WO2006056774A2 - Emulsifier-free wellbore fluid - Google Patents

Abstract

A wellbore fluid is described having a continuous non-aqueous phase and inorganic particles wherein said particles are coated with essentially permanently adsorbed amphiphilic molecules causing a lipophilic behavior of said particles under operating conditions.

Description

Emulsifier-free Wellbore Fluid

This invention relates to wellbore fluids comprising a continuous non-aqueous liquid phase having dispersed therein inorganic powder particles.

BACKGROUND OF THE INVENTION

The wellbore fluids of this invention are for use in the construction or repair of oil, gas, injection, water or geothermal wells. Thus the term "wellbore fluids" encompasses drilling fluids ("muds") , completion fluids, workover fluids, packer fluids, and the fluids used in remedial treatments. The main function of the particles dispersed in the wellbore fluid is to increase the fluid's density, although inorganic powders are contemplated that serve other functions in wellbore fluids such as the bridging solids that are used to seal pores or fractures in subterranean rock formations. The terms "oil-based fluid" or "oil based mud" as used herein are meant to refer to any wellbore fluids with a continuous non-aqueous liquid phase.

Drilling muds are used to carry rock cuttings out of the wellbore and to the surface. Other functions of drilling muds are to cool and lubricate the drill bit, protect against blowouts by counteracting downhole formation pressure, maintain a stable borehole and to prevent loss of fluids to the formations drilled.

Both water-based and oil-based muds are used as drilling muds. Water-based muds are generally cheaper and less toxic than those based on oil but the latter possess many operational advantages particularly for the drilling of high angle, long reach and high pressure/high temperature wells. As in an increasing number of jurisdictions cutting disposal is strictly regulated, the advantages of water-based mud are slowly diminishing. However conventional oil-based muds (OBMs) also suffer from a number of undesirable characteristics. Oil may be retained on the drilled cuttings which has environmental implications. In addition the presence of emulsifiers and other oil wetting agents which are essential components of conventional OBMs can alter the wettability of reservoir formations thereby reducing their permeability to oil.

Emulsifiers and oil wetting agents are added to conventional OBMs to emulsify the water phase in the oil phase and to ensure that all of the solids in the mud are wetted by the oil . Oil- based wellbore fluids are, almost without exception, formulated using comparatively low molecular weight emulsifiers and oil- wetting agents . Normally a brine phase is dispersed in the continuous oil phase as a water/oil (W/0) emulsion using emulsifiers such as oleic acid or tall oil fatty acids, usually in the form of their calcium soaps. Alternative W/0 emulsifiers that are commonly used include amidoamines or imidazolines manufactured by the condensation of fatty acids such as those mentioned above with a polyalkyleneamine such as triethylenetetramine or tetraethylenepentamine. The alkylamidoamines or alkylimidazolines can be further reacted with, for instance, maleic anhydride in order to introduce carboxylic acid functionality to these molecules. Supplementary oil-wetting agents are also used such as lecithin or the calcium soap of dodecylbenzenesulphonic acid.

A water/oil (W/0) emulsifier is normally added even when an oil- based wellbore fluid is formulated without a dispersed brine phase to counter the inadvertent contamination of the fluid by water such as a subterranean brine flow.

The emulsifier can, however, damage the reservoir. For example, the majority of formations drilled are naturally water-wet but when contacted by the emulsifiers present in OBMs can easily be changed to oil-wet. The ability to eliminate such emulsifiers would allow cuttings and reservoir formations drilled using OBMs to remain in their natural, water-wet state. This would lead to both lower oil retention and to smaller reductions in permeability- . . .. .

Unfortunately it is important that the weighting agent of the wellbore fluid remains oil wet. This is especially true for the surface of barite particles such as the barium sulphate powder that is normally used to increase the density of the wellbore fluid. Oil-wetting agents must be added to disperse the weighting agent in the oil and to prevent the particles from becoming water-wet. If the weighting agent becomes water-wet, the particles will aggregate and clump together, causing rapid separation from the fluid and a potentially dangerous loss of density and hydrostatic head.

