phase Barite 163 g Weighting Agent Calcium Chloride 60 g Water Activity Balance (82% pure) RM-631 1 g Rheology Modifier 1 - available from Baroid Limited The fluid formed has the following properties: Density :: 1.38kg/m3 (11.51b/gal) Oil/Water Ratio : 50/50 Water Phase Salinity : 250,000 ppm calcium chloride Base oil A has the following composition and properties: wt % n-C11 9.1 n-C12 21.7 n-C13 36.2 n-C14 31.3 n-C15 1.4 n-C16 0.04 > C16 trace aromatics < 0.1 Pour point = -9 C Flash point = 91 C Kinematic Viscosity at 40 C = 1.75cSt The properties of base oil A compared with some commercially available North Sea base oils is illustrated in Table 2 below.

TABLE 2

Base Oil Density Flash Point Aromatics Aniline Kin. Pour Point g/ml &commat; 15 C C (PMCC) % (Max) Point C Viscosity C cst at 40 C Test Procedure ASTM D129 ASTM D93 ASTM D2887 ASTM D611 ASTM D445 ASTM D97 Base Oil A 0.759 91 0.1 93 1.75 9 BP 83 HF 0.790 95 4.5 88 2.40 -10 Fina DMF 120 0.820 74 2.3 73 1.72 nd Fina DMF HF 0.820 103 3.9 82 2.90 -18 Total DFl 0.800 77 0.5 77 1.73 -39 Total HDF 0.814 100 6.0 86 3.20 -30 Shellsol D90 0.805 95 0.1 78 2.00 -20 Shellsol D70 0.792 72 0.5 78 1.62 -30 Clairsol 350 0.798 78 4.1 76 1.89 -35 Clairsol 450 0.815 93 4.4 88 3.40 -20 Clairsol 350MHF 0.783 94 2.0 78 2.20 -20 PETROFREE 0.860 179 0.0 na 6.00 -30 nd - not determined na - not applicable It can be seen from this table that base oil A compares very favourably with the other oils which are used to formulate well-bore fluids with respect to the combination of characteristics it possesses.

Example 2 The properties of a laboratory barrel (350ml) of the fluid of Example 1 were measured in accordance with API RP 13B-2. The properties of the fluid were also measured when contaminated with 35g Hymod clay, 10% by volume seawater and 10% by volume carnalite brine. The results are shown in Table 3 below.

TABLE 3 MEASURED PROPERTIES

Fluid of invention Fluid + Hymod Fluid + Fluid + Clay Seawater Carnalite Brine Hot Rolled (250 F) hours - 16 16 16 16 Fann 35 Readings at 120 F:: 600 rpm 85 101 159 118 96 300 rpm 55 61 104 72 59 200 rpm 43 45 85 54 45 100 rpm 33 30 61 35 30 6 rpm 16 13 26 14 11 3 rpm 14 12 24 13 10 Plastic Viscosity, cp 28 40 55 46 37 Yield Point, lb/100ft2 27 21 49 26 22 10 sec Gel, lb/100ft2 14 13 30 13 9 10 min Gel, lb/100ft2 20 20 40 20 10 Electrical Stability v 288 345 209 122 130 HPHT Filtrate, at 250 F, ml - 2.8 3.0 3.6 3.2 This table shows that the well-bore fluid of the invention is very stable to contaminants. It should be noted that the well-bore fluid exhibits very good, low rheological properties even though the water content of the fluid is 50% by volume. This shows how the low kinematic viscosity of the n-alkane mixture base oil contributes to the ability of the fluid to tolerate high added concentrations of water, and water as a contaminant.

Example 3 Following the same procedure as Example 1 a laboratory barrel (350ml) of well-bore fluid was prepared. The ingredients were mixed in the order listed in Table 4 below. Base oil A was the same as that used in Example 1.

