GB2115459A - Drilling fluids and methods of using them - Google Patents

Abstract

0il based drilling fluids in which the oil of the oil base is substantially non-toxic to marine life include, as gelling agent, a hectorite gelling agent such as dimethyldioctadecyl ammonium hectorite. The fluids are of particular value for carrying out of a subsea bore hole debris that is to be dumped in the sea while contaminated with the oil. The oil of the oil base may be naphthenic or paraffinic oils.

Description

SPECIFICATION Drilling fluids and methods of using them Drilling fluids are used to carry debris, such as drill cuttings, out of a bore hole during the drilling of the hole or during other operations within the hole. Thus the fluids are circulated down the hole and carry the debris up the hole. Throughout this specification we use the term "drilling fluids" in the generic sense to mean the fluids (sometimes called muds) that are intended to be used during the actual drilling of an oil well or other bore hole as well as the fluids that are intended to be used at other stages, for instance the work over or completion of a well, such other fluids sometimes being known as work over fluids or packer fluids.

The debris that is carried from the bore hole by the drilling fluids is separated from the fluid at the head of the hole and the fluid is recycled. The debris may be dumped.

The drilling fluids consist of a liquid phase and often contain also a solid phase dispersed in it, for instance a weighting agent such as barytes. The liquid phase may consist of water in which various minor additions may be dissolved or dispersed, e.g. various gelling agents and dispersing agents.

However it is often found that best results are obtained, especially during drilling, when the liquid phase includes oil, the fluids then being referred to as oil based drilling muds or fluids. Thus the liquid phase may consist of oil or it may be a mixture of oil and water, for instance an oil-in-water emulsion or a water-in-oil emulsion.

Numerous oils have been proposed for use as the oil in the liquid phase of drilling muds. There have been some proposals to use vegetable or other edible oils but mineral oils have generally been considered as more satisfactory and cost effective. Various mineral oils have been proposed. A typical disclosure is in British Patent Specification No. 1,467,841 in which it is stated that the oil may be diesel oil, crude oil, kerosene or other aliphatic hydrocarbons or mixtures. Another appears in l US Patent Specification No.2,969,321 in which the proposed oils are topped crude oils, gas oils, kerosene, diesel fuels, heavy alkylates and fractions of heavy alkylates.Despite all these numerous proposals the oil was generally chosen having regard primarily to avaiiability and cost effectiveness and as a result the oil that is used in practice is generally diesel oil.

Despite the actual use of diesel oil in practice there are some examples in the literature of particular oils other than diesel oils. For instance various asphaltic, paraffinic and naphthenic oils are exemplified in US Patent Specification No. 2,698,833 and in US Patent Specification No. 3,840,460 there is an example of an oil base that is a blend of sulphurised lard oil, chlorinated paraffin and a naphthenic mineral oil. The oils exemplified in US Patent 2,698,833 generally appear unsatisfactory by todays safety standards because of their generally low flash points and the oil exemplified in US Patent 3,840,460 suffers from the cost and other disadvantages incurred in the use of oils other than mineral oils.

When the drill cuttings or other debri are separated from the drilling fluid, e.g. at the well head, the resultant separated debris will still be contaminated with the fluid phase of the drilling mud, and therefore with the oil if it is an oil based drilling mud. When the drilling is at sea the further treatment of the contaminated debris can create a problem. If the contaminating oil is toxic to marine life and the contaminated debris is simply dumped into the sea then this dumping contaminates the sea unacceptably. Diesel oil has been shown to be toxic to marine life and so debris contaminated with diesel oil has to be washed before dumping but this requires extra apparatus on the rig or drilling platform and results in the generation of washings contaminated with oil, which in turn then have to be separated or treated further before they can be discharged.

In U.S. Patent No. 3,594,317 the problems arising from the anti-pollution regulations concerning the use of oils in drilling muds are discussed and it is stated that it has become necessary to find materials other than oil which will provide the attributes of oil in drilling mud. The proposal in that specification is to use decyl alcohol as a component of an aqueous based mud. Whilst this may avoid pollution problems decanol is not a satistactory and cost effective alternative to oil in drilling muds, especially in the more difficult bore holes where sticking of, for instance, the drill pipe is a particular risk.