The U.S. patent no. 5,376,629 discloses oil based drilling fluids free of emulsifiers or oil-wetting agents, the muds comprising a weighting agent having a siloxane or silane coating thereon. The objectives were similar to some of those of the present invention. For example avoiding the conversion from water-wet to oil-wet of producing formation rock surfaces leads to the maintenance of optimal permeability to oil .

The basic principle of 5,376,629 is to coat a hydrophobic layer of a silane or a siloxane onto weighting agent particles, the layer then being immobilised by the polymerisation and cross- linking of the silane or siloxane brought about by hydrolysis by any moisture present and/or by a heating step. The preferred weighting agent was barite. This type of hydrophobised barite suffered from several disadvantages:

- The polysiloxane layer on the surface of the barite could be dislodged by exposure of the drilling fluid to high-shear rates such as are encountered in mud pumps or when the fluid passes through bit nozzles. In contrast to their chemisorption onto hydroxylated mineral surfaces such as silica, silanes adsorb less strongly onto non-hydroxylated surfaces such as barium sulphate. Barite has a low energy surface with no tendency to form covalent bonds of the type X-O-Si where X is the solid substrate surface. Therefore the polymerised oil-wetting siloxane layer around each particle was not strongly adsorbed and could be "chipped off" by inter-particle collisions at high shear rates. This allowed partial water-wetting of the barite with consequent suspension instability.

- Silanes and siloxanes are relatively expensive.

- There could be hazards during the formation of the polysiloxane coating due to hydrochloric acid or methanol released from chlorosilanes or methoxysilanes (respectively) .

- The silane coating of the particles necessitated a factory- based process consisting of the addition of the silane to a dry mix of the powder in air at high shear rates, and a heating step at 12O⁰C to cross-link the silane coating.

The U.S. patent 6,017,854 describes an attempt to formulate simplified non-aqueous drilling fluids where the properties of viscosity / suspending ability and fluid loss control are obtained from a hydrophobic polystyrene-polyolefin-polystyrene block copolymer that has not been further functionalised to contain polar adsorptive groups. An example of such absent functional groups is the dicarboxylic acid grouping that could have been introduced by grafting this polymer with maleic anhydride.

The oil-based fluids described in this patent do not contain an additive that is designed to maintain the weighting agent in an oil-wet condition. In fact the importance of the oil wetness of the weighting agent is not appreciated. In Example 1 of the patent the simplified drilling fluid contains only oil, a non- polar styrene-ethylene/butylene-styrene block copolymer, barite powder and REV DUST (a powdered mineral designed to simulate drilled solids) . If this formulation were to be used in a well a serious problem of water-wetting of the solid particles would be anticipated. A certain amount water contamination is almost inevitable in any drilling or remedial operation. This would water-wet the barite causing potentially dangerous aggregation and separation of the particles.

The U.S. patent no. 4,776,966 discloses the use of certain block or graft copolymers in oil-based drilling fluids as emulsifying agents for brine in oil. The advantages of amphiphilic block copolymers as agents for dispersing solid particulate matter in the oil-based drilling fluid are not mentioned or appreciated.

The dispersing agent claimed by this patent is a surfactant containing a hydrocarbon chain having 30—500 carbon atoms as the hydrophobic component, and a polar component (which is not a polymeric block) that adsorbs on to the particulate solid surface.

In view of the above, the invention aims at solving the many problems exacerbated by the relatively low molecular weight mobile emulsifiers and oil-wetting agents that are present in conventional non-aqueous wellbore fluids, particularly oil-based muds (OBM) . These problems include:

- Oil-wetting of a producing formation can be caused by emulsifiers and oil-wetting agents (hereafter "surfactants") dissolved in the mud filtrate that penetrates the permeable rock such as sandstone or limestone. A change of wettability to oil- wet results in reduction in the permeability of the rock to oil and thus reduced production rates. - On occasions surfactant-induced emulsification of formation water with the filtrate can cause an "emulsion block" that restricts production.

- Surfactant-induced oil wetting of the wellbore wall and steel casing frequently results in poor bonding of the cement pumped into the annulus between the casing and the wellbore wall.

