IE894056L - The use of selected ester oils in drilling fluids,¹particularly for the offshore development of oil and gas¹sources (i) - Google Patents

Abstract

The invention concerns the use of selected esters which are free-flowing and pumpable over the temperature range 0-5 DEG C as the oil phase in invert oil muds suitable for use in the development of oil and gas fields and which do not damage the environment. The esters are prepared from monofunctional C2 to C12 alcohols and unsaturated C16 to C24 monocarboxylic acids, the latter with one or more olefinic double bonds. The oil muds contain, in a closed oil phase, a dispersed aqueous phase as well as emulsifiers, weighting agents, fluid-loss additives and, if required, other customary additives plus an alkali reserve subject to the condition that use is not made of significant quantities of strong, hydrophilic bases such as an alkali-metal hydroxide and/or diethanolamine, and chalk, if added, is limited to a maximum of about 2lb/bbl of flushing oil. [EP0374671A1]

Description

65152 1 The use of selected ester oils in drilling fluids, particularly for the offshore development of oil and gas sources fl) This invention relates to new drilling fluids based on ester oils and to invert drilling muds based thereon which combine high ecological compatibility with good stability and performance properties. One important application for the new drilling fluids is in offshore drilling for the development of oil and/or gas sources, the particular object of the invention in this regard being to provide technically useful drilling fluids of high ecological compatibility. The use of the new drilling fluids is of particular importance in, but is not limited to, the offshore sector. The new drilling fluids may also be used quite generally for land-supported drilling, including for example geothermal drilling, water drilling, geoscientific drilling and mine drilling. In this case, too, the ester-based drilling fluids selected in accordance with the invention basically simplify ecotoxic problems to a considerable extent.

It is known that liquid drilling fluids for sinking bores in rock and bringing up the rock cuttings are slightly thickened, water-based or oil-based fluid systems. Oil-based systems are being increasingly used in practice, particularly in offshore drilling or in the penetration of water-sensitive layers.

Oil-based drilling fluids are generally used in 65152 2 the form of so-called invert emulsion muds which consist of a three-phase system, namely: oil, water and finely divided ge of additives, including in particular emulsifiers and emulsifier systems, weighting agents, 5 fluid loss additives, alkali reserves, viscosity regulators and the like, for stabilizing the system as a whole and for establishing the desired performance properties. Full particulars can be found, for example, in the Article by P. A Boyd et al entitled 10 "New Base Oil Used in Low-Toxicity Oil Muds" in Journal of Petroleum Technology, 1985, 137 to 142 and in the Article by R.B. Bennet entitled "New Drilling Fluid Technology - Mineral Oil Mud" in Journal of Petroleum Technology, 1984, 975 to 981 and the literature cited 15 therein.

Oil-based drilling fluids were originally made from diesel oil fractions containing aromatic constituents. For the purposes of detoxification and reducing the ecological problems thus created, it was 20 then proposed to use hydrocarbon fractions substantially free from aromatic compounds - now also known as "nonpolluting oils" - as the continuous oil phase, cf. the literature cited above. Although certain advances were achieved in this way through 25 elimination of the aromatic compounds, a further reduction in the environmental problems caused by drilling fluids of the type in question here seems to be urgently required. This applies in particular to the sinking of offshore wells for the development of 30 oil and gas sources because the marine ecosystem is particularly sensitive to the introduction of toxic and non-readily degradable substances.

The relevant technology has for some time recognized the significance of ester-based oil phases 35 for solving these problems. Thus, US-PSS 4,374,737 and - 3 - 4,481,121 describe oil-based drilling fluids in which nonpolluting oils are said to be used. Non-aromatic mineral oil fractions and vegetable oils of the peanut oil, soybean oil, linseed oil, corn oil and rice oil type, and even oils of animal origin, such as whale oil, are mentioned 5 alongside one another as nonpolluting oils of equivalent rank. The ester oils of vegetable and animal origin mentioned here are all triglycerides of natural fatty acids which are known to be environmentally safe and which, ecologically, are distinctly superior to hydrocarbon fractions, even where they have been de-aromaticized. 10 Interestingly, however, not one of the Examples in the US patents cited above mentions the use of such natural ester oils in invert drilling muds of the type in question here. Mineral oil fractions are used throughout as the continuous oil phase.

US-4,631,136 also seeks to provide oil based drilling fluids of 15 increased ecological suitability using triglycerides of plant origin. It is proposed that use be made preferably of those glycerides, which have a minimum content of 2 or 3 ethylenes in the unsaturated carboxylic acid residues. In addition the information is given that known antioxidants and products which are synergetic therewith should be used 20 in the ester oils to increase the effectiveness of the antioxidants. As a result of these measures, a thickening of the system arising from oxidation produced polymerization or interlinking should be prevented.