Recent tests in USA have indicated that the mineral seal oil available in USA from US refineries under the trade name Mentor 28 can be used in place of diesel oil as the oil in an oil based drilling fluid and that the resultant fluid is less toxic to marine life than fluids based on diesel oil.

Also we have discovered that certain other oils, especially naphthenic oils having low aromatic contents, have acceptably low toxicity, and indeed are much less toxic than Mentor 28 as supplied in USA.

The viscosity characteristics required of conventional muds are well known and the viscosity of the oil of the oil base in an oil based drilling mud is very significant in determining the viscosity of the mud.

Diesel oils have been regarded as having particular convenient viscosity properties, and this is one reason why they have been used so extensively.

It is standard practice to adjust the rheological properties of oil based and other drilling fluids by including gelling agents in them. A variety of materials have been proposed as gelling agents. The most widely used gelling agents are bentonites, for instance the material commercially available as DMB (drilling mud bentonite) and the products available as DMS, Sedapol 155 or Sedapol 44, or Claytone 34, Claytone 40, Claytone 1 MG and Perchem A2/31/1.

We have now surprisingly found that gelling agents which are effective with diesel oil and other conventional mineral oils are inadequately effective when the mineral oil is a non-toxic oil as defined above especially when it is one of the oils having particularly low toxicity and low viscosity, as discussed below. It seems that components, or fractions containing components, that contribute to the toxicity of diesel oil may contribute also to the gelling properties and so their absence results in rather poor gelling properties being obtained when using conventional widely used gelling agents, such as the bentonites.

Possibly fractions or components of fractions that are missing from the non-toxic oils may interfere with the gelling properties of conventional gelling agents.

Whatever the cause, we find that for good gelling properties with non-toxic oils it is necessary to select a particular type of gelling agent in order to obtain satisfactory results at economic dosages. In particular we select organophilic hectorite gelling agents.

Accordingly an oil based drilling fluid according to the invention has, as the oil of its oil base, a non-toxic mineral oil as defined above and includes, as gelling agent, an organophilic hectorite gelling agent. The fluid is especially suitable for carrying debris out of a subsea bore hole, prior to dumping of the debris in the sea while still contaminated with the oil.

The organophilic hectorite gelling agent may be present as a salt with an inorganic cation but preferably is present as a quaternary ammonium salt of a hectorite.

The gelling agent may be a naturally occurring hectorite or synthetic hectorite, for instance as descibed in British Patent Specification No. 1054111. If it is a synthetic hectorite it preferably includes exchangeable organic ammonium cations as described in British Patent Specification No. 1121501.

The preferred materials may be described as tetraalkylammonium hectorites, as described in British Patent Specification No. 121501. One to three of the alkyl groups are preferably short chain alkyl groups (e.g. C1 S most preferably C1~3, typically methyl) and one to three of the alkyl groups are preferably long chain alkyl groups te.g. C,0~2s, typically C1422, most preferably Cur8).

A preferred material is dimethyldioctadecyl ammonium hectorite, preferably Bentone 38 or lmvitone 1 or Imvitone 2, which are derivatives of naturally occurring hectorite.

The oil may be more viscous than diesel and may be for instance Mentor 28. Preferably however the oil of the oil base should at 50C, and generally also at 200 C, have a viscosity less than the viscosity of diesel oil. This is particularly important because of the low ambient temperatures encountered in many offshore drilling operations and the difficulties that follow from funnel and plastic mud viscosities that may be too high at ambient temperatures unless oils of very low viscosity are used. Generally the viscosity at 50C is below 15, preferably below 10, for instance 1 to 7 cSt.

The viscosity at 200C should be low, generally below 1 5 and preferably below 10, most preferably below 8. It is normally at least 1, typically from 3 to 8 and often 4 to 7 cSt. The oil of the oil base generally has a viscosity at 400C of below 6 cSt and preferably below 5.5 cSt. The viscosity is often in the range 1 to 5.5, for instance 3 to 5. However there are indications that best results are obtained with very low values, preferably 1.2 to 3.8 cSt.