- Cuttings produced at a drilling bit often consist of water-wet shale. The oil-wetting surfactants cause penetration of the oil into porous shale, especially along planes of weakness. The surfactants thus tend to disperse the cuttings into particles small enough that they cannot be easily removed. This surfactant-induced dispersion leads to a build up of undesirable "low-gravity solids" (LGS) and high viscosity. The only remedy with conventional OBM is to dilute the mud with oil leading to the unwanted generation of extra fluid volume. The build up of LGS presents difficulties that inhibit the multiple re-use of the OBM.

- An OBM sometimes becomes contaminated by quantities of water or brine, usually as a result of a subterranean water flow. This can happen when the drilling operation is being run underbalanced and there is a need to separate the produced fluid from the drilling fluid, . In a conventional oil-based fluid, the mobile surfactants attempt to incorporate into the oil phase a large volume fraction of the contaminating water as emulsified droplets, resulting in a high viscosity sludge or slop. It is difficult to separate the phases for re-use or disposal. Again oil dilution and unwanted volume increase is required to restore the fluid's properties.

- Because the surfactants in conventional OBM are mobile and can relocate onto new surfaces (such as a water flow emulsified into the mud) , the weighting agent (usually powdered barite) can lose some of its oil wetting surfactant layer and become water-wet. This is a difficult and dangerous situation because the water- wet barite rapidly agglomerates ("flips") and separates from the fluid. This can result in a loss of hydrostatic pressure downhole and a blowout or wellbore collapse.

- In conventional OBM the surfactants have a deleterious thinning effect on the viscosity and gels conferred by the addition of organically modified clays. This effect is especially apparent after high temperature exposure when the viscosity at low shear rate and gel strength can substantially disappear. This can result in potentially dangerous sedimentation of the weighting agent.

- The mobility of OBM surfactants allows them to partition to some extent into water when the fluid is discharged (for instance as contamination adhering to cuttings) into a body of water such as the sea. Surfactants are usually quite toxic towards marine flora and fauna, and are a major contributor to the poor acute toxicity results of conventional oil-based fluids towards test organisms such as Skeletonema Costatum, a marine alga.

- It is quite difficult to remove the oil-based drilling fluid adhering to rock cuttings after they are separated using a shaker screen. The oil-wetting surfactants cause not only adhesion of the fluid to the cuttings' surface but also imbibition of oil into porous rock cuttings. Attempts to use cuttings washing techniques to reduce the discharge of oil to the environment have not been very successful for the above reasons. It has often been necessary to resort to expensive and potentially dangerous high temperature distillation techniques to clean the cuttings so that they are fit for disposal.

It is believed that surfactants, and the lime required to activate many of them, contribute strongly towards the human skin irritation frequently associated with OBM. Clearly there is a real need to address these problems that are largely caused by the mobility of the surfactants used.

SUMMARY OF THE INVENTION

In general terms, the present invention provides a wellbore fluid having a continuous non-aqueous phase and inorganic particles wherein said particles are coated with an adsorbed immobile layer giving the particle a lipophilic character.

This invention relates to wellbore fluids comprising a continuous non-aqueous liquid phase having dispersed therein inorganic powder particles that are coated with an adsorbed immobile lipophilic layer which is resistant to attrition, desorption and dissolution, particularly over a long period of use in wellbore operations such as drilling, completion, packer fluid placement, well repair or remedial treatments.

The oil-wetting components of the immobile anchored oil-wetting layer on the dispersed inorganic powder particles cannot significantly relocate to adsorb onto other surfaces such as cuttings (or smaller particulate drilled solids), the producing formation or tubular steel goods, and cannot partition into a watercourse and thence adsorb onto marine flora or fauna.

In this way the wellbore fluids of this invention overcome or minimise the problems associated with conventional emulsifiers and oil-wetting agents.

Thus the problems of formation damage caused by wettability change or emulsion blocking, poor cement bonding, surfactant- induced dispersion of drilled solids, inability to separate water contamination, poor rheological properties and stability at high temperature, toxicity to freshwater or marine species, difficulty in washing cuttings, and skin irritation can all be overcome or minimised by the wellbore fluids of this invention. The main function of the coated particles dispersed in the wellbore fluid is to increase the fluid's density, although similarly coated inorganic powders are contemplated that serve other functions in wellbore fluids such as the bridging solids that are employed to seal the entrances to pores or fractures in subterranean rock formations .