The investigations on which the present invention is based have shown that the use of readily degradable oils of vegetable and/or 25 animal origin, which was considered in the prior art, is not feasible for practical reasons. The rheologic properties of such oil phases cannot be controlled for the wide temperature range required in practice of 0 to 5°C on the one hand up to 250°C and higher on the other hand. 30 In its general descriptive part, US-PS 4,481,121 mentions not only triglycerides, but also a commercial product "Arizona 208" of the Arizona Chemical Company, Wayne, N.J., which is a purified isooctyl-monoalcohol ester of high-purity tall oil fatty acids. An ester of a monofunctional alcohol and monofunctional carboxylic acids, 35 mentioned for the first time here, is described as equivalent to triglycerides of natural origin and/or de-aromaticized hydrocarbon fractions.

The cited US patent does not contain any copyable 4 Examples relating to the use of such an ester of monofunctional components.

The teaching of the present invention is based on the observation that it is in fact possible to produce 5 oil-based invert drilling fluids of the type in question here based on ester oils of high environmental compatibility which correspond in their storage and in-use behavior to the best of the hitherto known oil-based drilling fluids, but have the additional 10 advantage of increased environmental compatibility.

Two key observations in this regard dominate the teaching according to the invention: The triglycerides accumulating in the form of natural oils are not suitable for the production of 15 mineral-oil-free oil-based invert drilling fluids, whereas the esters of monofunctional carboxylic acids with monofunctional alcohols derived from those oils or fats are suitable for the production of such drilling fluids. The second key observation is that ester oils 20 of the type in question here do not in fact show the same in-use behavior as the mineral oil fractions used hitherto based purely on hydrocarbons. In practical application, the ester oils of monofunctional components in question here undergo partial hydrolysis, 25 resulting in the formation of free fatty acids. These free fatty acids react in turn with the alkaline constituents always present in drilling fluids of the type in question here, for example with the alkali reserve used to prevent corrosion, to form the 30 corresponding salts. However, salts of highly hydrophilic bases and the acids in the range from about to C ^commonly encountered in fats and oils of natural origin are known to be compounds having comparatively high HLB values which lead in particular 35 to the formation and stabilization of o/w emulsions.

Use is made of this to a very considerable extent in the field of detergents and cleaning preparations. However, the formation of undesirably large quantities of such o/w emulsifier systems must interfere with the w/o emulsions required for solving the problem addressed by the invention and, hence, lead to problems. The teaching of the present invention as described in the following shows how invert drilling fluids based on ester oils can be effectively used in practice despite these difficulties inherent in the system.

In a first embodiment, therefore, the present invention relates to the use of selected esters -flowable and pumpable at temperatures in the range from 0 to 5*C - of monofunctional C alcohols and 2-12 olefinically mono- and/or polyunsaturated C ^ ^ monocarboxylic acids or mixtures thereof with small quantities of other, more especially saturated, monocarboxylic acids as the oil phase or at least a substantial pari: of the oil phase of invert drilling muds which contain in a continuous oil phase a disperse aqueous phase and also emulsifiers, weighting agents, fluid loss additives, thickeners, and, if desired, other standard additives together with an alkali reserve, with the proviso that strong hydrophilic bases, such as alkali hydroxide and/or diethanolamine, are not used in significant quantities. Lime (calcium hydroxide) is often added as the alkali reserve, more especially for protection against inrushes of C02 and/or H2S into the drilling fluid and hence for protection against corrosion. An addition of lime such as this may be used as the alkali reserve in accordance with the invention. However, it is important to ensure that only comparatively small quantities of this alkaline component are incorporated. In the preferred em- bodiment of the invention, the maximum addition of lime is of the order of 2 lb/bbl (lime/oil mud) and is thus distinctly below the quantities typically used in practice in oil-based invert drilling fluids. 5 In another embodiment, the invention relates to mineral-oil-free invert drilling fluids which are suitable for the offshore development of oil and gas sources and, in a continuous oil phase based on ester oil*;, contain a disperse aqueous phase together with 10 emulsifiers, weighting agents, fluid loss additives, thickeners, and, if desired, other standard additives. The new drilling fluids are characterized in that the oil phase consists at least substantially of esters of monofunctional C2 alcohols and olefinically mono-15 and/or polyunsaturated c ^monocarboxylic acids and in that the w/o emulsion is mildly alkalized and, where lime is added, this alkali reserve preferably does not exceed quantities of about 2 lb/bbl (lime/oil mud) . The lime content is preferably slightly below this 20 limit.

