GB2542656A

GB2542656A - Filter cake treatment

Info

Publication number: GB2542656A
Application number: GB1611383.9A
Authority: GB
Inventors: Atkinson Brian
Original assignee: Calkem (uk) Ltd
Current assignee: Calkem (uk) Ltd
Priority date: 2015-07-01
Filing date: 2016-06-30
Publication date: 2017-03-29
Also published as: GB201611383D0; GB201511557D0

Abstract

A composition for the treatment of filter cake derived from oil-based drilling fluids. The composition comprises an aqueous solution of an amine carboxylate, a surfactant and an oil solubilising co-solvent. In preferred embodiments, the composition comprises about 15% w/w of amine carboxylate, preferably as ammonium formate. The surfactant may be a betaine, a quaternary ammonium compound or an amine oxide. Preferred surfactants include cocoamidopropyl amine oxide and cocoamidopropyl betaine. The oil-solubilising co-solvent may comprise a glycol ether or a C1 to C4 alcohol or diol. A method of using the composition comprises pumping the composition into a bore hole or well bore and leaving the composition for at least 4 hours.

Description

Filter Cake Treatment

The present invention relates to the treatment of filter cake, in particular to compositions for the reduction and removal of filter cake.

Filter cake is the residue which is deposited on permeable medium such as rock when a slurry such as a drilling fluid, is forced against the medium under pressure. This occurs particularly in borehole drilling, especially in the oil and gas exploration industries. Liquid in the slurry passes through the permeable medium as a filtrate, leaving the solid component of the slurry forming a filter cake as a surface to the permeable medium.

There are two types of drilling fluid - oil-based drilling fluids or muds (OBM) and water-based drilling fluids or muds (WBM). OBM have a non-aqueous (and water insoluble) external phase and an internal (dispersed) aqueous phase, whereas WBM employ water (either fresh or as a brine) as the external phase. This allows for the use of OBM in situations where water reactive or water soluble materials are present in the formation to be drilled. OBM mainly comprise a non-aqueous external (continuous) phase (this can be a hydrocarbon such as diesel or paraffinic oil, a poly-a-olefin, an olefin or a fatty acid ester) and an internal (dispersed) phase brine (usually a strong concentration of sodium or calcium chloride). The ratio of external to internal phase can be in the range 100:0 to 50:50 depending upon the fluid requirements. In addition to this, weighting agents (usually barite or calcium carbonate), viscosifiers (usually organophilic clays or synthetic polymers), lime, fluid loss additives (usually asphaltic materials, synthetic polymers or lignitic materials) and emulsifiers (polyamides and fatty acid derivatives) are used to provide the fluid characteristics required. WBM use either fresh water or a brine (usually sodium or potassium chloride but other salts can be used depending upon the fluid requirements), viscosifiers (natural polymers e.g. Starch, guar gum, synthetic polymers e.g. Xanthan gum, poly anionic cellulose), fluid loss additives (starches, poly anionic celluloses, acrylate copolymers etc., ph control (sodium hydroxide, soda ash) and weighting agents (normally barite or calcium carbonate). In addition, a variety of other additives can be employed such as defoamers, corrosion inhibitors, shale stabilisers and so on.

The choice of which fluid to use relies on several factors (composition of formation to be drilled, cost of drilling and environmental). Usually, OBM is used when drilling a formation that contains either reactive or water soluble materials (such as shale or salts). If drilling through water soluble salts, then OBM has to be used. If drilling through shale, it is (sometimes) possible to use WBM by adding a variety of "shale stabilisers" or "shale hydration inhibitors" to the fluid. OBM can offer environmental problems (in disposal) and are generally more expensive than WBM. Thus, WBM are generally preferred and OBM are used where there is a specific technical need.

The differences between filter cakes derived from an OBM or WBM are due to the various chemicals used. The main difference is that filter cakes from OBM contain oil-wetted materials and are insoluble in water whereas those from WBM contain water-wetted materials and are soluble in water. OBM filter cakes comprise oil-wetted solids (weighting agents and organophilic clay), fluid loss additive and oil soluble emulsifiers. WBM filter cakes comprise water-wetted solids (weighting agents and clay) and various polymers (natural or synthetic).

Filter cake may be desirable in certain circumstances, but is often undesirable as it can cause sticking and other drilling or extraction problems. Filter cake can cause formation blocking and blocking of screens in horizontal wells.