These inorganic particles have preferably a weight average particle diameter of less than about 10 microns, preferably less than about 5 microns, and preferably less than 3 or 2 microns.

Preferably the wellbore fluid is substantially free of mobile emulsifiers or oil-wetting agents that are able to migrate through the oil phase to stabilise water droplets or oil-wet new surfaces such as freshly exposed rock formations .

The placement of the oil-wettable layer on the particles is achieved by using molecules or polymers that can be characterised as being amphiphilic. The amphiphilic coating agent has thus at least one, preferably two or more polar sections and one or more lipophilic sections. Sections as defined herein are parts or groups within the molecule or polymer that provide functionality. In particular, polar section are sections, more preferably terminal sections, of the molecule that include a polarized bond.

The polar section or sections effect the adsorption of the coating onto the particle surface. The lipophilic sections form what can be defined as the coating or shell around the particle thus determining the character of its interactions with the surrounding chemical environment. The lipophilic sections are preferably hydrocarbyl in nature, i.e., have a carbon backbone chain.

The adsorbed immobile layer may be a monolayer. In preferred embodiments of the invention immobilisation is effected using two possible methods:

1. The oil wetting agent is an amphiphilic block copolymer having alternating lipophilic blocks and hydrophilic blocks that contain adsorptive polar functional groups such as carboxylate, sulphate ester, sulphonate, phosphate ester, phosphonate and polyoxyethylene groupings. The strong adsorption and occlusion of the polar block (s) allows the placement and immobilisation of the oil-wettable layer to occur spontaneously and concurrently.

2. The oil-wetting agent is a molecule that comprises at least one polar adsorptive group chosen for its strong adsorption onto the inorganic powder in question, at least one lipophilic group that is predominantly hydrocarbyl in nature, and at least one reactive group capable of forming chemical links to at least one neighbouring molecule. The adsorbed oil-wetting coating around each particle is immobilised in a separate step by cross-linking between neighbouring molecules via the action of a suitable cross-linking agent, catalyst, or an energy or radiation source

A combination of these two variants is possible where the amphiphilic block copolymer comprises also reactive groups that are capable of forming cross-links to one or more neighbouring adsorbed block copolymer molecules.

The adsorption and immobilisation process is based predominantly not on the establishing of a non-polar covalent chemical bond between the substrate or the surface of the particle and the amphiphilic agent. Hence, it is different in character from silane or siloxane based bonds. Instead, the adsorption process of the present invention is a surfactant or ligand adsorption based predominantly on electro-static interactions, dentate bonding, or, in rarer cases, on hydrogen bonds between the particle surface and the amphiphilic molecule. Preferably the polar group effecting the immobilisation is directly linked to a carbon atom which in turn is preferably linked to the ^est of the aitiphiphilic molecule through further carbon-carbon links.

The oil-wettable coating is applied to the surfaces of the inorganic particles, either during or after any grinding that may be required. The coating can be placed onto the surface of the inorganic particles either in a dry powder process, or in a process where the particles are suspended in a liquid. The liquid can be aqueous, or a non-aqueous liquid that is compatible with the non-aqueous liquids that are suitable as the base fluid of the wellbore fluid.

In the variant of a coating process performed in a water phase, after adsorbing and immobilising the coating, the water must be removed from the product prior to use. When the inorganic particles are coated with a hydrophobic layer they tend to flocculate because of hydrophobic attraction between the particles. This facilitates the separation of the coated material by filtration. The wet filter cake may then be dried and milled to give a dry powder product.

An advantage of dry powder products is that they can be added to any non-aqueous base fluid that is suitable for the wellbore fluid, enabling a wide choice of base fluids without any non- aqueous fluid contamination. The choice of base fluid is usually made to comply with local environmental restrictions, Pour Point and Flash Point properties to match expected conditions, and thermal or hydrolytic stability of the base fluid for high temperature applications .

