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

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muds or fluids. Thus the liquid phase may consist of oil or it may be a mixture of oil and water, for instan¬ ce 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 consi¬ dered 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 US Patent Specification No. 2,969,321 in which the proposed oils are topped crude oils, gas oils, kerosene, diesel fuels, heavy alkylates and frac¬ tions of heavy alkylates. Despite all these numerous proposals the oil was generally chosen having regard primarily to availability 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 prac¬ tice there are some examples in the literature of par¬ ticular 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 naph¬ thenic 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 debris are

ITU££- $ ^•

separated from the drilling fluid, e.g. at the well head the resultant separated debris will still be contaminate 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 befo re 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 satisfactory 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. However Mentor 28 can b expensive and the toxicity of the fluids

tested in USA still appears to be too high to be freely acceptable for dumping in the North Sea. Also it appears that -the toxicity of the fluid may vary according to the source from which the Mentor 28 is obtained, and this renders it impracticable as a base for non-toxic drilling fluids. Another problem is that Mentor 28 is rather more viscous than diesel oil.

According to the invention we have now surprisingly found that naphthenic oils are, as a class, especially suitable for use as the oil of oil based drill ing fluids intended for carrying debris out of a subsea bore hole, prior to dumping of the debris in the sea whil still contaminated with the oil.

Naphthenic oils may be derived rom naphthenic crude and it seems that they can be mucϋ 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 from 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 naphth enic 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 compou ds 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 amoutit of naphthenic (cycloaliphatic) co - pounds. 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. The naphthenic oil preferably has a characteri¬ sation factor of less than 12.0 and preferably from 11.8 to 11.0 or even down to 10.0.

Preferably the oil has an aromatic content of less than 5%, preferably less than 4% and most preferably 0.2 to 3.5%. The aromatic content of an oil may be recorded by. various test methods. . Typically it may be determined as the percentage by volume of the oil that is provided by aromatic compounds. It can be mea¬ sured 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 hydrocar¬ bon content of the oil. The contents of naphthenic and paraffinic compounds may be determined in the same manner. Suitable--methods, are ASTM D2140-66 and ASTM D2007. 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. In general they should not be such as to increase the aromatic content above the values quoted since it seems that high aromatic contents are often associated with toxicity. Mentor 28 in USA seems to have an aromatic content of above 10%. However 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 ben¬ zene rings and generally 5 or more, (especially benzo- pyrene and 1,2,5,6 dibenzanthracene) and some lower molecular weight compounds such as toluene, xylenes, phenanthrenes and possibly also naphthalenes. If these are absent then the total aromatic content can be higher than the 5% mentioned above and may be as

high as 10 or even 12%. It is however safest, from the toxicity point of view, to have the aromatic content as low as possible, preferably below 2.5%.

The naphthenic oil is preferably substantially colourless and substantially odourless. It must of course comply with safety regulations and in practice this means that it must have a flash point of at least 60°C, preferabl 66°C or more.

We have established that it is desirable, * especially for subsea drilling, that the oil of the oil base should at 5°C, and generally also at 20°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 5°C is below 15, preferably below 10, for instance 1 to 7 cSt. The. viscosity at 20 ^C C should be low, generally below 15 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 ha a viscosity at 40°C of below 6 cSt and preferably below 5.5 cSt. This viscosity is often in the range 1 to 5.5, for instance 3 to 5. However there are indica¬ tions that best results are obtained with very low value preferably 1.2 to ^' 3.8 cSt.

The oil preferably has a viscosity at 100°C of from 0.6 - 2:5, generall .0.7 to 1.4 cSt. All

.viscosity measurements herein are kinematic viscosity values obtained by ASTM 445.1P71.

The initial boiling point of the distillation range of the oil used as the oil base is preferably belo 250°C. The A.P.I, gravity value of the oil is generally at least 15 and is normally below 35.

Four naphthenic oils suitable for use in the

invention are 60 Solvent Pale and KL55 (also sold 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 15°C 30.2 32.2

Density g/cm ³ at 15°C 0.875 0.864

Flash Point - closed cup 145°C 142°C

Pour Point -57°C -54°C

Colour (Sabolt) 24 15

Viscosity cSt at 40°C 7.7 6.6

Viscosity cSt at 100 ^C C 2-2.1 1.9-2

Distillation - Initial

Boiling Point 275°C 294°C

Final

Boiling

Point 350°C 329°C(95%)

Aniline Point 76 °C 80°C

Sulphur Content 0. 1-0 .2% less than 0.1

Paraffinid- 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 20°C - 0.860

Kinematic Viscosity at 20°C cSt 6

Kinematic Viscosity at 40°C cSt 3.6

Viscosity at 100°C cSt 1.1

Flash Point (PMCC) °C T15

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 —

Viscosity at 20°C 2.3 cSt Density" at 15°C 0.788 ^' ASTM D1298

Distillation Range °C ASTM D 86

Initial Boiling Point 200

50% Distils at -221

Dry Point 248 '

Flash Point °C 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)

Upper Explosive Limit 7. 1

(% volume in air)

Autoignition Temperature °C 230

Naphthenic content 40% v/v isoparaffin content 20% v/v n-paraffin content 40% v/v

Threshold Limit 200 by calculati

Value (TLV) pp

Other oils having similar analys ^' is may be used especially other naphthenic solvents , for instance having characteristics similar to Clairsol 350.