The ester oils selected in accordance with the invention which are intended to form the entire continuous oil phase of the invert drilling muds or at least a substantial part thereof are discussed first in 25 the following.

As already stated, an important criterion lies in the choice of esters which may be assigned to the class of reaction products of monofunctional carboxylic acids with monofunctional acohols. In addition, however, it 30 is intended in accordance with the invention exclusively or at least predominantly to use carboxylic acids within this class. The carboxylic acids may be derived from unbranched or branched hydrocarbon chains, particular significance being 35 attributed to the linear chains. Monocarboxylic acids 7 of this type and of the C, . to C. , range in question 16 d 4 here and esters thereof are unsuitable as predominantly saturated hydrocarbon compounds on account of their comparatively high solidification points. Even then, 5 however, esters of the type in question here are flovable and pumpable down to temperatures of 0 to 5*C providing an adequate level of olefinically unsaturated ester constituents is guaranteed. In the preferred embodiment of the invention, therefore, esters of the 10 described type of which more than 70% by weight and preferably more than 80% by weight are derived from olefinically unsaturated c-\^2k carboxy^ic acids are used. Important natural starting materials are carboxylic acid mixtures which contain at least 90% by 15 weight olefinically unsaturated carboxylic acids in the C range mentioned here. The unsaturated carboxylic acids may be mono- and/or polyolefinically unsaturated. Where carboxylic acids or carboxylic acid mixtures of natural origin are used, the double ethylenic double 20 bond in particular and, to a lesser extent, even a triple ethylenic double bond per carboxylic acid molecule plays a certain part in addition to a single ethylenic double bond in the molecule. Particulars of this are given in the following. 25 In conjunction with the choice of esters of monofunctional reactants in accordance with the invention, the choice of such a comparatively highly unsaturated carboxylic acid component in the ester oils ensures that the ester oils and, ultimately, the final 30 invert emulsions show the rheologic properties required in practice, particularly at relatively low temperatures. The comparatively highly unsaturated ester oils containing 16 to 24 C atoms in the monocarboxylic acid component, which are used in 35 accordance with the invention, have solidification 8 points (pour point and setting point) below -10'C and more especially below -15*C in the preferred embodiment. Despite this high mobility at low temperatures, the molecular size of the ester oil 5 prescribed in accordance with the invention ensures that the flashpoints of the ester oils are sufficiently high, being at least 80*C, but generally exceeding a temperature limit of approximately 100*C. Ester oils having flashpoints above 160*C are preferred. Ester 10 oils of the described type showing high mobility, even at low temperatures, and having flashpoints of 185*C or higher may be produced without difficulty.

In conjunction with these high - flashpoints determined by the size of the molecule, it is possible 15 at the same time to ensure that the viscosity values are within the required limits. Thus, preferred ester oils of the described type show a Brookfield (RVT) viscosity at a temperature of 0 to 5*C of not more than 55 mPa.s and preferably of at most 45 mPa.s or lower. 20 It is possible to adjust values of 30 or even higher, for example in the range from 20 to 25 mPa.s, at temperatures in the range indicated.

Among the unsaturated ester oils suitable for use in accordance with the invention, there are two sub-25 classes of particular importance.

The first of these sub-classes is based on unsaturated C ^monocarboxylic acids of which no more than about 35% by weight are diolef inically and, optionally, polyolefinically unsaturated. In their 30 case, therefore, the content of polyunsaturated carboxylic acid residues in the ester oil is comparatively limited. Within this sub-class, however, it is preferred that at least about 60% by weight of the- carboxylic acid residues are monoolefinically 35 unsaturated. 9 In contrast to the first sub-class described above, the second sub-class of ester oils of practical significance is derived from C 1 ^ ^ ^ monocarboxylic acid mixtures of which more than 4 5% by weight and 5 preferably more than 55% by weight are derived from diolefinically and/or polyolefinically unsaturated acids within the C range mentioned.