In the variant of the invention where there remains in the coated product an amount of non-adsorbed or non-cross-linked oil-wetting agent, this may readily be removed using adsorbent materials that are chosen for their affinity for the wetting agent. For instance free fatty acids or carboxylated polymers may readily be removed using macroporous basic magnesium oxide granules slurried with the product. After a period of exposure the granules can readily be removed along with the excess oil- wetting agent by, for instance, screening. The adsorbed and immobilised layer on the particles remains intact. This process could be performed either before or during use in a wellbore fluid.

In the variant of a cross-linkable oil-wettable coating, a cross-linkable oil-wetting agent, hereinafter also referred to as CLOWA, is adsorbed onto the surface of the mineral particles.

Having formed a closely packed oil-wettable layer on the surface, the CLOWA molecules are caused to inter-link by the introduction of a suitable cross-linking agent, catalyst, or an energy or radiation source. The hydrophobic coating becomes not only a continuous cross-linked layer, but also remains strongly adsorbed onto the particles via the polar anchor groups. In this way the desired resistance to attrition, desorption and dissolution is achieved.

The cross-linkable oil-wetting agents comprise at least one hydrophobic alkyl, aryl, or alkylaryl hydrocarbyl grouping having at least 4 carbon atoms, at least one polar group capable of strong adsorption onto the inorganic particles, and at least one group capable of forming chemical links to at least one neighbouring CLOWA molecule via the action of a suitable cross- linking agent, catalyst, or an energy or radiation source.

In non-aqueous fluids, by virtue of the amphiphilic nature of the CLOWA, any excess not adsorbed will assemble to form inverse micelles, the polar groups being concentrated at the core of the micelles surrounded by a shell of hydrophobic moieties. These CLOWA micelles can themselves be cross-linked by the action of the suitable cross-linking agent, catalyst, or an energy or radiation source. The CLOWA molecules thus become "fixed" in the micelle with the polar groups hidden. They cannot therefore readily relocate to adsorb onto new surfaces and the advantages of this invention can be maintained. Similarly, if an invert emulsion of a dispersed aqueous phase is formed using a CLOWA as the emulsifier prior to cross-linking, the adsorbed CLOWA film around the emulsion droplets can also then be immobilised during the cross-linking step. This allows the use of an invert emulsion whilst maintaining a fluid that is substantially free of unbound and active surfactants.

In any event for reasons of economy the amount of CLOWA in excess can be minimised by ascertaining by simple tests the least dose required for efficient particle coating for the materials in question.

Any remaining excess non-adsorbed / non cross-linked CLOWA would be quickly removed by adsorption onto new surfaces during wellbore fluid applications. For instance, during drilling, fresh mineral surfaces like cuttings or drilled solids would adsorb excess CLOWA without however depleting the cross-linked layer on the inorganic weight material surfaces. This immobilised layer shows very little tendency to desorb by virtue of its multiply linked nature and its multiple anchor points. The advantages of having no free or mobile emulsifiers/wetting agents can thus be realised.

Optionally, specific adsorbents can be employed to "scavenge" any "free" CLOWA, either prior to application in the wellbore fluid or during use.

Free amphiphilic block co-polymers in accordance with the first variant of the invention can be removed when necessary using similar methods.

It is hence a advantage of the present invention to provide a wellbore fluid that is either inherently or with minimal modification or treatment substantially free of reactive emulsifiers or dispersants, such that only a very limited amount of damaging emulsifiers or dispersants are present in the fluid at its place of application.

It is envisaged that the lipophilic mineral particles of this invention will have applications outside those in wellbore fluids.

These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a chemical system in accordance with a first example of the invention;

FIG. 2 is a schematic illustration of a chemical system in accordance with a second example of the invention; and

FIG. 3 illustrates a chemical structure suitable for the purpose of the present invention.

EXAMPLES(S) OF THE INVENTION

A range of different chemical species can be used for the purpose of this invention. The compounds which can lead to a spontaneous formation of a resilient coating as envisaged by the first variant of the invention as described above are termed amphiphilic block copolymers. The chemistries which include crosslinking are further below described in the section "Cross- linkable Oil-Wetting Agents" together with suitable crosslinkers . A further section describes potential compositions for wellbore fluids followed by examples of the above. The term "molecular weight" where used herein means weight average molecular weight.