The oil preferably has a viscosity at 1000C of from 0.6-2.5, generally 0.7 to 1.4 cSt. All viscosity measurements herein are the kinematic viscosity values obtained by ASTM 445/1 P7 1.

Toxicity can be observed by determining the effect of a selected amount of the oil in sea water on brown shrimps (Crangon crangon). Healthy shrimps are maintained in aerated sea water at 1 50C in the presence of a selected concentration of the oil and the mortality of the shrimps after varying periods is observed. On this test diesel oil gives high mortality, e.g. above 50% and often 90 to 100% at a concentration of 100 pI/I after 24 hours. The oils used in the invention give substantially no mortality (for instance below 10% and preferably below 1%) at 24 hours when present in amounts of 100 yI/I and preferably also substantially no mortality when used in amounts of 333 tLI/I for 24 hours.Preferably the mortality at 96 hours at 100 ulll is also low, generally below 30% and preferably below 15% and preferably also the mortality at 333 all at 96 hours is in the same range, most preferably below 15%.

Generally the toxicity is such that at least 50% of the brown shrimps survive for at lest 5 days at oil concentrations of at least 333,ul/l and often of at least 1000 /l. A typical diesel oil, No. 2 diesel oil, results in only 50% survival after as little as 5.6 hours at a concentration of 100 Vl.

We believe that some low molecular weight aromatic compounds are non-toxic and that the toxicity probably arises from the presence of some or all of the polynuclear aromatic compounds, where poly represents at least 4 benzene rings and generally 5 or more, (especially benzopyrene and 1,2,5,6 dibenzanthracene) and some lower molecular weight compounds such as toluene, xylenes, phenanthrenes and possibly also naphthalenes.

The oil is preferably substantially free of, for instance, benzopyrene and other aromatic compounds that cause toxicity. By this we mean that the oil is either totally free of benzopyrene and other toxic compounds or contains them in such small amounts that the toxicity of the oil is not raised unacceptably.

Because of the uncertainty of the nature of some of the aromatic compounds in oils containing a significant aromatic content it is preferred that the oil has an aromatic content of less than 5%, preferably less than 4% and most preferably 3.5% or less. The aromatic content of an oil may be recorded by test methods such as CSL 606-4, ASTM D2007 or ASTM D2140--66. Typically it may be determined as the percentage by volume of the oil that is provided by aromatic compounds. It can be measured by calculating the proportion of carbon atoms in the oil that are present in aromatic compounds, based on the total proportion of carbon atoms in the hydrocarbon content of the oil.

With many oils there is a significant increase in toxicity between the preferred oils used in the invention, typically having aromatic contents of 0.2 to 3.5%, preferably below 2.5%, and oils containing higher contents, for instance 7 to 12% aromatics. For instance Mentor 28 in USA seems to have an aromatic content above 1 0% and is found to be more toxic than is desirable. However if the oil is free of toxic aromatic compounds then the total aromatic content can be higher than 5% and may be as high as 10 or even 12%.

Preferred oils for use in the invention are naphthenic or paraffinic oils having low aromatic content.

Naphthenic oils may be derived from naphthenic crude and it seems that they can be much less toxic to marine life than diesel oil and US Mentor 28. The naphthenic oil may be obtained by blending two or more oils of which at least one generally is derived form naphthenic crude. For instance a blend may be formed of an oil derived from naphthenic crude and a paraffin oil, provided that the final blended oil can still be classified as a naphthenic oil. Naturally when blends are formed the blending oil must not be such as to introduce toxic components, and this is discussed in more detail below.

Suitable naphthenic crude for use as the source of some or all of the naphthenic oil is Venezualan crude. The oil may have been hydrogenated during its production from naphthenic or other crude to convert aromatic compounds to naphthenes.

Naphthenic oils are a well recognised class of oils clearly distinguished from paraffinic oils. They are characterised by the fact that they contain less than about 70% paraffinic (aliphatic) compounds and a substantial amount of naphthenic (cycloaliphatic) compounds. For instance at least 25% and preferably at least 35 or 40% of the oil is provided by naphthenic compounds. Best results appear to be provided when the oil contains 30 to 60%, preferably 45 to 60%, naphthenic compounds, but higher amounts (for instance up to 70% or 80%) or lower amounts (for instance 25 to 30 up to 45%) are sometimes suitable. The paraffinic content is preferably not more than 65%, or 70% at the most.

