CA2058636C - Use of selected ester oils in water-based drilling fluids of the o/w emulsion type and corresponding drilling fluids with improved ecological acceptability - Google Patents

Abstract

The use is disclosed of water-dispersible esters, which are fluid or at least plastically deformable at working temperature and have flash points of at least 80°C, from saturated and/or unsaturated carboxylic acids with up to 36 carbon atoms and mono- and/or polyhydric aloohols, as at least the main constituent of the dispersed oil phase of water-based O/W-emulsion drilling fluids which are suitable for the environmentally friendly development of geological formations and which contain, if desired, insoluble, finely particulate weighting agents for the formation of water-based O/W-emulsion drilling mode and/or further additives, such as emulsifiers, fluid-loss additives, wetting agents, alkali reserves and/or auxiliary substances for the inhibition of drilled rock of high water-sensitivity.

Description

-1- _.2058636 Use of selected ester oils in water-based drilling fluids of the O/W- emulsion tvpe and corresponding drilling fluids with improved ecological acceptabilitx The invention discloses new drilling fluids based on water-based 0/W-emulsions and O/W-emulsion drilling muds based thereon, which are distinguished by high ecological acceptability and at the same time good standing and application properties. An important area of use for the,new drilling mud systems is in off-shore wells for the development of petroleum and/or natural gas deposits, the aim of the invention being particularly to make available drilling fluids with high ecological acceptability which can be used in industry. The use of the new drilling mud systems admittedly has particular significance in the marine environment, but is not limited thereto.
The new mud systems can also be put to quite general use in land-based drilling, i.e. also for the development of petroleum and/or natural gas deposits hare. They are, however, new valuable working agents, for example, also in geothermal wells, in water bore-holes, in the drilling of geoscientific bores and in drilling for the mining industry. It is also essentially true hare that the associated ecotoxic problems are substantially simplified by the new water-based O/W-drilling fluids selected according to the invention.
The Prior Art Liquid mud systems used in the sinking of rock bores for bringing up the loosened drill cuttings are known to be flowable systems, thickened to a limited extent, which can be assigned to one of the three following classes:
Purely aqueous drilling fluids; drilling mud systems based on oil, which as a rule are used in the form of so-called invert emulsion muds, and represent preparations of the W/O-emulsion type in Which the aqueous phase is distributed as a heterogeneous fine dispersion in the continuous oil phase. The third class of the known drilling fluids is composed of water-based O/W-emulsions, i.e. fluid systems which contain a heterogeneous, finely-dispersed oil phase in a continuous aqueous phase. The invention discloses improved systems of this latter type.
X;

zo5ss~s The application properties of the drilling fluids of such O/W-emulsion systems take an intermediate position between the purely aqueous systems and the oil-based invert fluids. The advantages, but also the disadvantages, of the purely aqueous systems are connected with the advantages and disadvantages of the oil-based invert-emulsions disclosed hitherto. Detailed information on this subject can be found in the relevant specialist literature, refer, for example, to the text book by George R. Gray and H.C.H. barley, "Ca~osition and Properties of Oil Well Drilling Fluids", 4th. edition, 1980/1981, Gulf Publishing Co~any, Houston, and the extensive specialist and patent literature cited therein and to the manual "Applied Drilling Engineering", Adam T.
Hourcpyne, Jr. et al., First Printing Society of Petroleum Engineers, Richardson, Texas (USA).
One of the main weaknesses of purely water-based drilling mud systems lies in the interaction of water-sensitive, particularly water-swellable, rock and/or salt formations with the aqueous drilling fluid and the secondary effects initiated thereby, in particular bore-hole instability and thickening of the drilling fluid. Many proposals are concerned with the reduction of this problem area and have, for example, resulted in the development of the so-called inhibitive water-based muds, cf. for exanple, "Applied Drilling Engineering", loc. cit., Chapter 2, Drilling Fluids, 2.4 and Gray and barley loc. cit., Chapter 2, in particular the sub-section on pages 50 to 62 (M~.uls for "Heaving Shale", Muds for Deep Holes, Non-Dispersed Polymer Muds, Inhibited Muds: Potassium Compounds).
In more recent practice, in particular drilling fluids based on oil, which consist of the 3-phase system oil, water and finely particulate solids and are preparations of the W/0-emulsion type, have succeeded in overoaning the difficulties described above. Oil-based drilling fluids were originally based on diesel oil fractions contain;ng aranatics. For detoxification and to reduce the ecological problems created thereby, it was then proposed to use hydrocarbon fractions which are largely free of aranatics - nvw also lu~wn as "non-polluting oils" - as the continuous oil phase, see in this regard, for example, the publications by E.A. Hoyd et al. "New Base Oil Used in Low Zbxicity Oil Muds", Journal of Petroleum Technology, 1985, 137 - 143 and R.B.