Filter cake breaker fluids are well known and typically involve acids, chelating agents, oxidisers or enzyme treatments, or combinations of these compositions. However, these systems, especially acid-based systems can be difficult to control. Additionally, filter cake formed during drilling operations with an oil-based drilling fluid is oil-wetted and these are difficult to remove with conventional surfactant-based materials.

Accordingly, there is a need for alternative filter-cake removal compositions for removal of filter cake derived from oil-based drilling fluids.

In its broadest sense, the present invention provides a composition for removal or dissolution of filter cake derived from oil-based drilling fluids, the composition comprising an amine carboxylate.

Put in other words, the present invention provides a composition for removal of carbonate salts in filter cake from an oil-based drilling fluid, the composition comprising an amine carboxylate.

Preferably, the composition further comprises an aqueous solvent, more preferably water.

Preferably, the composition further comprises a surfactant.

Preferably, the composition further comprises an oil-solubilising co-solvent.

Preferably, the composition further comprises from 10 to 50% w/w of amino carboxylate; more preferably from 10 to 30 % w/w; even more preferably about 15% w/w.

Preferably, the amine group of the amine carboxylate is of formula NR1R2R3 wherein Ri, R2 and R3 are selected from H or Ci to C4 alkyl and may be the same or different.

Preferably, Ri, R2 and R3 are each selected from H, methyl and ethyl and may be the same or different, and may be substituted.

Preferably, the amine group is ammonia, methylamine, dimethylamine, trimethylamine, diaminoethane, diethylene triamine, ethanolamine, diethanolamine or triethanolamine.

Preferably, the carboxylate group of the amine carboxylate is a Ci to C5 carboxylate, more preferably a Ci to C4 carboxylate, even preferably a Ci to C3 carboxylate, most preferably a Ci or C2 carboxylate group.

Preferably, the carboxylate is methanoate, ethanoate, propionoate, malate, lactate, citrate or maleate, more preferably the carboxylate group is a methanoate group.

Preferably, the amine carboxylate is ammonium formate.

Advantageously, the surfactant is an amphoteric surfactant.

Preferably, the surfactant is a betaine, a quaternary ammonium compound or an amine oxide.

In one embodiment, the surfactant is an amine oxide, preferably at least one amine oxide of formula R4RsR6N+-0' or a mixture of thereof wherein R4 is an alkyl group, preferably a C10 to Cis alkyl group.

In an alternative embodiment, the amine oxide is based on at least one starting fatty acid having a C10 to Cis chain and at least one dialkylaminoalkylamine of formula R5R6 N fCH2}n NH2; wherein n is from 1 to 5, preferably from 2 to 4, more preferably 3.

Rs and R6 are Ci to C3, preferably methyl or ethyl, and may be the same or different.

Alternatively, the amine oxide is at least one amine oxide of formula R7CONH-(CH2)n-N+(CH3)2 Ο-, where R7 is C10 to Cis and n=l to 5, preferably 2-5 and more preferably 3.

In certain embodiments, preferably the surfactant is cocoamidopropyl amine oxide.

In alternative embodiments, the surfactant is a quaternary ammonium compound, preferably a trialkyl ammonium halide, optionally substituted, preferably an alkyl dimethyl ammonium halide, a dialkyl methylammonium halide, a benzyl ammonium halide or an alkoxylated quaternary ammonium compound.

In yet further alternative embodiments, the surfactant is advantageously a betaine, preferably a dimethyl amino alkyl betaine or dimethyl amido propyl alkyl betaine.

Preferably, the alkyl group of the alkyl betaine is a Cio to Cig alkyl group.

Preferably, the surfactant is cocoamidopropyl betaine.

Preferably, the composition comprises surfactant in an amount of from 1 to 30% v/v; more preferably from 10 to 25% w/w; even more preferably from 10 to 20% w/w.

Preferably, the oil-solubilising co-solvent comprises at least one glycol ether.