Amphiphilic Block Copolymers

In the following description of the hydrophilic and lipophilic blocks the use of the term "block" in the singular is not meant to imply that a plurality of such blocks may not be present, and vice versa. Suitable amphiphilic block copolymers may be di- block or tri-block or greater.

In the example of FIG. 1, there is shown a particle 10 surrounded by amphiphilic block copolymers 11 with the schematic structure L-H₃, with L denoting the lipophilic blocks and H the hydrophilic blocks. The shown tri-block of hydrophilic blocks H are chosen such that they adsorb strongly and spontaneously onto the inorganic particle 10. In its absorbed state the lipophilic tails L of amphiphilic block copolymers 11 give the particle a liphophilic character.

Preferably the lipophilic block is predominantly hydrocarbyl • in nature. But not all hydrophobic blocks are suitable lipophilic blocks . For instance polyalkylene oxide hydrophobes such as polypropylene oxide are insufficiently lipophilic and do not confer the desired properties to the wellbore fluids. Therefore products from the well-known large range of EO/PO block copolymers are not suitable. Although hydrophobic, polydimethylsiloxane block segments are not preferred, and they are not as lipophilic as hydrocarbyl segments.

Typically suitable hydrophobic blocks have a molecular weight in the range 400 - 40,000, preferably 1,000 to 20,000. They could be comprised of polymers of ethylene, propylene, butadiene, isoprene, styrene, alphaolefins, or any random copolymer of the foregoing monomers. Homopolymers or copolymers of vinylic ester or amide monomers having a hydrocarbyl grouping of at least 4 carbon atoms (such as lauryl methacrylate and N- dodecylacrylamide) are also suitable. Optionally the block copolymer also contains reactive groups that can form cross¬ links with neighbouring adsorbed block copolymer molecules.

As shown in FIG. 1, the hydrophilic block comprises any hydrophilic polymeric segment having sufficient polar anchor groups H to adsorb strongly onto the inorganic particles 10. The nature of the anchor groups H should be chosen for their strong adsorption on to the substrate concerned. For instance polymers formed from monomers bearing carboxylate, sulphate ester, sulphonate, phosphate ester, phosphonate and polyoxyethylene groupings adsorb well onto barium sulphate particles by virtue of their coordination with barium atoms at the surface of the particle. Amino- or quaternary ammonium anchor groups are better suited to, for example, silica particles.

The hydrophilic block can be polyethylene oxide having a molecular weight of at least about 260 and an upper molecular weight limit consistent with the Hydrophobic-Lipophilic Balance (HLB) of the block copolymer being less than about 12, and preferably less than about 10. Thus for an polyethylene-jbloα/c- poly (ethylene oxide) di-block copolymer having an ethylene block of molecular weight 10,000, an HLB of about 10 or less is obtained if the molecular weight of the polyethylene oxide block is restricted to a maximum of about 10,000.

The HLB system or classification of non-ionic surfactants or dispersants is a semi-empirical method to predict what type of surfactant properties a molecular structure will provide.

It was introduced by William C. Griffin in 1949 and 1954. The HLB system is based on the concept that some molecules have hydrophilic groups, other molecules have lipophilic groups, and some have both. Weight percentage of each type of group on a molecule or in a mixture predicts what behavior the molecular structure will exhibit. The HLB is calculated forming the percentage of molecular weight of the hydrophilic portion of the molecule and dividing this percentage by a scaling factor to keep the range of HLB numbers small. Using 5 as this scaling factor the range of workable surfactants becomes 0.5 to 19.5 Water-in-oil emulsifiers have a low HLB numbers, typically around 4. Solubilising agents have high HLB numbers. Oil-in- water emulsifiers have intermediate to high HLB numbers.

Preferably, where the inorganic particles are barium sulphate the hydrophilic block comprises carboxylate or sulphonate groups. For instance polyacrylic acid or poly(styrenesulphonic acid) blocks (or their neutralised salts) can be used. Again the molecular weight of the hydrophilic block is restricted such that the HLB of the copolymer is below about 12, preferably below about 10.