Bennet "New Drilling Fluid Technology - Mineral Oil Mud", Journal of Petroleum Technology, 1984, 975 - 981 and the literature cited therein.
Drilling fluids of the water-based 0/W-eqnulsion system type have also hitherto used pure hydrocarbon oils as the dispersed oil phase, cf.
here, for exanple, Gray, barley loc. cit., p. 51/52 under the section "Oil nnulsion Muds" and the tabular stunnary on p. 25 (Tables 1-3) with details of water-based emulsion fluids of the salt-water mud, lime mud, gyp mud and C~CLS mud type .
In this context in particular it is known that water-based 0/W-emxlsion fluids represent a substantial improvement in many regards to the purely water-based drilling mud systems. Particularly in more recent times, however, the advantages and disadvantages of such water-based emulsion fluids have also been examined critically in ca~~arison with the oil-based invert-systems. Thie is due to the considerable ecological reservations felt towards the oil-based invert drilling fluids camanly used today.
These ecological reservations can be subdivided into two problem areas:
In addition to the basic constituents, i.e. oil and water, all drilling fluid systems based on water and/or oil require a large number of additives for the establishment of the desired application properties.
The following can be mentioned here purely by way of example:
emulsifiers or emulsifier systems, weighting agents, fluid-loss additives, wetting agents, alkali reserves, viscosity regulators, in some cases auxiliary agents for the inhibition of drilled rock w2th high water-sensitivity, disinfectants and the like. A detailed stunnary can be found, for example, in Gray and barley, loc. cit., Chapter 11, "Drilling Fluid Catponents". The industry has developed additives which currently appear ecologically harmless, but also additives which are evologically questionable or even ecologically undesirable.
The second problem area is caused by the oil phases used in such drilling fluids. Even the hydrocarbon fractions which are largely free from ara~atics, currently known as "non-polluting oils", are not oonpletely harmless when released into the environment. A further 2o5s636 reduction in the enviiror~nental problems, which are caused by the fluid oil phases of the type referred to here, appears urgently necessary.
This is true in particular for the sinking of off-shore wells, e.g., for the develo~nent of petroleum or natural gas deposits, because the marine eco-system reacts particularly sensitively to the introduction of toxic and poorly degradable substances.
There have recently been some proposals for reducing these latter problems. For example, the US Patent Specifications 4,374,737 and 4,481,121 disclose oil-based invert-drilling fluids in which non-polluting oils are to be used. The following can be mentioned together as of equal value as non-polluting oils: mineral oil fractions which are free from aromatics and vegetable oils, such as peanut oil, ' soybean oil, linseed oil, worn oil, rice oil or even oils of animal origin, such as whale oil. These named ester oils of vegetable and animal origin are all, without exception, triglycerides of natural fatty acids, which are known to have high environmental acceptability and are clearly superior from the evological point of view to hydrocarbon fractions - even when these cb not contain aromatic hydrocarbons.
In the above US Patent Specifications, however, not one oonc~ete example describes the use of such natural ester oils in invert drilling fluids. Without exception, mineral oil fractions are used as the continuous oil-phase. In fact, oils of vegetable and/or animal origin of the type mentioned here are not considered for invert drilling fluids for practical reasons. The Theological properties of such oil phases cannot be controlled over the wide temperature range generally required in practice, from 0 to 5°C on the one hand, up to 250°C
and more on the other.
The ALplicant's other proposals A series of publications and co-pending Canadian Applications by the Applicant describe the use of easily biodegradable and ecologically harmless ester oils as the continuous oil phase in W/O-invert drilling mud systems. Refer here in particular to the publications DE 38 42 659 and DE 38 42 703 and the modifications of the ester oils that can be used according to the details of the co-pending Canadian Applications 2,047,697, filed March 1, 1990 and 2,047,706, filed March 1, 1990 (DE 39 07 391 and DE 39 07 392).
The subject of these co-pending Applications is the use of ester oils based on selected monocarboxylic acids and monocarboxylic acid mixtures and monofunctional and optionally polyfunctional alcohols as the continuous oil phase in W/O-invert systems. The co-pending Applications show that, using the esters and ester mixtures disclosed therein, not only can satisfactory Theological properties be established in the fresh drilling fluid, but it is also possible by the additional use of selected, known alkali reserves in the drilling fluid, for work to be carried out without fearing undesired thickening effects when there is a partial ester hydrolysis.
An important further development of such invert drilling fluids based on ester oils is the subject of the Applicant's co-pending Canadian Application 2,009,689, filed February 9, 1990 (DE 39 03 785).
The teaching of this co-pending Application starts with the concept of also using a further additive in invert drilling fluids based on ester oils which is suitable for keeping the desired Theological data of the drilling fluid in the required range, even when in practice larger and larger amounts of free carboxylic acids are formed by partial ester hydrolysis. The co-pending Application provides for the additional use of basic amine compounds, which are capable of forming salts with carboxylic acids and have a marked oleophilic nature and at most limited water-solubility, as additives in the oil phase.
The invention~roblem and its technical solution The present invention starts with the problem of providing drilling mud systems of the highest, and in this form previously unknown, ecological acceptability which simultaneously have good application properties and which also in particular enable satisfactory application in problem areas. In the invention a conscious decision has therefore been made to reject the oil-based type of invert drilling mud systems and to return to the oil-modified water-based type of O/W-emulsion systems. The auxiliary agents described in the cited co-pending Applications and Publications of the ~~~8636 Applicant, and the ecological advantages associated theres~rith, are now, however, also to be used in this class of drilling mud systems.
In a first embodiment, the aim of the invention is therefore to make use of the advantages that O/W-emulsion mud systeqns have against purely water-based drilling fluids, but at the same time to replace the mineral-oil phase at least to a substantial amount - preferably co~pletely - with ecologically harmless ester oils.
In a further approach, the invention aims also to reduce the ecological concerns of the second problem area, i.e. that associated with the additives and auxiliary agents in drilling fluids, by selecting fran the wide range of additives known in this field, at least to a large extent and preferably in all cases, those auxiliary agents which are distinguished by their ecologically harmless nature.
The subject of the invention is accordingly in a first embodiment the use of water-emulsifiable esters which are fluid and/or at least plastically deformable at working temperature and have flash points of at least 80°C, frown saturated and/or unsaturated carboxylic acids with up to 36 carbon atoms and mono- and/or polyhydric alcohols as at least the predc~ninant constituent of the dispersed oil-phase of water-based O/W-emulsion drilling fluids, which are suitable for an envi.rornrentally friendly develapnent of geological formations, and, if desired, contain insoluble, finely particulate weighting agents for the formation of water-based O/W-e~lsion drilling mode and/or further additives, such as emulsifiers, fluid-loss additives, wetting agents, alkali reserves and/or auxiliary substances for the inhibition of drilled crock with high water-sensitivity.
In a further embodiment, the invention relates to water-based O/W-emulsion drilling fluids, which in a continuous aqueous phase contain, in stable dispersion, an oil phase in amounts of about 5 to 50 % by weight - the percentage by weight referred to the sum of the unweighted water phase and oil phase - together, if required, with dissolved and/or dispersed auxiliary substances of the type mentioned and are characterized in that at least the largest part of the dispersed oil phase is formed by water-emulsifiable ester oils which are fluid or at ;r , _ _,-least plastically deforznable at working temperature, and have flash points of at least 80oC, from saturated and/or unsaturated carboxylic acids with up to 36 carbon atoms and mono- and/or polyfunctional alcohols.
Both embodiments of the teaching according to the invention include the additional preferred step of also using for the water-based emulsion drilling fluids or emulsion drilling mode, at least to a large extent, those inorganic and/or organic auxiliary and loading substances which are at least predominantly ecologically and toxicologically harmless.
And thus, for example, in the most important eqnbodiments of the invention the use of auxiliary agents based on soluble, toxic heavy-metal compounds is awided.
The preferred embodiments of the invention The mixture ratios of the ester oil/water phases cover the usual range for the previously lazaan O/W-emulsion drilling fluids based on mineral oils. The lower limit values for the oil phase are usually at least about 5 % by weight, or preferably between about 5 and 10 % by weight, e.g., therefore 7 or 8 % by weight - each percentage by weight referred to the total weight of the fluid phases ester oil + water, each in the unweighted state. Minim~un amounts of the order given ensure that use can be made of the characteristic peculiarities of an O/W-emulsion fluid. The upper limit value for the oil content is usually about 50 %
by weight or even slightly higher, e.g., a maximum of about 65 % by weight. Assuming that the droplet size of the dispersed oil phase is sufficiently even, the range with the most dense packing is therefore already achieved, and thus conversion into the fluid type of the W/0-invert fluids is obvious or appears logical.
The upper limit for the ester~il content in the O/W-fluids according to the invention is generally determined by oost/benefit considerations and is, for exanple, about 45 % by weight, preferably less, e.g., about 40 % by weight.
An ester-oil amount from about 10 to 40 % by weight - percentage by weight calculated ae before - and in particular amounts of the ester ~~58636 _8-oil fran about 15 to 35 % by weight facilitate the exploitation of many - lmown and not previously described - advantages of such emulsion fluids. Oil contents of, for example, 20 % by weight, or in an extreme case, 30 % by weight provide the basis of high-quality drilling fluids which approach, at least very closely, the oil-based invert fluids in the way they function, but require very much less ester~il phase.
The definition of the ester oils used acoordinQ to the invention The following general considerations apply initially for the selection and adaptation of the estex oils for each application:
The ester oils should be fluid at ambient temperature, but also under their conditions of use, with the range of materials thought to be fluid, however, also including materials which are at least plastically deformable at ambient t~rQerature and which soften to became fluid at the usually high working tett~eratures. For reasons of easier processability, ester oils with solidification values (pour point and setting point) below 10°C and usefully below OoC are preferred in practice. Corresponding ester oils with solidification values not above -5°C can be particularly suitable. One should take into account here the fact that the drilling fluids are usually produced on site using, for example, sea water at va~paratively low water-temperatures.
For reasons of industrial safety, it must be a requirement that the ester oils have flash points of at least 80°C, fir, higher flash points of at least 100°C and substantially higher values are preferred, for example, those above 150 or 160 °C.
A further important requirement for the optimal employment of the subject of the invention is that the ester oils have a biologically or ecologically acceptable constitution, i.e. in particular are free from undesired toxic constituents. In the preferred embodiment of the invention, ester oils are accordingly used which are free from araratic constituents and in particular have saturated and/or olefin-unsaturated, straight-chain and/or branched hydrocarbon chains. The use of oa~onents containing cycloaliphatic structural constituents is possible from ecological points of view, but for reasons of cost will y .... ..