The most important monoethylenically unsaturated carboxylic acids within the range in question here are 10 hexa-dec^noic acid (palmitoleic acid (C ) , oleic acid 1 6 (C1Q) , the related ricinoleic acid (CjQ) and erucic acid (C i • The most important di-unsaturated carboxylic acid within the range in question here is linoleic acid (C^g) while the most important triethylenically 15 unsaturated carboxylic acid is linolenic acid (C1Q)« Selected individuals of the ester type formed from an unsaturated monocarboxylic acid and a monoalcohol may be used as the ester oil in accordance with the invention. One example of such esters are the esters 20 of oleic acid, for example of the oleic acid isobutyl ester type. So far as the rheology of the system is concerned and/or for reasons of availability, it is frequently desirable to use acid mixtures. This is of importance so far as meeting the above-stated 25 specifications of the two sub-classes for preferred ester oils is concerned.

As already mentioned, the first of these two subclasses is distinguished by the fact that its content of di-unsaturated and polyunsaturated acids is limited 30 and does not exceed about 35% by weight. Vegetable oils of natural origin, of which the hydrolysis or transesterification gives mixtures of carboxylic acids or carboxylic acid esters of the type required here, are-for example palm oil," peanut oil, castor oil and, 35 in particular, rapeseed oil. Suitable rapeseed oils 10 are both traditional types of high erucic acid content and also the more modern types of reduced erucic acid content and increased oleic acid content.

Ester oils of the first sub-class which correspond 5 to this definition are particularly important for the simple reason that problems possibly arising in their case from the lack of stability to oxidation are reduced. In practice, the drilling fluid is of course continuously pump-circulated and, in the process, is 10 brought constantly into contact with atmospheric oxygen, often over a large area and at at least slightly elevated temperatures, for the purpose of separating out the rock cuttings brought up, for example by sieving. 15 However, carboxylic acid mixtures of the second sub-class mentioned above are also of considerable practical significance for use in accordance with the invention. This is attributable not least to their broad accessibility from natural fats of animal and/or 2 0 vegetable origin. Classic examples of oils which have a high content of or Cjg_22 carboxylic acids and which, at the same time, contain at least about 45% of at least diethylenically unsaturated carboxylic acids are cottonseed oil, soybean oil, sunflower oil and 25 linseed oil. The tall oil acids isolated during the recovery of cellulose also fall within this range. However, starting materials of the last type are generally distinguished by more or less large additional contents of resin constituents. A typical 30 animal starting material for the production of corresponding carboxylic acid mixtures is fish oil, particularly herring oil.

As already mentioned, the ester oils used in accordance with the invention may be certain selected 35 individual esters corresponding to the above 11 definition. However, mixtures of esters of corresponding monocarboxylic acids and monoalcohols will normally be present. In this regard, the scope of the invention encompasses above all those mixtures which, on the one hand, meet the viscosity requirement according to the invention and of which, on the other hand, at least 50 to 51% comprise the monofunctional esters of the olefinically mono- and/or polyunsaturated Cl6-2 4 carboxYlic acids. Ester constituents and, in particular, carboxylic acid esters of monofunctional alcohols and monofunctional carboxylic acids of different constitution may be present as minor constituents of the mixture providing the mixture has the required property profile. This is important where carboxylic acid mixtures of natural origin are used. Natural starting materials such as these generally also contain more or less large proportions of saturated carboxylic acids, often including linear ^ car boxylic acids. Saturated fatty acids of this type and their esters readily give rise to rheologic difficulties on account of their comparatively high melting points. According to the invention, therefore, saturated C1& carboxylic acids preferably make up no more than about 20% by weight and, in particular, no more than about 10% by weight of the ester oils.

By contrast, the presence of, in particular, saturated carboxylic acids containing less than 16 carbon atoms and, more especially, from 12 to 14 carbon atoms is more acceptable. In small quantities, the contents of such lower, fully saturated fatty acids often present in natural starting materials are frequently valuable mixture components in the context of the problem addressed by the invention. Their esters are not vulnerable to oxidation under practical in-use conditions and their rheologic properties 12 promote the objective of the invention, namely to * replace the pure hydrocarbon oils hitherto solely used in practice by ester oils or ester oil fractions.

The alcohol radicals of the esters or ester 5 mixtures according to the invention are preferably derived from straight-chain and/or branched-chain saturated alcohols, particular significance being attributed to alcohols containing at least 3 C atoms and, more especially, to alcohols containing up to 10 about 10 C atoms. The alcohols may also be of natural origin, in which case they have normally been obtained from the corresponding carboxylic acids or their esters by hydrogenating reduction. However, the invention is by no means limited to starting materials of natural 15 origin. Both on the monoalcohol side and on the monocarboxylic acid side, the starting materials of natural origin may be partly or completely replaced by corresponding components of synthetic origin. Typical examples of alcohols are the corresponding oxo alcohols 20 (branched alcohols) and the linear alcohols obtained by the Ziegler process. Similarly, monocarboxylic acid components present in particular in carboxylic acid mixtures may be derived from petrochemical synthesis.