Preferably, the glycol ether is at least one of monoethylene glycol methyl ether (methyloxitol), diethylene glycol methyl ether (methyl dioxitol), triethylene glycol methyl ether (methyl trioxitol), tetraethylene glycol ethyl ether (ethyl tetraoxitol), monoethylene glycol ethyl ether (ethyloxitol), diethylene glycol ethyl ether (ethyl dioxitol), triethylene glycol ethyl ether (ethyl trioxitol), tetraethylene glycol ethyl ether (ethyl tetraoxitol), monoethylene glycol Propyl ether (propyloxitol), diethylene glycol propyl ether (propyl dioxitol), triethylene glycol propyl ether (propyl trioxitol), monoethylene glycol butyl ether (butyloxitol) (butyl cellosolve) and diethylene glycol butyl ether (butyl dioxitol) (butyl carbitol).

Preferably, the composition comprises glycol ether in an amount of from 1 to 30% v/v; preferably from 10 to 25% w/w; more preferably from 10 to 20% w/w.

Typically, the filter cake is a filter cake formed in a bore hole or well bore.

In a further aspect, the present invention also provides the use of an amine carboxylate in the removal of filter cake in a bore hole or well bore. Preferably, the amine carboxylate is an amine carboxylate as defined above.

In a yet further aspect, the present invention also provides a method of reducing formation damage in a filter cake removal process, the method comprising the steps of obtaining a composition as above; pumping the composition into a bore hole or well bore; leaving the composition within the bore hole or well bore for a period of at least about 4 hours, preferably at least about 4 to 5 hours, more preferably at least about 4 to 6 hours.

The above and other aspects of the present invention will now be described in further detail with reference to the following examples.

The present invention seeks to provide a water-based fluid that will remove filter cake left following the use of an oil-based drilling fluid (invert oil emulsion mud). The filter cake formed during drilling operations with an oil-based drilling fluid is oil-wetted and difficult to remove with conventional surfactant-based materials.

Our invention uses a solution of an amine carboxylate, which acts as a delayed-action acid generator, to dissolve carbonate salts present in the filter cake. The solvent is conveniently water, obtainable locally. Accordingly, the water may be fresh water or saltwater. A co-solvent, suitably a glycol ether solvent, aids dissolution of any oil present in the filter cake and an amphoteric or cationic water-soluble surfactant water-wets any solids present. The slow release of acid helps to minimise formation damage that often occurs when strong acids are employed to remove calcium carbonate.

The process for removal of the filter cake is as follows. The composition of the present invention is formulated and spotted into the well bore after drilling has been completed. Whilst the fluid is in the well bore, the oil-wetted solids, mainly consisting of carbonate salts with minor amounts of organophilic clay, emulsifiers and fluid-loss additives from the drilling fluids become water-wetted due to the action of the surfactant. The amine carboxylate salt undergoes a double decomposition reaction with calcium carbonate to yield the amine carbonate. The calcium salt of the carboxylic acid of the amine carboxylate is selected so that the solubility of its calcium salt in water is high.

At the higher temperatures seen in the well bore, the amine carbonate so formed decomposes to form the parent amine, carbon dioxide and water. In this way, the solid calcium carbonate of the filter cake is effectively taken into solution and the solid material of the filter cake removed.

The reaction is a two stage reaction as follows:

CaC03 + 2NR3H+ R'COO" Ca(R'COO)2 + (NR3H)2C03 (i) (NR3H)2C03 2NR3 + C02 + 02 + H20 (ii) R is H or an alkyl group and may be the same or different within an NR3group. R' is H or an alkyl group. Preferably, the alkyl group is a Ci-Ce group, more preferably Ci - C5, and may be linear or branched.

The selection of the appropriate formulation to be used is determined in the laboratory prior to use to ensure good water (or brine) solubility of all the components.

An important factor affecting the selection of an appropriate carboxylic acid is the solubility (in water) of the ammonium and calcium salts of the carboxylic acid. In general, the solubility of the ammonium salt presents no problems and so is not a particular consideration. However, the solubility of the calcium salt can be problematic. The solubility in water of calcium salts of carboxylic acids decreases as the carbon chain length increases. For example, calcium lactate (C3) has a solubility of 8.88 g per 100ml of water, whereas calcium gluconate (0ε) has a solubility of 3 g/lOOml of water. It is important in the inventive process that the calcium salts formed during the process are water soluble, as any insoluble calcium salts could case blockages in either the formation or the screens. For this reason, C1-C5 carboxylic acids are preferred, namely: formic, acetic, propionic, butyric, pentanoic, tartaric, oxaloacetic, malonic, succinic, maleic, fumaric, glutaconic, glyoxylic, glycolic, lactic or citric, preferably lower alkyl, C1-C3, more preferably methanoic or ethanoic.