Naphthenic and paraffinic contents can be determined as above.

The naphthenic oil preferably has a characterisation factor of less than 12.0 and preferably from 11.8 to 11.0 or even down to 10.0.

Naphthenic oil derived from suitable naphthenic crude can have a satisfactorily low aromatic content but if the oil is obtained by blending then the oils blended into the naphthenic oil must not be such as to introduce toxic components and so the oil that is blended with the naphthenic should also be substantially free of toxic aromatic compounds.

Low odour kerosenes and other paraffinic oils having a low aromatic content are often suitable.

The mineral oil (or blend of mineral oils) is preferably substantially colourless and substantially odourless. It must of course comply with safety regulations and in practice this means that is must have a flash point of at least 600C, preferably 660C or more.

The initial boiling point of the distillation range of the oil used as the oil base is preferably below 25000. The A.P.I. gravity value of the oil is generally at least 1 5 and is normally below 35.

Four naphthenic oils suitable for use in the invention are 60 Solvent Pale and KL 55 (also known as Prospect 5) from J. O. Buchanan of Renfrew, Scotland, POLY-X-HP35 supplied by Burmah Castrol Company and Clairsol 350 supplied by Carless Solvents of Hackney Wick, London. Typically analyses of these oils are as follows::- 60 Solvent Pale KL55 Gravity A.P.I. at 150C 30.2 32.2 Density g/cm3 at 15 C 0.875 0.864 Flash Point-closed cup 1450C 1420C Pour Point -57 C -54 C Colour (Sabolt) 24 15 Viscosity cSt at 400C 7.7 6.6 Viscosity cSt at 1000C 2-2.1 1.9-2.0 Distillation-Initial Boiling Point 2750C 2940C Final Boiling Point 350 C 329 C(95%) Aniline Point 760C 800C Sulphur Content 0.1-0.2% less than 0.1 Paraffinic Content 48.2% 53.9% Naphthenic Content 48.5% 42.2% Aromatic Content (ASTM D2140) 3.2% 3.9% Characterisation Factor 11.6 11.5 POLY-X-HP35 Colour, Saybolt +20 Density at 200C 0.860 Kinematic Viscosity at 200C cSt 6 Kinematic Viscosity at 400C cSt 3.6 Viscosity at 100 C cSt 1.1 Flash Point (PMCC) C 115 Pour Point C -66 Sulphur Content % 2.2 Aniline Point 91#1 C Aromatic Content Atoms 6% Naphthenic Carbon Atoms 54% Paraffinic Carbon Atoms 40% Clairsol 350 Typical Properties Test Method Odour Good Colour Water White Densityati50C 0.788 ASTM D1298 Distillation Range OC ASTM D 86 Initial Boiling Point 200 50% Distils at 221 Dry Point 248 Flash Point OC 74 ASTM D 93 Kauri Butanol Value 28 ASTM D1133 Aromatic Content v/v 0.2% CSL 606-4 Low Explosive Limit 0.6 (% volume in air) Viscosity at 200C 2.3 cSt Upper Explosive Limit 7.1 (% volume in air) Autoignition Temperature OC 230 Naphthenic content 40% v/v Isoparaffin content 20% v/v n-paraffin content 40% v/v Threshold Limit 200 by calculation Value (TLV) ppm Other oils having similar analyses may be used especially other napthenic solvents, for instance having characteristics similar to Clairsol 350.

Any of these oils can be used individually or blends can be formed of two or more of these oils or of one or more of these oils with another oil, for instance a paraffinic oil. A suitable blend is formed of 40 to 90, preferably 60 to 80, parts by volume of a naphthenic oil with a paraffinic oil, provided the blend still has a sufficiently high naphthenic content to be classed as a naphthenic oil.

A suitable oil for use in the invention is formulated by blending 70 parts by volume of 60 Solvent Pale Oil and 30 parts by volume of Clairsol 350. The resultant blended naphthenic oil has the following properties.