However, the advantages of starting materials of 25 natural origin lie in particular in their proven lower toxicologic values, their ready degradability and their ready accessibility. The natural destruction of the used oil mud ultimately required presupposes that ester oils of the type described herein be both aerobically 3 0 and anaerobically degradable.

However, one important limitation is associated with the use of these ester oils in invert oil muds of the type in question. This limitation arises out of the- difficulty mentioned" at the beginning that, in 35 principle, the carboxylic acid esters are vulnerable to 13 hydrolysis and, accordingly, have to behave differently than the pure hydrocarbon oils hitherto used.

Invert drilling muds of the type in question contain the finely disperse aqueous phase, normally 5 together with the continuous oil phase, in quantities of from about 5 to 45% by weight and preferably in quantities of from about 5 to 25% by weight. Particular significance is attributed to the range of about 10 to 25% by weight of disperse aqueous phase. 10 This precondition from the constitution of conventional drilling muds also applies to the ester-based invert drilling muds in question here. It is clear that, in continuous practical operation, disturbances of the equilibrium can occur in the multiphase system as a 15 result of partial ester hydrolysis.

The situation is complicated by the fact that, in practice, drilling muds of the type in question here always contain an alkali reserve. This alkali reserve is particularly important in affording protection 20 against corrosion caused by unexpected inrushes of acidic gases, particularly COz and/or H2S. The danger of corrosion to the drill pipe requires the safe establishment of pH values at least in the mildly alkaline range, for example in the range from pH 8.5 to 25 9 and higher.

In oil muds based on pure hydrocarbon fractions as the oil phase, strongly alkaline and, at the same time, highly hydrophilic inorganic or organic additives are generally used in practice without any difficulty. 30 Particular significance can be attributed to the alkali hydroxides and, in particular, to sodium hydroxide on the one hand or to highly hydrophilic organic bases, diethanolamine and/or triethanolamine being particularly typical additives for binding impurities 35 of H2S. In addition to and/or instead of the highly 14 hydrophilic inorganic and organic bases mentioned here, lime or even more weakly basic metal oxides, especially • zinc oxide or comparable zinc compounds, are particularly important as the alkali reserve. Lime in 5 particular is widely used an inexpensive alkalizing agent. It may safely be used in comparatively high quantities of, for example, from 5 to 10 lb/bbl (lime/oil mud) or even higher.

The use of ester-based oil muds of the type 10 described here requires a departure from standard practice so far as these variables are concerned. It is of course necessary in this case, too, to ensure that the pH value of the drilling mud is kept at least in the mildly alkaline range and that a sufficient 15 quantity of alkali reserve is available for unexpected inrushes of, in particular, acidic gases. At the same time, however, the ester hydrolysis should not be undesirably promoted and/or accelerated by such an alkali content. 2 0 Thus, in the preferred embodiment of the invention, no significant quantities of highly hydrophilic, inorganic and/or organic bases are used in the oil mud. In particular, the invention does not use alkali hydroxides or highly hydrophilic amines of the 25 diethanolamine and/or triethanolamine type. Lime may be effectively used as the alkali reserve. In that case, however, it is best to limit the max-imum quantity of lime used in the drilling mud to around 2 lb/bbl or slightly lower, for example to between about 30 l and 1.8 lb/bbl (lime/drilling mud). In addition to or instead of lime, it is also possible to use other known alkali reserves, including in particular the less basic metal oxides of the zinc oxide type and other comparable zinc compounds-. However, even where acid-35 binding agents such as these are used, it is important 15 not to use excessive amounts to prevent unwanted 3 premature ageing of the drilling mud accompanied by an increase in viscosity and hence a deterioration in the rheologic properties. The particular aspect of the 5 teaching according to the invention in question here prevents or at least limits the formation of unwanted quantities of highly active o/w emulsifiers to such an extent that the favorable rheologic properties are maintained for sufficiently lor.:; periods in operation, 10 even in the event of thermal ageing. In relation to the recommendations of the prior art which have hitherto remained in the realm of theoretical considerations, this represents a significant surplus which actually enables the low toxic properties of 15 ester oils of the type in question to be utilized in practice for the first time.