Amphiphilic block copolymers can be difficult and expensive to synthesise. It can be more convenient and cheaper to graft polar groups on to a preformed non-polar block copolymer. For instance the commercial tri-block copolymers styrene-ethylene/butylene- styrene (SEBS) , styrene-isoprene-styrene (SIS) , and styrene- butadiene-styrene (SBS) can be grafted with maleic anhydride (MA) . Then after hydrolysis a plurality of dicarboxylic acid groups are introduced which are very effective anchor groups for barium sulphate. The degree of grafting is restricted so that the HLB of the product does not exceed 12, and preferably does not exceed 10.

In the case of MA-grafted SIS and SBS, after adsorption of the block copolymer onto the substrate, the residual double bonds of the neighbouring polyisoprene or polybutadiene chains will be in close proximity. This allows them to be cross-linked by, for instance, a free-radical process. The strength and resistance to desorption of the adsorbed layer can in this way be further enhanced. Alternatively, certain groups in a preformed non-polar block copolymer can be chemically modified to become polar groups. For instance some of the styrene groups in SEBS can be readily sulphonated leaving the random ethylene/butylene copolymer block untouched. The degree of sulphonation is restricted such that the HLB of the product is less than 12, preferably less than 10.

Cross-linkable Oil-Wetting Agents

In the example of FIG. 2, there is shown a particle 20 surrounded by cross-linkable oil-wetting agents 21 with the schematic structure L-X-H, with L denoting the lipophilic sections, H the polar anchor group and X a cross-linkable group of the agent. The shown hydrophilic groups H are chosen such that they adsorb strongly and spontaneously onto the inorganic particle 20. In its absorbed state the lipophilic tails L of amphiphilic block copolymers 21 give the particle a lipophilic character.

To obtain a cross-linked oil-wettable coating, a cross-linkable oil-wetting agent (CLOWA) is first adsorbed onto the surface of the inorganic particles. Then the adsorbed layer is cross-linked via the action of a suitable cross-linking agent, catalyst, or an energy or radiation source on section X.

The cross-linked lipophilic coating L around each particle can be envisaged as a protruding hydrocarbyl chain network that is both anchored to the particle at many locations and cross-linked at many locations. The oil-wetting agent is thus not able to desorb, nor oil-wet fresh surfaces, nor emulsify added water.

The cross-linkable oil-wetting agent comprises at least one lipophilic alkyl, aryl, or alkylaryl grouping having at least 4, and preferably at least 8 carbon atoms, and at least one polar group capable of strong adsorption onto the inorganic particles. For example, strong adsorption onto barium sulphate is provided by carboxylate, phosphate ester, phosphonate, sulphate ester, sulphonate and polyoxyethylene groups. Other groups may be more suitable for other minerals such as silica for which anchor groups containing an amine or quaternary ammonium group are more effective.

The CLOWA further comprises at least one group capable of forming cross-links to one or more neighbouring adsorbed CLOWA molecules via the action of a suitable cross-linking agent, catalyst, or an energy or radiation source. These cross-linkable groups can be vinyl, allyl or other unsaturated groups capable of forming cross-links via, for instance, a free-radical process. They can be polyunsaturated groups present in the hydrocarbyl chain of a fatty acid such as linoleic acid capable of forming cross-links during an air-blowing process.

The cross-linkable groups can be amino groups capable of forming cross-links with an aldehyde linking agent such as glutaraldehyde, or with an amide-forming cross-linking agent such as a polyanhydride, or with an epoxy-bearing molecule to provide epoxide cross-links.

In a reversal of the foregoing the cross-linkable groups on the CLOWA can be aldehyde, anhydride, or epoxy groups which will react with the appropriate cross-linking agents, preferably polyfunctional cross-linking agents. Hydroxide groups on the CLOWA can be cross-linked using low molecular weight silanes such as trimethoxypropyl silane, or with titanate and zirconate esters, or by using isocyanate-bearing cross-linking agents forming urethane links.