Similarly, the selection of a suitable amine is based upon the solubility of the amine-carboxylate salts in water and, following the second of the pair of reactions above, of the amine itself in water; as well as the decomposition rate of the amine carboxylate. As is the case with the carboxylic acid, solubility decreases with increasing carbon chain length. For example, propylamine (C3) has a solubility in water of ca 50%, whereas hexylamine (Οβ) is almost insoluble in water. It is important in the process of the invention that the amine released during the final decomposition stage of the process is soluble in water to avoid any problems with formation damage and screen blocking. For this reason, the amine is preferably selected from Co (i.e. ammonia) to C4, namely: methylamine, ethylamine, propylamine, butylamine, monoethanolamine, diethanolamine and triethanolamine.

Any combination of the above listed carboxylic acids and amines can be employed. A preferred amine-carboxylate salt is ammonium formate. Decarboxylation of the resultant ammonium carbonate from the first stage of the reaction occurs at a comparatively low temperature of about 50-60°C. Ammonium formate has a low molecular weight and is also low in price.

Usage concentrations are from 10-50% w/w of amine carboxylate in solvent. A surfactant or mixture of surfactants is advantageously included. The selection of an appropriate surfactant is based on the surfactant's ability to water-wet any solids present in the filter cake (especially calcium carbonate) such that the slowly released acid(s) are able to react (and hence dissolve) any calcium carbonate present. In order to water-wet the solids, the surfactant should be cationic in nature and also be water soluble. Surfactants that meet these two conditions include: betaines, quaternary ammonium compounds and amine oxides. Due to the low environmental impact of betaines and amine oxides, they are preferred in the formulations to be employed.

For dimethyl alkyl amino-based betaines, the preferred products are based on a starting amine of between Cio and Cis, so includes: dimethylamino capryl betaine (Cio), dimethylamino dodecyl betaine (Cn), dimethylamino lauryl betaine (C12), dimethylamino tridecyl betaine (C13), dimethylamino myristyl betaine (C14), dimethylamino pentadecyl betaine (C15), dimethylamino palmityl betaine (Ci6), dimethylamino heptadecyl betaine (C17) and dimethylamino stearyl betaine (Cis).

Naturally or synthetically-occurring mixtures of the above, such as dimethylamino cocobetaine are equally suitable.

For dimethyl amidoalkyl alkyl betaines, the preferred products are based on a starting fatty acid of Cio to Ci8 and dimethyaminopropylamine, thus including: dimethyamidopropyl capryl betaine (Cio), dimethylamidopropyl undecyl betaine (Cn), dimethylamidopropyl lauryl betaine (C12), dimethylamidopropyl tridecyl betaine (C13), dimethylamidopropyl myristyl betaine (C14), dimethylamidopropyl pentadecyl betaine (C15), dimethylamidopropyl palmityl betaine (Cie), dimethylamidopropyl heptadecyl betaine (C17) and dimethylamidopropyl stearyl betaine (Cis).

For dimethyl alkyl amino-based amine oxides, the preferred products are based on a starting amine of between Cio and Cis: dimethylamino capryl amine oxide (Cio), dimethylamino undecyl amine oxide (C11), dimethylamino lauryl amine oxide (C12), dimethylamino tridecyl amine oxide (C13), dimethylamino myristyl amine oxide (C14), dimethylamino pentadecyl amine oxide (C15), dimethylamino palmityl amine oxide (Ci6), dimethylamino heptadecyl amine oxide (C17) and dimethylamino stearyl amine oxide (Cis).

For dimethyl amidoalkyl alkyl amine oxides, the preferred products are again based on a starting fatty acid of C10 to Cis and dimethylaminopropylamine. Thus the list of preferred products is: dimethyamidopropyl capryl amine oxide (C10), dimethylamidopropyl undecyl amine oxide (C11), dimethylamidopropyl lauryl amine oxide (C12), dimethylamidopropyl tridecyl amine oxide (C13), dimethylamidopropyl myristyl amine oxide (C14), dimethylamidopropyl pentadecyl amine oxide (C15), dimethylamidopropyl palmityl amine oxide (Ci6), dimethylamidopropyl heptadecyl amine oxide (C17) and dimethylamidopropyl stearyl amine oxide (Cis).