The esters based on olefinically unsaturated monocarboxylic acids defined in accordance with the invention, which flow and can be pumped at temperatures 20 in the range from 0 to 5*C, generally make up at least about half the continuous oil phase of the drilling mud. However, preferred oil phases are those in which esters or ester mixtures of the type according to the invention are very much predominantly present. In one 25 particularly important embodiment of the invention, the oil phase consists almost entirely of such ester oils. Components suitable for mixing with the ester oils defined in accordance with the invention are, in ♦ particular, selected other ester oil fractions which 30 are described in the parallel Irish Application No. 4057/89. The invention also encompasses mixtures with such other selected ester oils.

The following rheologic data apply to the rheology of preferred invert drilling muds according to the 16 invention: plastic viscosity (PV) in the range from about 10 to 60 mPa.s and preferably in the range from about 15 to 40 mPa.s, yield point (YP) in the range from about 5 to 40 lb/100 ft2 and preferably in the 5 range from about 10 to 25 lb/100 ft2, as measured at 50'C. Full information on the determination of these parameters, on the measurement techniques used and on the otherwise standard composition of the invert oil muds described herein can be found in the prior art 10 cited above and, for example, in "Manual of Drilling Fluids Technology" published by one of the joint Applicants, BAR0ID DRILLING FLUIDS, INC., cf. in particular the Chapter entitled "Mud Testing - Tools and Techniques" and "Oil Mud Technology", which is freely available to 15 interested experts. In the interests of fullness of disclosure, the following summary observations may be made: Emulsifiers suitable for use in practice are systems which are capable of forming the required w/o 20 emulsions. Selected oleophilic fatty acid salts, for example those based on amidoamine compounds, are particularly suitable, examples being described in the already cited US-PS 4,374,737 and the literature cited therein. One particularly 25 suitable type of emulsifier is the product marketed under the name of "EZ-MUL" by the joint Applicant,BAR0ID DRILLING FLUIDS, INC.

Emulsifiers of the type in question here are marketed in the form of concentrates and may be used, 30 for example, in quantities of from about 2.5 to 5% by weight and more especially in quantities of from about 3 to 4% by weight, based in each case on the ester oil phase.

In practice, organophilic lignite in particular 35 is used as a fluid-loss additive and, hence, in 17 particular for forming an impervious coating in the form of a substantially water-impermeable film over the walls of the well. Suitable quantities are, for example, in the range from about 15 to 20 lb/bbl or in 5 the range from about 5 to 7% by weight, based on the ester oil phase.

In drilling muds of the type in question here, the thickener normally used to create viscosity is a cationically modified, finely divided organophilic bentonite which 10 may be used in particular in quantities of from about 8 to 10 lb/bbl or in the range from about 2 to 4% by weight, based on the es-ter oil phase. The weighting agent normally used in practice to establish the necessary pressure equalization is barite which is 15 added in quantities adapted to the particular conditions to be expected in the well. For example, it is possible by addition of barite to increase the specific gravity of the drilling mud to values of up to about 2.5 and preferably in the range from about 1.3 to 20 1.6.

In invert drilling muds of the type in question here, the disperse aqueous phase is charged with soluble salts, generally calcium chloride and/or potassium chloride, the aqueous phase preferably being 25 saturated with the soluble salt at room temperature.

The emulsifiers or emulsifier systems mentioned above may also be used to improve the oil wettability of the inorganic weighting materials. In addition to the aminoamides already mentioned, alkyl 30 benzenesulfonates and imidazoline compounds are mentioned as further examples. Additional information on the relevant prior art can be found in the following literature references: GB 2,158,437, EP 229 912 and DE 32 47 123. 35 in addition to the advantages already mentioned, 18 the drilling fluids based in accordance with the invention on the co-use of ester oils of the described type are also distinguished by distinctly improved lubricity. This is particularly important when the 5 path of the drill pipe and hence the well deviate from the vertical during drilling, for example at considerable depths. In such cases, the rotating drill pipe readily comes into contact with the well wall and embeds itself therein. Ester oils of the type used as 10 oil phase in accordance with the invention have a distinctly better lubricating effect than the mineral oils hitherto used, which is an important advantage of the teaching according to the invention. 15 EXAMPLES EXAMPLE 1 An invert drilling mud is prepared using an uiidistilled isobutyl rapeseed oil ester as continuous 20 oil phase. This rapeseed oil ester is based on a mixture of predominantly unsaturated, straight-chain carboxylic acids which correspond substantially to the following distribution: 60% oleic acid, 20% linoleic acid, 9 to 10% linolenic acid, olefinically unsaturated 25 C20-22 monocarboxylic acids approximately 4%, remainder saturated monocarboxylic acids predominantly in the 18 range.