_ g -be of less significance in practice.
Carboxylic acid esters of the type in question here are subject to a limited degree, as the highly dispersed oil-phase in a continuous aqueous phase, to hydrolytic ester-cleavage with liberation of the ester-forming constituents carboxylic acid and alvohol. ~ closely linked issues should be taken into account as regards the application properties of the ester oils in the sense of the invention, namely vonsiderations of the possible toxicity of the liberated components, in particular of the alcohol ca~onents, when inhaled and also the change in the oo~c~osition of the emulsion fluid and any associated change which may take place in its application properties.
For oa~rehension of the teaching according to the invention, these considerations should be examined separately for each of the ester-forming basic constituents - on the one hand the aloohols and on the other the carboxylic acids.
According to the invention, both m~nohydric alcohols and polyhydric alcohols are suitable as the ester-forming alcohol components, and any mixtures of these types can also be used. A further distinction can be made between alaohols acvordi.ng to their solubility behavior in water.
The aloohols can be water-soluble and/or water-insoluble.
In a first group, polyhyriric alcohols are to be considered.
Particularly preferred here are the industrially easily available la~w~er, polyfunctional aloohols with 2 to 5, preferably 2 to 4 hydroxyl groups and in particular 2 to 6 carbon atoms, which forni esters with a suitable rheology.
Characteristic representatives are ethylene glycol, the prnpanediols and particularly glycerin.
Polyhydric alcohols of the type referred to here are distinguished by high water-solubility and at the same time by such low volatilization values that considerations of the exclusion of toxic hazards when inhaled cb not usually apply.