Mixed betaines are particularly suitable, based on mixed fatty acids. For example, cocoamidopropyl betaine is especially preferred, being derived from cocodimethylamine which, in turn, is derived from coconut oil, a naturally occurring fatty acid source comprising a mixture of capryl, lauryl and myristyl fatty acids. Naturally or synthetically-occurring mixtures are equally suitable. Our preferred surfactants are dimethylamidopropyl coco betaine and dimethylamino coco amine oxide.

Preferred surfactant usage concentrations are from 1 to 30% v/v. A co-solvent is included to solubilise or emulsify any oil present on the filter cake which aids in the dissolution of any acid soluble solids. Preferred co-solvents are those with a high flash point and low environmental toxicity. The co-solvents must also be water soluble and have minimal effect on the solubility of any salts present in the formulations used.

Of the numerous co-solvents available, preferred co-solvents which meet the above criteria are the glycol ethers, derived by the reaction of an alcohol with either ethylene, propylene or butylene oxide. Due to the nature of glycol ethers, they can be precipitated from aqueous solutions as the temperature increases (i.e. they reach a "cloud point" and solutions become "cloudy"). Any precipitation of these materials could interfere with the process as they would coat the solids present in the filter cake and may prevent good dissolution of any such solids. For this reason, glycol ethers are preferably selected from: monoethylene glycol methyl ether (methyloxitol), diethylene glycol methyl ether (methyl dioxitol), triethylene glycol methyl ether (methyl trioxitol), tetraethylene glycol ethyl ether (ethyl tetraoxitol), monoethylene glycol ethyl ether (ethyloxitol), diethylene glycol ethyl ether (ethyl dioxitol), triethylene glycol ethyl ether (ethyl trioxitol), tetraethylene glycol ethyl ether (ethyl tetraoxitol), monoethylene glycol Propyl ether (propyloxitol), diethylene glycol propyl ether (propyl dioxitol), triethylene glycol propyl ether (propyl trioxitol), monoethylene glycol butyl ether (butyloxitol) (butyl cellosolve) and diethylene glycol butyl ether (butyl dioxitol) (butyl carbitol).

Our preferred product is monoethylene glycol butyl ether or diethylene glycol monobutylether. Preferred usage concentrations are from 1 to 30% v/v.

Initial trials of compositions were conducted on a small scale against a lg chip of marble (CaCOs). Various amine carboxylates were tested at a range of temperatures typical of what would be found in a well bore. The results are shown in Table 1.

Table 1

The above results demonstrate that both ammonium formate and di-ammonium citrate are active as acid release agents at higher temperatures. Tri-ammonium citrate has low activity for our purposes as an acid releaser and although mono-ammonium citrate is effective its pH in solution is too low. The low pH would be likely to lead to the same problems as exhibited by mineral acids and may lead to a too rapid dissolution of the filter cake, resulting in formation damage.

As a consequence of these initial trials, ammonium formate was selected for further work. We then conducted trials of compositions based on ammonium formate against a filter cake model having the following composition:

Diesel oil 183ml

Secondary Emulsifier (Calkem S5082) 7.5g Calkem UK Limited

Organophilic clay (Bentone 42) lOg Elementis USA

Lime 2g EMEC Egypt

CaCh aqueous solution 30%w/w 78.4ml EMEC Egypt

Asphaltic fluid loss additive (EMEC TONE) 2.5g EMEC Egypt

CaC03 (99%) 130g EMEC Egypt

The emulsifier is present to produce and stabilise the emulsion formed between the oil and aqueous phases. Lime activates the emulsifier and gives the fluid a high pH. CaCh provides an osmotic pressure in the aqueous phase to inhibit any reactions with shale that may be present in the formation. A fluid loss additive is used to minimise drilling fluid losses to the formation whilst drilling and CaCC>3 is used to provide an increased density to the drilling fluid.

The components were mixed on a Hamilton Beach mixer for 1 hour followed by hot rolling at 65°C for 16 hours. The resultant mixture was subjected to HPHT (High Pressure-High Temperature) filtration at 500psi (34atm/3450kPa) for 30 minutes to produce a filter cake.