Typical Properties Aniline Point 75.40C Flash Point 960C Pour Point below -500C Viscosity at 400C 4.1 9 cSt Distillation range Initial boiling point 2140C 10% boiling 2360C 50% boiling 2920C 90% boiling 3200C Final boiling point 3350C Estimated aromatic content 2.37% Specific gravity 0.849 Other paraffinic or naphthenic oils having similar properties may be used. One such other oil is the product sold by Norol of Norway under their Trade Name Lampeparafin. Another is the paraffinic oil sold in the United Kingdom as Mentor 28.

The oil base of the drilling fluid may consist of the described mineral oil or it may be a blend of the described mineral oil and water. At least 1% by volume of this blend must be the mineral oil and generally the amount of oil is at least 30% by volume based on water plus oil, with the amount preferably being from 51 to 99%, most preferably 60 to 95% by volume oil, with the balance to 100% by volume being water. Depending upon the emulsifiers present and the amounts of oil and water the fluid may be a water-in-oil emulsion or an oil-in-water emulsion.

The water used for forming the fluid may be fresh water or sea water and may contain dissolved salts such as sodium chloride or calcium chloride, up to saturation concentrations. Thus the fluid may be an oii-in-water emulsion in which the water is a sodium chloride brine. An advantage of the use of the defined oils is that emulsions formed from them tend to be more stable than the corresponding emulsions formed from other, relatively non-toxic, mineral oils such as various paraffinic oils.

The drilling fluids may contain other additives as is conventional in oil based drilling fluids and these additives may be dissolved or dispersed in the oil base. Thus they may contain one or more emulsifiers, for instance, polymerised organic acids such as the product sold by the Applicant under the Trade Name Carbo-tec L and oil soluble amide polymers that are wetting agents and supplementary emulsifers, such as the product sold by the Applicant under the Trade Name Carbo-Mul. The amount of any emulsifiers is generally from 0.1 to 10% (of the commercial emulsifier) by volume, most preferably 1 to 5% by volume, based on the total volume of oil and water, or 1 to 20%, preferably 2 to 5% based on the water.

The mud may contain high molecular weight organic polymers and inorganic bridging agents, such as the mixtures sold by the Applicants under the Trade Name Carbo-Trol. Lime hydrate may be dissolved in the water.

The drilling fluids will, in particular, generally contain a large amount of weighting material, such as barite, iron oxide, siderite or calcite. The amount of weighting aid is generally from 100 to 400 grams per 100 cc drilling fluids, for instance 200 to 500 pounds per barrel. In general the amount of hectorite gelling aid required for optimum properties in the fluids is greater than the amount that would be required, of either hectorite gelling aid or other gelling aid, in conventional drilling fluids, for instance being from 1.5 to 2.5 times the amount required for optimum properties when the oil of the oil base is diesel oil.

The amount of the hectorite gelling aid is typically from 1 to 10, preferably 1.25 to 4, grams gelling aid per 100 cc fluid. An alternative way of expressing the amount is as 3 to 1 5, most preferably 5 to 9, pounds gelling aid per barrel drilling fluid.

The following are examples of the invention.

EXAMPLE 1 A drilling fluid was prepared containing 212 cc Pale Oil 60 as defined above, 7 cc blown tall oil emulsifer, 5 cc of oil soluble amide polymer as a secondary emulsifier, 53 cc water containing 25% calcium chloride, 6 g lime hydrate, 7 g of a blend of high molecular weight organic polymers and inorganic bridging agents, 358 g barite and 6 g dimethyldioctadecyl ammonium hectorite.

Its properties were measured before and after hot rolling at 1 220C for 1 7 hours. (H/R). It was labelled mud No. 1. Corresponding drilling muds, labelled Muds 2 to 6, were made up from the same recipe except that the hectorite was replaced by equal amounts of bentonites, namely Claytone 40, Claytone 34, Drilling Mud Bentonite, Sedapol 44 and Sedapol 155. The properties of these muds were recorded and the results are set out in the following table. ES is electrical stability.