The rapeseed oil ester used has the following characteristic data: density (20*C) 0.872 g/cm3; pour 30 point below -15*C; flash point (DIN 51584) above 180*C; acid value (DGF-C-V 2) 1.2; viscosity at 0*C 32 mPa.s, viscosity at 5*C 24 mPa.s; no aromatic compounds.

An invert drilling mud is conventionally prepared using the following mixture constituents: 35 230 ml rapeseed oil fatty acid ester 5 10 15 20 25 30 19 25 ml water 6 g organophilic bentonite (GELTONE, a product of BAROID DRILLING FLUIDS, INC.) 0.2 g lime 6 g water in oil errulsifier ("EZ-MUL", a product of BAROID DRILLING FLUIDS, INC.) 340 g barite 9.2 g CaCl2 x 2H20 20 g organophilic lignite ("DURATONE" a product of BAROID DRILLING FLUIDS, INC.! Plastic viscosity (PV), yield point (YP) and gel strength after 10 seconds and 10 minutes are first determined on the material before ageing by viscosity measurement at 50*C.

The invert drilling mud is then aged for 16 h at 125 "C in an autoclave in a so-called "roller oven" to determine the effect of temperature on the stability of the emulsion. The viscosity values are then redetermined at 50*C.

The following results are obtained: Unaged Aged material material Plastic viscosity (PV) 35 62 Yield point (YP) 21 24 Gel strength (lb/100 ft2) 10 seconds 12 12 10 minutes 14 15 COMPARISON EXAMPLE 1 .

Another invert drilling mud is prepared in the same way as in Example 1, except that on this occasion 20 the quantity of lime is increased to 4 g, i.e. drastically beyond the limit of approximately 2 lb/bbl. c Once again, the viscosity values and gel strength of the material are determined before and after ageing. 5 The following results are obtained: Unaged Aged material material 10 15 Plastic viscosity (PV) 41 cannot be measured 20 Yield point (YP) 22 cannot be measured 25 Gel strength (lb/100 ft ) 10 seconds 10 minutes 11 17 74 72 EXAMPLE 2 30 Another invert drilling mud is prepared with a continuous oil phase. The oil phase consists of distilled oleic acid isobutyl ester which has the following characteristic data: density (20*C) 0.86 g/cm3; viscosity (20*C) 8 to 10 mPa.s; pour point below 35 -25*C; flash point (DIN 51584) above 185*C; acid value (DGF-C-V 2) below 1; no aromatic compounds.

A drilling mud of the following composition is prepared: 210 ml isobutyl oleate 40 6 g fatty-acid-based emulsifier (INVERMUL , a product of BAROID DRILLING FLUIDS, INC.) 21 6 - g organophilic bentonite (GELTGME II, a product of BAROID DRILLING FLUIDS, INC.) 13 g organophilic lignite (DURATONE, a product 5 of BAROID DRILLING FLUIDS, INC.) 1 g lime 3 g water in oil emulsifier (EZ-MUL, a product of BAROID DRILLING FLUIDS, INC.) 270 g barite 10 58.2 g saturated aqueous CaCl2 solution Plastic viscosity, yield point and gel strength after 10 seconds and 10 minutes are determined before and after ageing (16 h at 125 *C in a roller oven) in the same way as in Example 1. The results obtained are 15 shown below. In the formulation used here, 1.2 kg lime substantially corresponds to the limit of 2 lb/bbl. 20 25 3 0 Unaged Aged material material 35 Plastic viscosity (PV) 46 41 Yield point (YP) 35 32 4 0 Gel strength (lb/100 ft2) 10 seconds 17 18 10 minutes 21 29 22 COMPARISON EXAMPLE 2 Another invert drilling oil emulsion is prepared using the formulation of Example 2, except that the addition of lime is increased to 2 g and hence to clearly beyond the limit of 2 lb/bbl. The plastic viscosity, yield point and gel strength of the material before and after ageing are shown in the following: Unaged material Aged material Plastic viscosity (PV) 53 46 Yield point (YP) 61 45 Gel strength (lb/100 ft2) 10 seconds 10 minutes 33 40 24 29 23

Claims (17)

1. The use of selected esters - flowable and pumpable at temperatures of 0 to 5*C - of monofunctional alcohols containing 2 to 12 C atoms and olefinically mono- and/or polyunsaturated monocarboxylic acids containing 16 to 24 C atoms or mixtures thereof with small quantities of other, in particular saturated, monocarboxylic acids as the oil phase or as at least a substantial part of the oil phase of invert drilling muds which are suitable for the environmentally safe offshore development of oil and gas sources and, in a continuous oil phase, contain a disperse axjueous phase and also emulsifiers, weighting agents, fluid-loss additives, thickeners, and* if desired, other standard additives together with an alkali reserve, with the proviso that significant quantities of strong hydrophilic bases, such as alkali hydroxide and/or diethanolamine, are not used and any addition of lime is limited to a maximum of about 2 lb/bbl oil mud.