TABLE 4

Ingredient ~ Amount Function Base Oil A 147 ml Base oil EZ MUL 2F1 20 g Emulsifier DURATONE HTt 11 g Filtration Control XP-101 (Experimental 3.5 g Filtration Control product) BENTONE 382 0.5 g Viscosifier SUSPENTONE1 4 g Viscosifer Lime 4 g Alkalinity Control Freshwater 28 ml Emulsified Phase Barite 589 g Weighting Agent Calcium Chloride 9.7 g Water Activity Balance (82% pure) RM-631 0.75 g Rheology Modifier 1 - available from Bariod Limited 2 - available from Rheox The fluid formed has the following properties: Density : 2.24kg/m3 (18.67 lb/gal) Oil/Water Ratio : 85/15 Water Phase Salinity : 250,000 mg/L calcium chloride EXAMPLE 4 Example 2 was repeated using a laboratory barrel of the well-bore fluid of Example 3. The contaminants used were Hymod clay (35g) and seawater (10% by volume). The fluid was hot rolled at the temperatures shown in Table 5 below.This table shows the properties of the fluid. TABLE 5 MEASURED PROPERTIES

Fluid of the Invention Fluid + Hymod Fluid + Seawater Clay Hot Rolled (350 F) hours - 16 - - - - - Hot Rolled (395 F) hours - - 16 - 16 - 16 Static Aged (395 F) hours - - - 16 - 16 - 16 600/300 rpm 130/75 132/78 131/74 165/98 182/111 183/120 136/85 168/109 200/100 rpm 58/38 61/40 58/37 74/48 88/62 97/68 67/27 87/61 6/3 rpm 12/10 16/14 14/12 17/15 28/27 28/24 16/14 24/20 Plastic Viscosity cp 55 54 57 67 71 63 51 59 Yield Point lb/100ft2 20 24 17 31 40 57 34 50 10 sec Gel lb/100ft2 12 16 22 18 26 28 22 23 10 min Gel lb/100ft2 21 21 24 26 32 36 25 28 Electrical Stability v 1393 1218 1484 1002 1215 1560 713 1211 HTHP fluid loss, ml &commat; 395 F 5.4 5.8 11.2 8.2 14.0 6.4 12.0 The results confirm the stability of the fluids of the invention even when exposed at high temperature to contaminants.

Claims (20)

1. Use as a base oil in a well-bore fluid of a mixture comprising at least 70t by weight of one or more normal alkanes having from 10 to 20 carbon atoms, wherein the mixture has a pour point of less than 0'C.

2. Use according to claim 1, wherein the mixture comprises upto 0.1% by weight aromatics.

3. Use according to claim 1 or 2, wherein the mixture comprises 30% by weight, or less, of one or more branched or cyclic alkanes having from 10 to 20 carbon atoms.

4. Use according to any one of claims 1 to 3, wherein the mixture comprises at least 90% by weight of one or more normal alkanes.

5. Use according to claim 4, wherein the mixture comprises at least about 98% by weight of one or more normal alkanes.

6. Use according to any one of the preceding claims, wherein the one or more normal alkanes have from 11 to 16 carbon atoms.

7. Use according to any one of preceding claims, wherein the mixture has a pour point of -6 C or less.

8. Use according to claim 7, wherein the mixture has a pour point of -9 C or less.

9. Use according to any one of the preceding claims, wherein the mixture has a flash point of at least 65"C.

10. Use according to claim 9, wherein the mixture has a flash point of at least 80"C.

11. Use according to any of the preceding claims, wherein the mixture has a kinematic viscosity at 40"C of from 1 to 10 cSt.

12. Use according to claim 11, wherein the mixture has a kinematic viscosity at 40"C of from 1 to 6 cSt.

13. Use according to claim 1 substantially as hereinbefore described.

14. A well-bore fluid comprising as base oil a mixture as defined in any one of claims 1 to 12 in combination with one or more of an emulsifier, oil-wetting agent, viscosifier, filtration control additive, rheology modifier thinner, weighting agent and/or other well-bore fluid additive.

15. A fluid according to claim 14 comprising from 20% to 100% by weight of base oil mixture, based on the total weight of the fluid.

16. A fluid according to claim 14 or 15 which is an all oil-based fluid.

17. A fluid according to claim 14 or 15 which is an invert emulsion with water, brine or a polar organic liquid which is insoluble in the base oil mixture.

18. A fluid according to claim 17, wherein the polar organic liquid is glycerol, methanol or propylene carbonate.

19. A fluid according to claim 14 substantially as described in Example 1 or 3.

20. A method of drilling a well which comprises using as drilling fluid the drilling fluid defined in any one of claims 14 to 19.