Polyhydric lower aloohols of the type referred to here can be used as totally esterified oil oo~ponents and/or as partial esters with some free hydroxyl groups and/or can be formed in the practical use of the emulsion fluid according to the invention. Provided that the partial esters formed retain the at least largely water-insoluble character of the oil phase, no substantial change takes place as regards the oil/water ratio in the emulsion fluid. The situation is only different when water-soluble hydrolysis products form - in particular therefore the free lower polyhydric aloohols. The changes which occur in practical operation in such emulsion fluids as a result of this process are, however, insignificant. Firstly, a oc~g~aratively high stability of the ester bond is ensured under the working conditions according to the invention. O/W-emulsion fluids are known to operate usually in the pH-range of approx. neutral to moderately alkaline, for example, in the pH-range of about 7.2 to 11 and in particular about 7.5 to 10.5, and thus for these reasons alone there is no aggressive hydirolytic attack on the ester bond. In addition, and moreover, the following is in particular also true:
In the practical use of the drilling fluid, and the associated driving forward of the bore into ever deeper earth strata, there is a continual consumption of the drilling fluid and in particular of the oil-phase used in the drilling fluid. ~ulsion fluids are knaan - and this is an i~ortant point of value in their use - for the fact that the emulsified oil phase clings on to solid surfaces and therefore both seals the filter bed to the wall of the bore shaft and hinders, or even prevents, interaction between the drilled rock and the aqueous phase of the drilling fluid. This continual consumption of drilling fluid, and in particular of the oil-phase necessitates a continual supply of oil-based mud. In practical operation, a state of equilibrium is therefore rapidly established in the drilling fluid which prevails for long periods of time facilitates a oontin~us operation.
Fmm acme viewpoints, further considerations should be taken into account when monohydric aloohols are used in the ester oils. Here only the lamer representatives of these alvohols are water-soluble or, in an unlimited quantity, water-miscible. In addition, however, volatility is a nat unimportant consideration in the case of these alcohols. In - m - I 2058636 the practical operation of a bore, at least moderately increased temperatures are rapidly established in the circulated drilling fluid and therefore the parts exposed by the ping to remove the drill cuttings have a temperature, for exanple, in the range of 50 to 70 °C.
Considerations of toxicological effects when inhaled must therefore be taken into account here. Even C4-aloohols, e.g.; isobutyl alcohol; can be so volatile under the operating conditions on the drilling platform that hazards to personnel must be taken into account. According to the invention, therefore, when ester oils are employed together with moryohydric alcohols, the lamer carbon number limit for these monohydric alcohols is preferably selected as 6, and working with esters of monofunctional alvohols with at least 8 carbon atoms can be particularly preferred.
The selection and limitation of the carbon number in the ester-forming alcohol, hvw~ever, at the same time has the follc~aing result as regards the oo~r~osition of the ester-oil phase when there is a partial hydrolysis during operation: The hydrolyzing parts of such ester oils r are converted to the free alcohol which remains as a practically water-insoluble mixture constituent in the dispersed ester-oil phase. This can bring about quite specific advantages for the functionability of the dispersed oil phase. Oil-based invert drilling- mud systems based on flowable, in particular monofunctional, alcohols with high ecological acceptability are disclosed in the Applicant's co-pending Canadian Application 2,051,624, filed March 29, 1990 (DE
39 11 238). The use of these alcohols as a material for O/W-emulsion fluids of the type comparable to that in the present invention is the subject of the co-pending Application by the Applicant. To this extent the teaching of the invention is connected with that of the co-pending Application and therefore this need not be dealt with in any further detail here.
Essentially, however, the above consideration also applies here, i.e.
that in practical use a quasi-static state adjusts rapidly through equilibrium forn~ation in the cocrposition of the ester-oil phase which is characterized by large contents of non-hydrolyzed ester oil.
A number of points must also be considered with regard to the carboxylic acids formed by the partial hydrolysis of the ester oil 2058~3~