Tests were then run by placing a ca 5g portion of filter cake in 100ml of test solution, heating the contents to the prescribed temperature and observing the effects of the test solution on the cake.

The results are shown in Table 2 against test solutions made up in fresh water, and in Table 3 against test solutions made up in brine.

Table 2

Table 3

The results show that combinations of ammonium formate, betaine surfactant and butyl carbitol solvent are efficient in dissolving the filter cake over a (relatively) long period of time (in comparison to more conventional mineral acids). This slow release of acid and slow dissolution of the filter cake should provide minimal formation damage during well treatments. The results also show that the dissolution rate is dependent upon the concentration of the ammonium formate and that the betaine surfactant and butyl carbitol solvent alone do not dissolve the filter cake. However, these components aid in solubilisation of the oil present in the filter cake and thus increase the rate of reaction of the acid with the filtercake. Thus a combination of the three ingredients can be mixed to produce a cleaning solution that can be "tuned" to enable complete dissolution of a filter cake over a known time giving complete removal of oily components (which are emulsified in the process).

Tests using ammonium formate as a single component give complete dissolution of the filter cake at a reduced reaction rate and any oil present is produced as a discrete layer and is not emulsified (showing that the filter cake has not been water wetted).

Example 14

Following these successful laboratory trials, a standard test composition comprising:

Water 50%

Ammonium formate (50% w/w in water) 30%

Cocoamidopropyl betaine (30% w/w in water) 10%

Butyl carbitol 10% was tested in field trials against various mud types in eight newly drilled horizontal oil wells. The wells had been drilled with both water based drilling fluids and oil based drilling fluids.

For field application, the desired formulation was blended in the mud plant to the required volume sufficient to treat the well. The formulation was then spotted into the open hole. The onset time and volume of losses of the fluid was monitored and the results are shown in Table 4.

Table 4

* ppg = lb per gallon; OBM = oihbased mud; WBM = water-based mud

As can be seen from the test results, the onset of losses occurred after a period of 4 to 6 hours at a rate of ca 10 bbl/hour. This shows that the spotted fluid is slowly leaking into the pore volume in the interior of the formation and that slow penetration and gradual removal (dissolution) of the filter cake is occurring. This, in fact, is beneficial as the fluid will contain some unreacted amino carboxylate which will further react with any calcium carbonate in the formation which will serve to increase the pore dimension in the near well bore zone leading to improved production rates (which, in practise is observed).

The trials showed that a delayed dissolution of the filter cake occurs, with the onset of dissolution occurring 4-5 hours after addition of the inventive composition. Following treatment of the well bore with our composition, flow of oil from the well was ca 3500 barrels per day, which is about twice as much as expected. No downhole screen fouling was observed even after six months, showing negligible formation damage.

Claims

Claims:

1. A composition for treatment of filter cake derived from oil-based drilling fluids, the composition comprising an aqueous solution of an amine carboxylate; a surfactant and an oil-solubilising co-solvent.

2. A composition as claimed in claim 1, comprising from 10 to 50% w/w of amino carboxylate; preferably from 10 to 30 % w/w; more preferably about 15% w/w.

3. A composition as claimed in claim 1 or 2, wherein the amine group of the amine carboxylate is of formula NR1R2R3 wherein Ri, R2 and R3 are selected from H or Ci to C4 alkyl and may be the same or different.

4. A composition as claimed in claim 3, wherein Ri, R2 and R3 are each selected from H, methyl and ethyl and may be the same or different, and may be substituted.

5. A composition as claimed in claim 4, wherein the amine group is ammonia, methylamine, dimethylamine, trimethylamine, diaminoethane, diethylene triamine, ethanolamine, diethanolamine or triethanolamine.

6. A composition as claimed in any preceding claim, wherein the carboxylate group of the amine carboxylate is a Ci to C5 carboxylate.

7. A composition as claimed in claim 6, wherein the carboxylate group is a Ci to C4 carboxylate, preferably a Ci to C3 carboxylate, more preferably a Ci or C2 carboxylate group.

8. A composition as claimed in claim 7, wherein the carboxylate is methanoate, ethanoate, propionoate, malate, lactate, citrate or maleate.