Mud No. 1 H/R H/R 2 H/R 3 H/R Mud Weight 14.5 14.5 14.5 14.5 14.5 14.5 14.5 Flow Prop. Test temp C 49 49 65 49 49 49 49 Fann Readings rpm 600 223 226 126 198 164 176 232 300 128 130 71 110 89 97 122 200 95 94 52 89 63 70 85 100 59 55 32 53 37 42 47 6 16 13 9 13 7.5 10 7 3 14 10 8 11 6 8 6 Plastic Viscosity cp 95 96 54 88 76 79 110 Yield Point g/100 cm2 16.5 17 8.5 11 7 9 6 Gel Strengths g/100 cm2 8/ 8.5/ 4/ 6/ 3.5/ 4.5/ 4.2/ 12.5 11.5 7.5 9.5 6.5 6.5 6 E.S. volts at 49 C 1430 1160 2000+ 1100 1370 1230

Mud No. 4 H/R 5 H/R 6 H/R H/R Mud Weight 14.5 14.5 14.5 14.5 14.5 14.5 14.5 Flow Prop. Test temp CC 49 49 49 49 49 49 65 Fann Readings rpm 600 211 185 162 170 199 172 129 300 115 104 89 91 112 103 70 200 84 74 64 64 82 74 51 100 51 | 43 39 36 50 43 30 6 6 12 10 9 6.5 12 8.5 7 3 10 8 7 5 10 7 6 Plastic Viscosity cp 96 81 73 79 87 69 59 Yield Point g/100 cm2 4 11.5 8 6 17.5 17 5.5 Gel Strengths 9/100 cm2 5.5/ 4/6 3.5/ 2.8/ 5/7.5 3.5/ 3.2/ 6.5 5.5 4.5 6.7 5.5 ES. volts at 490C 1460 1100 1300 1070 1450 1040

It will be apparent from this table that mud 1 had the best ratio viscosity:yield point before and after hot rolling, this indicating that it has the best gel properties before use and when in use down hole.

When the oil of mud 1 was tested for toxicity by the method described above, it was found that after 96 hours it caused about 3% fatality at 333 yI/I and up to 1 5% fatality after 120 hours. In the same tests number 2 diesel oil gives 93% fatality after 24 hours and 100% fatality after 72 hours at 100 ,uVl.

EXAMPLE 2 A drilling fluid, labelled Mud 7, is prepared from the same materials as Mud 1 in Example 1 except that the oil is replaced by POLY-X-HP35, the amount of blown tall oil emulsifier is increased to 10 cc and the amount of secondary emulsifer is reduced to 2 cc.

Muds 8 and 9 are prepared from the same materials as mud 7 except that the hectorite gelling agent is replaced in mud 8 with Perchem A2/31/1 and in mud 9 by Claytone IMG. The properties of these muds were recorded and the results are set out in the following table.

In these test the hot rolling is for 17 hours at 150 C. Perchem A2/3 1/1 and Claytone IMG are bentonites.

Mud No. 7 H/R H/R Mud Weight 14.5 14.5 14.5 Flow Properties Test Temp CC 49 49 60 Fann Readings RPM 600 105 145 98 300 65 82 53 200 51 59 37 100 35 34 21 6 6 13 4 2 3 12 2.5 2 Plastic Viscosity cp 40 63 45 Yield Point g/100 cm 12.5 9.5 4 Gel Strengths g/100 cm 7/11.5 1.7/4.8 1/1.5 E.S. Volts 2000+ 1210 1230 HT/HP at 150 C/500 PSI 1.8 cc

Mud No. 8 H/R H/R Mud Weight 14.5 14.5 1 4. Flow Properties Test Temp CC 49 49 60 Fann Readings @PM 600 74 103 70 300 39 55 35 200 28 37 24 100 17 20 13 6 4 2.5 2 3 3 1.5 1.5 Plastic Viscosity cp 35 48 35 Yield Point g/100 cm2 2 3.5 0 Gel Strengths g/100 cm2 2.5/4 1.2/1.8 1/2.5 E.S. Volts 1460 1180 1250 HT/HP at 150 C/500 PSI 9.4 cc