fluid.
Here it is possible, depending on the specific constitution of the carboxylic acids used, to distinguish between two basic types -without there being a rigid transition between the tux: carboxylic acids which give rise to carboxylic acid salts with an ~lsifier effect, and those which give rise to inert salts.
The respective chain-length of the liberated carboxylic acid molecule is in particular decisive here. Moreover, the salt-forming ration usually present in the alkali reserves of the drilling fluid should also be considered.
In general the following rules apply: Lower carboxylic acids, for example those with 1 to 5 carbon atoms, give rise to the formation of inert salts, for exanple, the formation of corresponding acetates or propionates. Fatty acids of higher chain-length and in particular those with frodn 12 to 24 carbon atoms result in o~OUnds with an emulsifier effect.
By the selection of suitable ester oils - and to a certain extent also the salt-forming rations in the emulsion fluid - the specific control of the secondary products in the emulsion fluid is therefore possible, which can also have considerable influence on the nature and effect of the emulsion fluid. The above also applies here: It is not only the dispersed organic phase, but also the aqueous phase, which is subject to continual consumption in practice and thus requires replacement. In stationary operation, therefore, rapidly controllable states of equilibrium will be established, even with regard to the reaction by-products based on the ester-forming carboxylic acids as discussed here.
General details of the definition of suitable ester oils In the sense of the invention, the corresponding reaction products of monocarboxylic acids with monofunctional and/or polyfunctional alcohols of the type given are preferred as the ester oils. The additional use of polyvalent carboxylic acids is not, haa~ever, excluded, but they are of less significance, in particular for reasons of cost.

.~ 2~58~~~

The carboxylic acids here can be of natural and/or synthetic origin, they are, as already indicated, preferably straight-chain and/or branched and optionally cyclic, but not aromatic in structure. The ester-forming carboxylic acids can be saturated and/or unsaturated, with unsaturated compounds here being understood, in particular, to be olefin-unsaturated compounds, which can be mono- but also poly-olefin-unsaturated. Olefin-unsaturated components can be of particular significance for adjusting the predetermined rheology values. It is known that olefinic longer-chain ca~ounds are suitable as corresponding saturated components for the forn~ation of esters with lower melting points.
The preferred range for the carbon number of the carboxylic acids extends from 1 to 36 and in particular from 2 to 36. For reasons of easy availability, an upper limit for the carbon number can be about 22 to 24. The respective chain length of the ester-forming carboxylic acid ca~onents is selected - depending on the nature of the alvohols oomponent(s) used - by consideration of the various issues already discussed, and refers not only, for example, to the ester and/or its rheology directly, but also to the reaction by-products formed in particular by partial hydrolysis.
Suitable aloohols are, as indicated, both monofunctional alcohols -provided the above limitations are taken into consideration - and polyfunctional alcohols, particularly lower polyfunctional alcohols with 2 to 6 carbon atoms and preferably with a maxinsun of 4 hydroxyl 9m~
The alcohol oatponents here can also be of natural and/or synthetic origin, they are straight-chain or branched and in particular in the case of the monofunctional aloohols are saturated and/or also olefin-unsaturated. Monofunctional alaohols have in particular up to 36 carbon atcms, preferably up to about 24 carbon atoms. Alcoh~ols with 6 to 18, in particular 7 to 15 carbon atoms, of natural and/or synthetic origin can only be of particular significance in the formation of the ester oils.
_ _--_ -~ f ~, . . _ ~: _ _ _~ .~___ . _ -~ ~ .... _ 14 _ 2058636 Particularly important ester oils in the sense of the use in the invention are the ecologically-acceptable ester oils as described in particular in the cited Publications and co-pending Applications DE 38 42 659, DE 38 42 703, 2,047,697 (DE 39 07 391) and 2,047,706 (DE 39 07 392). To complete the invention disclosure, the essential characteristics of these ester oils or ester mixtures are briefly summarized below.
The dispersed ester-oil phase accordingly contains carboxylic acid esters from at least one of the following sub-classes:
a) Esters from C1_5-rrnnocarboxylic acids and mono- and/or polyfunctional alcohols, in which radicals from monohydric aloohols have at least 6, preferably at least 8 carbon atans, and the polyhydric aloohols preferably have 2 to 6 carbon atoms in the molecule, b) esters fran monocarboxylic acids of synthetic and/or natural origin with 6 to 16 carbon atoms, in particular esters of corresponding aliphatic-saturated monocarboxylic acids and mono-and/or polyfunctional alvohols of the type indicated under a), c) esters of olefin mono- and/or poly-unsaturated monocarboxylic acids with at least 16, in particular 16 to 24 carbons atoms, and in particular m4nofunctional straight-chain and/or branched aloohols.
The latter esters of olefinic mono- and/or poly-unsaturated monocarboxylic acids with at least 16 carbon atans (c) are preferably assigned to at least one of the following sub-classes:
cl) esters which are derived by rrore than 45 % by weight, preferably by more than 55 % by weight frncn di- and/or poly~lefin-unsaturated C16-24-~n~°xylic acids, c2) esters which are derived by not more than 35 % frcm di- and poly-olefin-unsaturated acids, and are preferably at least about 60 %
by weight mono-olefin-unsaturated.
The raw materials for obtaining many of the monocarboxylic acids in these sub-classes, in particular those with a higher carbon number, are -15 - ~ I 2058636 vegetable and/or animal oils. Coconut oil, palm kernel - oil and/or babassu oil, can be mentioned in particular as materials used for obtaining mc~nocarboxylic acids mainly in the range of up to 18 carbon atat~s, and with essentially saturated components. ~anples of vegetable ester oils, in particular for olefinic mono- and optionally poly-unsaturated carboacylic acids with from 16 to 24 carbon atoms, are palm oil, peanut oil, castor oil and in particular rapeseed oil.
Carboxylic acids of animal origin of this type are in particular corresponding mixtures of fish oils, such as herring oil.
The teaching of the invention expressly includes also and in particular the use of m~nocarboxylic acid triglyoerides and therefore in particular also the use of corresponding glyceride oils of natural origin. Here, haaever, the following must be considered: Natural oils and fats usually occur in a form so highly contam;nated, for example, with free carboxylic acids or other accompanying substances, that there is as a rule no question of immediately processing them in O/W-emulsion fluids of the type referred to here. If such natural materials are added in the commercially available form to water-based drilling fluids, then almost immediately, such a large aimunt of foam forms in the drilling fluid being used as to constitute a serious hindrance or even to result in the drilling fluid being unusable. This may not be the case if cleaned and/or synthetically produced selected triglyoerides are used in the dispersed oil phase. The teaching according to the invention can be realized without exception with these. In principle, ha~aever, with such esters of high-grade aloohols one must always anticipate a not inconsiderable tendency towards foam formation. Partial esters of glycerin - the mr~no- or di-glycerides -are known to be effective emulsifier components.
As already indicated, it is not only the comparatively low-viscosity ester oils as in the disclosure of the cited Publications and co-pending Applications of the Applicant in the field of invert drilling fluids which are based on ester-oils, which are suitable for the purposes of the invention, but within the framework of O/W-emulsion fluids, in particular comparatively viscous ester oils can be of advantage as the dispersed phase. They are, for example, valuable auxiliary agents for sealing the finest pores in the filter cake of the bore shaft, or in rendering gy inert swellable rack. The lubricating ability of such ester oils of oatparatively high viscosity even at elevated te~eratures in the bore shaft, and in particular also in deviated bore-holes, is in some cases distinctly better than that of the comparatively low-viscosity ester oils. A dispersed ester oil phase of catQaratively hick-viscosity ester oils does not cause any detrimental effect on the drill-technology, the rheology of the system as a whole is determined by the continuous aqueous phase. In this sense it may be preferred to use ester oils as the dispersed phase which have a Brookfield viscosity of up to about 500,000 mPa.s or even higher, for ale, up to about 1 million mPa.s or even 2 million mPa.s (determined at roan te~perature).
This constitutes an important extension of the teaching in the noted Publications and co-pending Applications of the Applicant in the field of oil-based invert drilling fluids based on ester-oils.
In one cinbodiment of the invention, branched-chain oc~onents and in particular alpha-branched-chain aloohols and/or carboxylic acids can be of particular significance. Branches of this type are known on the one hand to influence the rheology, the esters formed by such chain-branching are usually more mobile. Moreover, such alpha-branching can, however, also promote increased hydrolysis stability under working conditions, this is therefore exploited in the invention.
The actueous phase All types of water are suitable for the production of the O/W-e~tlsion fluids according to the invention. These can therefore be based on fresh water and in particular' also on salt water - particularly sea water for use in off-shore wells.
Additives in the emulsion fluid In principle, all the additives used in co~~parable drilling fluid types can be considered, which are usually added in ~nnection with a quite specific desired range of drilling fluid properties. The additives can be water-soluble, oil-soluble and/or water- or oil-dispersible.
Classical additives for water-based O/W-atwlsion fluids can be:

~0~863fi emulsifiers, fluid-loss additives, structure-viscosity-building soluble and/or insoluble substances, alkali reserves, agents for the inhibition of undesired water-exchange between drilled formations - e.g. water-swellable clays and/or salt strata - and the water-based drilling fluid, wetting agents for better adhesion of the emulsified oil phase to solid surfaces, e.g. for improving the lubricating effect, but also for the improvement of the oleophilic seal of exposed rock formations, or rock surfaces, disinfectants, e.g. for inhibiting bacterial attack on such O/W-emulsions, and the like. For details, reference should be made here to the relevant prior art, as described in detail in the specialist literature cited above, refer here in particular to Gray and barley, loc. cit., Chapter 11, "Drilling Fluid Co~c~onents". We will therefore only cite extracts below:
Finely-dispersed additives for increasing the fluid density: Barium sulfate (barite) is widely used, but also calcium carbonate (calcite) or the mixed carbonate of calcium and magnesium (dolomite).
Agents for building up the structure-viscosity, which at the same time also act as fluid-loss additives: Bentonite in particular should be mentioned here which is kno~m to be used in water-based fluids in a non-modified form and is therefore ecologically safe. For salt-water fluids other oo~parable clays, in particular attapulgite and sepiolite, are of considerable significance in practice.
The additional use of organic polymer ornpounds of natural and/or synthetic origin can also be of considerable in~ortance in this context. The following should in particular be mentioned here: starch or chemically modified starches, cellulose derivative, such as carboxymethylcellulose, guar gum, xanthan gum, or also purely synthetic water-soluble and/or water-dispersible polymer oa~pounds, such as in particular the polyacrylamide compounds of high molecular weight with or without anionic or cationic modification.
Zhinners for viscosity-regulation: So-called thinners can be of organic or inorganic nature; exanple of organic thinners are tannins and/or quebracho extract. Further e~les are lignite arxi lignite derivative, particularly lignosulfonates. As indicated above, in a _ ___~_.._..:~...-.~.~ _ __.. . . _ _ _ _. ___ preferred embodiment of the invention the use of toxic cutponents is particularly to be excluded, and in particular the corresponding salts with toxic heavy metals, such as chromium and/or copper. Polyphosphate compounds are exiles of inorganic thinners:
Ffiulsifiers: For the teaching according to the invention, two features in particular should be considered. It has emerged that a stable dispersion of ester oils is very much more easily produced than the corresponding dispersion of pure mineral oils as used in the state of the art. This in itself is a first simplification. Furthermore, it should be vonsidered that, when longer-chain carboxylic acid esters are used, by the partial saponification of the ester oils under the additional effect of suitable alkali reserves, effective O/W-emulsifiere are also formed which contribute to the stabilization of the system.
Additives which inhibit undesired water-exchange with, for example, clays: The additives known from the state of the art for use in water-based drilling fluids can be oonsi.dered here. In particular, halides and/or carbonates of the alkali and/or alkaline-earth metals, with particular importance given to corresponding patassiiun salts, optionally in vatibination with lime. Reference is made for example to the appropriate publications in "Petroleum Engineer International", September 1987, 32 - 40 and "World Oil", Nov~ber 1983, 93 -97.
Alkali reserves: inorganic and/or orc~nic bases adapted to the total behavior of the fluid can be considered, in particular corresponding basic salts or hydroxides of alkali and/or alkaline-earth metals and orcganic bases .
In the field of organic bases, a conceptual distinction must be drawn between water-soluble orcganic bases - for example, compounds of the diethanol~ine type - and practically water-insoluble bases of marked oleophilic character as described in the Applicant's co-pending Application 2,009,689 (DE 39 03 785) cited above as additives in invert drilling muds based on ester oil. The use of such oil-soluble bases in the framework of the present invention in particular falls within the new teaching. Oleophilic bases of this type, which are distinguished in particular by at least one longer hydrocarbon radical with, for example, 8 to 36 carbon atoms, are, however, not dissolved in the aqueous phase, but in the dispersed oil phase. Here these basic oc~onents are of multiple significance. On the one hand they can act immediately as alkali reserves. On the other, they give the dispersed oil droplets a certain positive state of charge and therefore result in increased interaction with the negative surface charges which can be found in particular in the hyd~hilic clays which are capable of ion-exchange. According to the invention one can thus influence the hydrolytic cleavage and the oleophilic sealing of water-reactive rock strata.
The quantity of auxiliary substances and additives used in each case moves essentially within the usual boundaries and can therefore be found in the cited relevant literature.
~sanples Firstly, a 6% by weight harogenized bentonite suspension is prepared using commercially available bentonite (not hydrophobized) and tap water, and a pH value of 9.2 to 9.3 is adjusted by means of a sodium hydroxide solution.
Starting with this pre-swollen aqueous bentonite phase, the individual ' components of the water-based ester-oil emulsion as in the 'following formulation are incorporated in successive stages of the process - each under intensive intermixing:
350 g 6 % by weight bentonite solution rM
1.5 g industrial carboxymethylcellulose low-viscosity (gelatin U 300 S9) 35 g sodium chloride 70 g ester oil (according to the definition given below) 1.7 g emulsifier (sulf. castor oil "Turkey-red oil", unless otherwise indicated) 219 g barite Viscosity measurements are carried out on the thus pregared O/W-2U58~35 emulsion fluids as follows:
Firstly, the plastic viscosity (PV), the yield point (YP) and the gel strength after 10 sec. and after 10 min. of the emulsion fluid are determined at 50°C on the unaged material.
The emulsion fluid is then aged for 16 hours at 125oC in an autoclave in the so-called "roller-oven" to examine the effect of temperature on the stability of the emulsion. Then the viscosity values at 50oC are determined once again. In each of the following exanples the nature of the ester oil used, details of the emulsifier, the values determined for the unaged and aged material and - if necessary - general comments are given.
Ele 1 Ester~il used: 2-ethylhexylester of a Cg_14-fatty acid mixture (essentially saturated) Turkey-red oil as the emulsifier unaged aged material material plastic viscosity [mPa.s] 8 14 yield point [Pa] 19.2 13.4 gel strength [Pa]
sec. 13.4 9.6 10 min. 16.3 23.0 Example 2 The formulation in Example 1 is repeated, but without using the emulsifier (Turkey-red oil).
The viscosity values measured on the unaged and aged material are as follows:
unaged aged material material plastic viscosity [mPa.s] 11 10 yield point [Pa] 16.8 18.2 gel strength [Pa]