Any of these oils can be used individually or blends can be formed of ws 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 75.4°C

Flash Point 96°C

Pour Point below -50°C

Viscosity at 40°C 4.19 cSt

Distillation range -

Initial boiling point 214°C

10% boiling 236°C

50% boiling 292°C

90% boiling 320°C

Final boiling point ^' 335°C

Estimated aromatic content 2.37%

Specific gravity 0.849

A characteristic of the defined oils is that, compared to the toxicity of diesel oil, they are substantially non-toxic if they are dumped in the sea in relatively small quantities, e.g. as contamina¬ tion on cuttings that have been separated from drilling fluid. Toxicity can be observed by determining the effect of a selected amount of the oil in sea water on brown shrimps (Grangon crangonQ . Healthy shrimps are maintained in aerated sea water at 15°C 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 μl/1 after 24 hours. The oils used in the invention preferably give substantially no mortality (for instance below 10% and preferably below 1%) at 24 hours when present in amounts of 100 μl/1 and preferably also substantially no mortality when used in amounts of ^' 333 μl/1 for ^' 24 hours. Preferably the mortality at 96 hours at 100 μl/1 is also low, generally, being below ^' 30% and preferably below 15% and preferably also the mortality at ^" 333 μl/1 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 least 5 days at oil concentrations of at least ^' 333 μl/1 and-often of at least 1000 μl/1. 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 μl/1.

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 oil-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 fluid and these additives may be dissolved or dispersed in the oil. base. Thus they may contain one or more emulsifi ers, 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 ag

and supplementary emulfieirs, 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 organ¬ ic 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.

It is standard practice to adjust the rheologi- cal 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 Sedapol 155 or Sedapol 44, or Claytone ^' 34 or Claytone 40. They can be used in the invention but better results are obtained by use of an organophilic hectorite.

This may be a naturally occurring hectorite or synthetic hectorite, for instance as described 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. 1121501. One to three of the alkyl groups are preferably short chain alkyl groups (e.g. C., _o most preferably C-_ ₃ , typically methyl) and

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one to three of the alkyl groups are preferably long chain alkyl groups (e.g. ^c - _j ø_25' typically C, _4-22 , most preferably C _1g ).

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

The amount of gelling aid is typically from 1 to 10, most preferably 1.25 to 4, grams gelling aid per 100 cc fluid. An alternative way of expressing the amount is as ^' 3 to 15, most preferably 5 to 9, pounds gelling aid per barrel drilling fluid. In general the amount of gelling aid required in the fluids of the invention is greater than the amount required in conventional drilling fluids, for instance being from

1.5 to 2.5 times the amount required when the oil is diesel oil.

The following are examples of the invention. Example 1 A drilling fluid is prepared containing 212 cc Pale Oil 60 as defined above, 7 cc blown tall oil emulsifier, 5 cc oil soluble amide polymer as secondary emulsifier, 53 cc water containing 25% calcium chloride, 6 g lime hydrate, 7g of a blend of high molecular weight organic polymers and inorganic bridging agents, 358 g barite and 6 g dimethyldioctadecyl ammonium hectorite. Its properties are measured before and after hot rolling at 122°C for 17 hours (H/R) . The results are set out in the following table. ES is electrical stability.

Mud No. 1 H/R H/R

Mud Weight 14.5 14.5 14.5

Flow Prop. Test temp °C 49 49 65

Fann Readings rpm 600 223 226 125

^' 300 128 130 71

200 95 94 52 ioo 59 55 32

6 16 13 9

^' 3 14 10 8

Plastic. Viscosity cp 95 96 54

Yield point g/100cm ² 16 17 8

Gel strength g/100cm ² 8/.1.2.5 6/11.5 4/7.5

E.S.. volts at 49°C 1430 1160

When the oil was tested for toxicity by the method described above, it was found that after 96 hours .it caused abou 3% fatality at ^' 333 μl/1 and up to 15% 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 μl/1, and the paraffinic oil sold as Mentor 28 gives about 59% fatality after 96 hours at 333 μl/1.

Example 2

A drilling fluid is prepared having the same composition as- above except that the amount of Pale Oil 60 is 149 cc and this is blended with 63 cc Clairsol 350. Very satisfactory downhole and toxicity properties ^■ are obtained when used in a subsea bore hole followed by filtration of the debris from the fluid and dumping of the debris in sea. Example ^" 3 A drilling fluid is prepared as in Example 1 except that POLY-X-HP-35 is used in place of the Pale Oil 60. The resultant fluid had particularly good proper¬ ties under high downhole temperature conditions. Example 4 A drilling fluid was prepared by blending 235 cc Clairsol 350, 5 cc primary emulsifier, 5cc secondary emulsifier, 9 grams gelling aid, 42 cc calcium chloride brine, 5 grams lime, 15 grams bridging aid and 309 grams barytes. This drilling fluid is a 13 pound per gallon mud having an 85:15 oil:water ratio and an internal phase activity of ^" 0.75. Its initial properties at 49°C were plastic, viscosity 22 cps, yield point 5.5 g/100cm ² and gel strength ^' 3/6.5g/100cm ² and after hot rolling at 65°C were plastic, viscosity 23 cps, yield point 7 g/ 100cm ² and gel strength 5/6.5 g/100 cm ² . It is particu- ^• larly suitable for use in subsea drilling where the sea temperature may be 5°C or lower. Example 5

A drilling fluid is prepared as in Example 2 using a bentonite gelling aid in place of the hectorite. The resultant fluid is less satisfactory when it is allowed to cool to, say, 5°C but still gives useful downhole properties.

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 solventjor, if it is an insulating oil, when its aromatic content is below 5% when measured by ASTM 2041. When measured by infra red the aromatic content may be below 10, preferably belo 6, for instance 0.1 to 5% (compared to about 12% for US Mentor 28 and 18-20% for Diesel).

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