9. A composition as claimed in claim 7, wherein the carboxylate group is a methanoate group.

10. A composition as claimed in any preceding claim, wherein the amine carboxylate is ammonium formate.

11. A composition as claimed in any preceding claim, wherein the surfactant is a cationic surfactant, preferably an amphoteric surfactant.

12. A composition as claimed in any preceding claim, wherein the surfactant is a betaine, a quaternary ammonium compound or an amine oxide.

13. A composition as claimed in claim 12, wherein the surfactant is an amine oxide, preferably at least one amine oxide of formula F^RsRelNT-O' wherein R4 is an alkyl group, preferably a C10 to Cis alkyl group.

14. A composition as claimed in claim 12, wherein the amine oxide is based on a starting fatty acid having a CIO to C18 chain and at least one dialkylaminoalkylamine of formula R5R6 N {Chhl· Nhh; wherein n is from 1 to 5, preferably from 2 to 4, more preferably 3.

15. A composition as claimed in claim 13 or claim 14, wherein R5 and R6 are Ci to C3, preferably methyl or ethyl, and may be the same or different.

16. A composition as claimed in claim 12, wherein the amine oxide is at least one amine oxide of formula R7CONH-(CH2)n-N+(CH3)2 O', where R7 is Cioto Cis and n=l to 5, preferably 2-5 and more preferably 3.

17. A composition as claimed claim 12, wherein the surfactant is cocoamidopropyl amine oxide.

18. A composition as claimed in claim 12, wherein the surfactant is a quaternary ammonium compound, preferably a trialkyl ammonium halide, optionally substituted, preferably an alkyl dimethyl ammonium halide, a dialkyl methylammonium halide, a benzyl ammonium halide or an alkoxylated quaternary ammonium compound.

19. A composition as claimed in claim 12, wherein the surfactant is a betaine, preferably a dimethyl amino alkyl betaine or dimethyl amido propyl alkyl betaine.

20. A composition as claimed in claim 19, wherein the alkyl group of the alkyl betaine is a Cio to Ci8 alkyl group.

21. A composition as claimed in claim 19 or claim 20, wherein the surfactant is cocoamidopropyl betaine.

22. A composition as claimed in any preceding claim, comprising surfactant in an amount of from 1 to 30% v/v; preferably from 10 to 25% w/w; more preferably from 10 to 20% w/w.

23. A composition as claimed in any preceding claim, wherein the co-solvent is based on a Ci to C6 alcohol or polyol, preferably a Ci to C4 alcohol or diol.

24. A composition as claimed in any one of claims 1 to 22, wherein the oil-solubilising co-solvent comprises at least one glycol ether.

25. A composition as claimed in claim 24, wherein the glycol ether is at least one of monoethylene glycol methyl ether (methyloxitol), diethylene glycol methyl ether (methyl dioxitol), triethylene glycol methyl ether (methyl trioxitol), tetraethylene glycol ethyl ether (ethyl tetraoxitol), monoethylene glycol ethyl ether (ethyloxitol), diethylene glycol ethyl ether (ethyl dioxitol), triethylene glycol ethyl ether (ethyl trioxitol), tetraethylene glycol ethyl ether (ethyl tetraoxitol), monoethylene glycol Propyl ether (propyloxitol), diethylene glycol propyl ether (propyl dioxitol), triethylene glycol propyl ether (propyl trioxitol), monoethylene glycol butyl ether (butyloxitol) (butyl cellosolve) and diethylene glycol butyl ether (butyl dioxitol) (butyl carbitol).

26. A composition as claimed in any one of claims 23 to 25, comprising co-solvent in an amount of from 1 to 30% v/v; preferably from 10 to 25% w/w; more preferably from 10 to 20% w/w.

27. A composition as claimed in any preceding claim, wherein the filter cake is a filter cake formed in a bore hole or well bore.

28. Use of an amine carboxylate in the treatment of filter cake derived from oil-based drilling fluids in a bore hole or well bore.

29. Use as claimed in claim 28, wherein the amine carboxylate is an amine carboxylate as defined in any one of claims 3 to 27.

30. A method of reducing formation damage in a filter cake removal process, the method comprising the steps of preparing a composition as claimed in any one of claims 1 to 27; pumping the composition into a bore hole or well bore; leaving the composition within the bore hole or well bore for a period of at least about 4 hours, preferably at least about 4 to 5 hours, more preferably at least about 4 to 6 hours.