sec. 18.2 16.3 10 min. 35.9 24.0 Even in the fresh formulation a slight droplet formation can be seen on the surface, after ageing the solid substanceto settle.
tends Exanple 3 Ester-oil used: Oleic-acid isobutylester Emulsifier: Turkey-red oil The viscosity values measured on the unagedaged drilling and the fluid are as follows:

unaged aged material material plastic viscosity [mPa.s] 11 13 yield point [Pa] 18.2 12.9 gel strength [Pa]

10 sec. 12.5 9.6 10 min. 15.8 18.7 . . ~ ~ - 22 - 205863b ~ple 4 The formulation in ale 3 is repeated, but now soybean lecithin "N
(oa:mercial product: Drilltreed ) is used as the emulsifier in a quantity of 1.7 g. The viscosity values measured on the unaged and aced drilling f laid are as follows unaged aged material material plastic viscosity [mPa.s] 11 12 yield point [Pa] 12.5 14.9 gel strength [Pa]
sec. 12.5 6.2 10 min. 8.6 16.3 ~tanple 5 With Turkey-red oil as the e:rnulsifier, p~pylene-glyaolamno-cleats is used as the ester oil. The following viscosity values are determined:
unaged aged material material plastic visvosity [mPa.s] 20 14 yield point [Pa] 20.1 16.3 gel strength [Pa]
10 sec. 15.8 12.5 10 min. 24.9 24.9 Fle 6 Again using Turkey-red oil as the emulsifier, a trimethylolpropane tri-fatty-acid ester is used as the ester-oil phase. The values measured for the unaged and aged material are as follows:

~g~ age _ material material plastic viscosity [mPa.s] 15 16 yield point [Pa] 17.7 15.8 gel strength [Pa]
sec. 13.9 12.9 10 min. 20.6 23.0 le 7 In the context of the teaching of ale 6, a corresponding glycerin tri-fatty-acid ester is used as the ester oil phase. The values determined for the unaged and aged emulsion drilling fluid are as follows:
unaged aged material material plastic viscosity [mPa.s] 13 12 yield point [Pa] 8.6 8.1 gel strength [Pa] ' 10 sec. 8.6 6.2 10 min. 9.1 5.8 ~anple 8 Using Turkey-red oil as the emulsifier, the residue, esterified with n-hexanol, from the fatty-acid dimerization (monomer fatty acid Alip~hat~
47 ) is incorporated as the ester-oil phase. The viscosity values determined for the unag~ed and aged material are as follows:

.~ . ~ ~~~8~3~

unaged aged material material plastic vis~sity [mPa.s] 14 16 yield pint [Pa] 17.2 14.4 gel strength [PaJ
sec. 12.0 9,1 10 min. 12.9 22.5

Claims (13)

CLAIMS:

1. A water-based, oil-in-water emulsion drilling fluid suitable for the development of geological formations, said drilling fluid consisting essentially of a. a continuous aqueous phase containing from about 5 to about 50% by weight of an oil phase dispersed in said aqueous phase, based on the weight of said oil phase and said aqueous phase, said oil phase comprising a water-emulsifiable ester oil which has a flash point of at least about 80°C, said ester oil being selected from the group consisting of an ester oil having been obtained from a C1-C5 monocarboxylic acid and a polyfunctional alcohol, and a C1-C36 monocarboxylic acid and a monofunctional alcohol having at least 6 carbon atoms, b. an emulsifier, c. a fluid-loss additive, d. a weighting agent, e. a viscosifier, and f. an alkali reserve component.

2. The water-based, oil-in-water emulsion drilling fluid as in claim 1 wherein said oil phase is present in an amount of from about 8 to about 40% by weight.

3. The water-based, oil-in-water emulsion drilling fluid as in claim 1 or 2 wherein said polyfunctional alcohol contains from 2 to 6 carbon atoms and up to 4 hydroxyl groups.

4. The water-based, oil-in-water emulsion drilling fluid as in claim 3 wherein said polyfunctional alcohol is selected from ethylene glycol, propanediol and glycerol.

5. The water-based, oil-in-water emulsion drilling fluid as in any one of claims 1 to 4 wherein said ester oil has a pour point and setting point of below about 0°C
and a flash point of at least about 100°C.

6. The water-based, oil-in-water emulsion drilling fluid as in any one of claims 1 to 5 wherein said ester oil has a Brookfield (RVT) viscosity of less than about 1 million mPas at about 20°C.

7. The water-based, oil-in-water emulsion drilling fluid as in any one of claims 1 to 6 having a pH of from about 7.2 to about 11.

8. The water-based, oil-in-water emulsion drilling fluid as in any one of claims 1, 2 and 5 to 7 wherein said ester oil is selected from the group consisting of:
(a) an ester of a C1-C5 monocarboxylic acid and a mono- or polyfunctional alcohol wherein said monofunctional alcohol contains at least 6 carbon atoms and said polyfunctional alcohol contains from 2 to 6 carbon atoms, (b) an ester of a C6-C16 aliphatically-saturated monocarboxylic acid and a monofunctional alcohol containing at least 6 carbon atoms, and (c) an ester of a C16-C24 mono- or polyolefinically unsaturated monocarboxylic acid and a monofunctional straight-chain or branched alcohol containing at least 6 carbon atoms.

9. The water-based, oil-in-water emulsion drilling fluid as in claim 8 wherein said component (c) is selected from (C1) an ester containing at least about 45%
by weight of a di- or polyolefinically unsaturated C16-C24 monocarboxylic acid, and (C2) an ester containing less than about 35% by weight of a di- or polyolefinically unsaturated C16-C24 monocarboxylic acid and at least about 60% by weight of mono-olefinically unsaturated C16-C24 monocarboxylic acid.

10. The water-based, oil-in-water emulsion drilling fluid as in any one of claims 1 to 9 containing up to about 10% by weight, based on the weight of said oil phase, of an oleophilic basic amine having-limited solubility in water, said amine being free from aromatic constituents and having at least one long-chain hydrocarbon radical containing 8 to 36 carbon atoms.

11. The water-based, oil-in-water emulsion drilling fluid as in any one of claims 1 to 10 wherein said continuous aqueous phase is selected from fresh water and water containing dissolved or suspended salts.

12. The water-based, oil-in-water emulsion drilling fluid as in any one of claims 1 to 3 and 5 to 11 wherein said ester oil is formed from at least one branched-chain component.

13. A process of developing a source of petroleum, natural gas or water by drilling, comprising drilling said source in the presence of a drilling fluid comprising the water-based, oil-in-water emulsion drilling fluid as in any one of claims 1 to 12.