2. The use claimed in claim 1, characterized in that esters of which more than 70% by weight, preferably more than 80% by weight and, more preferably, more than 90% by weight are derived from olefinically unsaturated C carboxylic acids are used. 16-24

3. The use claimed in claims 1 and 2, characterized in that the esters used in the drilling mud have solidification points (pour point and setting point) below -10*C, preferably below -15*C, and flash points above 100*C and preferably above 160*C.

4. The use claimed in claims 1 to 3, characterized in that the esters used in the drilling mud have a Broook-field (RVT) viscosity at 0 to 5*C of no more than 55 mPa.s and preferably of no more than 45 mPa.s.

5. The use claimed in claims 1" to 4, characterized in that no more than 35% by weight of the unsaturated Cl6~ 24 2^ monocarboxylic acids present in the ester are derived from di- and polyolefinically unsaturated acids ( and at least about 60% by weight are preferably monoolefinically unsaturated. 5 6. The use claimed in claims 1 to 4, characterized in that more than 4 5% by weight and preferably more than 55% by weight of the Cj^ ^ monocarboxylic acids present in the ester mixture are derived from di- and/or polyolefinically unsaturated acids.

6. 1.0

7. The use claimed in claims 1 to 6, characterized in that residues of saturated C16- jg carboxylic acids present in the ester mixture make up no more than about 20% by weight and, in particular, no more than about 10% by weight, but preferably contain fewer carbon atoms, preferably 12 to 14 carbon atoms. 15

8. The use claimed in claims 1 to 7, characterized in that the carboxylic acids present in the ester or ester mixture are at least predominantly linear and are preferably of vegetable and/or animal origin, being derived in particular from triglycerides of vegetable 20 origin and/or fish oils and fats.

9. The use claimed in claims 1 to 8, characterized in that the esters are used in drilling fluids which, in the invert drilling mud, contain the finely disperse aqueous phase together with the continuous ester-based 25 oil phase in quantities of from about 5 to 4 5% by weight and preferably in quantities of from about 5 to 25% by weight.

10. The use claimed in claims 1 to 9, characterized in that the alcohol radicals of the esters used are 30 derived from straight-chain and/or branched, saturated alcohols preferably containing from 3 to 10 C atoms.

11. Mineral-oil-free invert drilling fluids which are suitable for the offshore development of oil and gas sources and which, in a continuous oil phase based on 35 ester oils, contain a disperse aqueous phase together - 25 - with emulsifiers, thickeners, weighting agents, fluid loss additives and, if desired, other standard additives, characterized in that the oil phase consists at least substantially of esters of monofunctional alcohols containing 2 to 12 C atoms and olefinically mono- and/or 5 polyunsaturated monocarboxylic acids containing 16 to 24 C atoms and in that the w/o emulsion is mildly alkalized and, where lime is added as alkali reserve, the quantity added should not exceed about 2 lb/bbl (lime/oil mud), and is preferably slightly smaller.

12. An invert drilling fluid as claimed in claim 11, characterized 10 in that the oil mud does not contain highly hydrophilic bases, such as alkali hydroxides, or highly hydrophilic amines, such as diethanolamine, and in that limited quantities of metal oxides of the zinc oxide type are optionally present as alkali reserve in addition to or instead of lime. 15

13. Drilling fluids as claimed in claims 11 and 12, characterized in that they have a plastic viscosity (PV)) in the range from about 10 to 60 mPa.s and a yield point (YP) in the range from about 5 to 40 lb/100 ft^, as measured at 50°C.

14. Drilling fluids as claimed in claims 11 to 13, characterized in 20 that their disperse water content is from about 5 to 45% by weight and preferably from about 10 to 25% by weight and, in particular, contains dissolved salts of the CaC^ and/or KC1 type.

15. Drilling fluids as claimed in claims 11 to 14, characterized in that the oil phase of the invert mud has a Brookfield (RVT) viscosity 25 at 0 to 5°C of below 50 mPa.s and preferably not exceeding 40 mPa.s.

16. The use according to claim 1 of selected ester oils in drilling fluids substantially as hereinbefore described with reference to the Examples.

17. Drilling fluids according to claim 11 substantially as 30 hereinbefore described with reference to the Examples. T0MKINS & CO. 35