Mud No. 9 H/R H/R Mud Weight 14.5 14.5 14.5 Flow Properties Test Temp C 49 49 60 Fann Readings RPM 600 101 107 79 300 61 56 M 200 46 40 28 100 31 22 15 6 11 2.5 2 3 10 2 1.5 Plastic Viscosity cp 40 51 38 Yield Point g/1 00 cm2 10.5 2 1.5 Gel Strengths g/100 cm2 6/9.5 1.5/2 1/1.5 E.S. Volts 2000+ 1190 1080 HT/HP at 150 C/500 PSI 21 cc

H/R - Hot rolled for 17 hours at 1 500C It is again apparent that mud 7 has the best properties, for instance the best ratios plastic viscosity:yield point before and after hot rolling and the best HT/HP values.

It should be noted that best results are obtained when the oil has an aromatic content of below 15, and preferably below 5 and most preferably below 1% by volume when measured by ASTM 2007 (especially when the oil is a naphthenic solvent) or, if it is an insulating oil, when its aromatic content is below 5% when measured by ASTM 2041. When measured by infrared the aromatic content may be below 10, preferably below 6, for instance 0.1 to 5% (compared to about 12% for US Mentor 28 and 18-20% for Diesel).

Claims (19)

1. An oil based drilling fluid in which the oil of the oil base gives a mortality of brown shrimps of below 10% when tested in aerated sea water at 1 50C for 24 hours at a concentration of 100 flti/l and in which the fuid includes an organophilic hectorite as gelling agent.

2. Afluid according to claim 1 in which the hectorite is a tetraalkylammonium hectorite in which one to three of the alkyl groups are C18 alkyl groups and one to three of the alkyl groups are C1025 alkyl groups.

3. A fluid according to claim 2 in which one to three of the alkyl groups are C13 alkyl and one to three of the alkyl groups are C,4~22 alkyl.

4. A fluid according to any preceding claim in which the hectorite is dimethyldioctadecyl ammonium hectorite.

5. A fluid according to any preceding claim in which the oil gives a mortality of brown shrimps of below 5% when tested in aerated sea water at 150C for 24 hours at a concentration of 333 1/1.

6. A fluid according to any preceding claim in which the oil of the drilling fluid is a naphthenic oil.

7. A fluid according to claim 5 in which the naphthenic oil is derived from a naphthenic crude or is a blend of an oil derived from a naphthenic crude with a paraffinic oil.

8. A fluid according to any preceding claim in which the oil of the oil has an aromatic content of less than 5%.

9. A fluid according to any preceding claim in which the oil of the oil base is substantially free of toxic polynuclear aromatic compounds.

10. A fluid according to any preceding claim in which the oil is substantially free of benzopyrene and 1,2,5,6 dibenzanthracene.

11. A fluid according to any preceding claim in which the oil is selected form 60 Solvent Pale, KL55, POLY-X-HP35 and Clairsol 350 and the oils having substantially the same properties as any of these, and blends of two or more such oils.

12. A fluid according to any preceding claim in which the oil has a viscosity at 400C less than 6 cSt.

13. A fluid according to any preceding claim in which the oil has a viscosity at 200C less than 10 cSt.

14. A fluid according to any preceding claim in which the oil has a viscosity at 40 C of from 1 to 5.5 cSt and at 200C of from 1 to 8 cSt.

1 5. A fluid according to any preceding claim in which the oil has a viscosity at 200C of from 1 to 7.

at 400C of from 1 to 5 and at 1000C of from 0.7 to 2.5 cSt.

16. A fluid according to any preceding claim in which the oil is less viscous at 200C than diesel oil.

17. A fluid according to any preceding claim in which the oil base consists of 30 to 100% by volume of the naphthenic oil and 70 to 0% water and the drilling fluid also includes drilling fluid additives selected from gelling agents, emulsifiers, bridging agents weighting agent and lime.

18. A fluid according to any preceding claim substantially as herein described with reference to any of the examples.

19. A method in which an oil based drilling fluid according to any preceding claim is used to carry debris out of a subsea bore hole and the debris is then dumped in the sea while contaminated with the fluid.