In the case of a CLOWA bearing a single cross-linkable group it is preferred to utilise a polyfunctional cross-linking agent in order to achieve sufficient extended linkage of the CLOWA molecules to achieve effective immobilisation of the adsorbed layer. Linoleic acid and linolenic acid, including their metal or ammonium salts, act as effective and affordable CLOWAs. After a layer of the fatty acid has been adsorbed onto, for example, the surface of a barium sulphate particle, the protruding hydrophobic tails and the multiple double bonds will be in close proximity. Linking of the double bonds can be brought about by introducing a free radical generating compound or a Lewis Acid catalyst, or preferably by air blowing a suspension of the coated particles, optionally in the presence of a transition metal catalyst such as a cobalt compound.

Other possible crosslinking methods for fatty acids include sulphur cures & other vulcanisation processes (Sulphur monochloride) .

A preferred polyunsaturated fatty acid is dehydrated castor oil fatty acid. It is commercially available at greater than 90% purity and is suitable for air-blown immobilised coatings. Dedico™ 5981 (ex Uniqema) contains over 90% linoleic acid and over 60% is conjugated linoleic acid (which is more reactive) . A small proportion of this material consists of non-crosslinkable fatty acids. After the coating is "cured" by air blowing the non-crosslinkable free fatty acids can be adsorbed (for instance) onto basic magnesium oxide granules . These can then be removed (along with the free fatty acids) by sieving.

Other suitable CLOWAs include the maleinised polybutadiene and methacrylated polybutadiene products available, for example, from Sartomer (part of Atofina) as RICON™ resins. These consist of a relatively low molecular weight (about 1,000 to 10,000) polybutadiene liquid that has a number (between about 1 and 10) of maleic anhydride or methacrylate groups grafted onto the chain. The methacrylate, maleic anhydride (or the dicarboxylic acid from hydrolysis of maleic anhydride) groups adsorb strongly onto many mineral surfaces, and are especially suitable for barium sulphate. The hydrophobic loops and tails of the polybutadiene chain contain a number (about 10 to 100) of vinyl groups some of which will be in close proximity to vinyl groups on neighbouring molecules after adsorption onto the particles. The proximal vinyl groups can be then cross-linked by for example, peroxide curing agents, optionally in combination with a reactive co-monomer such as divinyl benzene or trimethylolpropane trimethacrylate.

Somewhat similar polyunsaturated maleinised polymers are available from the Kuraray Co. Ltd as LIR-403™ and LIR-410™. These are liquid isoprene rubber that has been grafted with maleic anhydride. In LIR-410 the MA groups are partially esterified with methanol. The molecular weight is reported in the region of 25,000.

Ricon 131 MA17™ and LIR-403™ have been found to be effective dispersants for barium sulphate powder in mineral oil, especially after the maleic anhydride groups are ring opened by hydrolysis to give dicarboxylic acid anchor groups. The adsorbed oil-wet layer can be crosslinked and immobilised by heating in the presence of a peroxide such as dicumyl peroxide or dibenzoyl peroxide.

The normal function of these resins in conjunction with mineral fillers is to provide links between the filler and an elastomer continuous phase. The functional groups adsorb onto the filler, whilst the vinyl groups can be cross-linked with the elastomer, thus providing improved properties to the finished article. In contrast, the resin as used herein is part of a persistent hydrophobic coating that is not formally linked to the continuous phase.

By utilising certain appropriate cross-link agents it has surprisingly been found that a relatively conventional particular class of Oil-Based fluid additive can be used as a cross-linkable oil-wetting agent. An exemplary type of additive is manufactured by condensing two moles of a fatty acid such as Tall Oil Fatty Acid (TOFA) with a polyethyleneamine such as triethylenetetramine or tetraethylenepentamine . The diamide thus produced contains two or three residual secondary amine groups. Maleic anhydride is added to introduce to the molecule carboxylate functionality to impart the property of good adsorption onto barite. The molecule thus produced will adsorb to deposit an oil-wet layer on barite particles and disperse them in oil very effectively.

After adsorption the residual secondary amine groups residing in the polar head groups are available for further reaction with, for instance, polyfunctional cross-linking agents such as glutaraldehyde, polyanhydrides, titanate esters such as TYZOR™ TnBT (tetra-n-butyl Titanate) , tetraalkyl zirconates, titanium and zirconium chelates, and low molecular weight silanes such as trimethoxypropyl silane. The polyfunctional cross-linking agents are preferred inasmuch as they will promote a more extensive network of linked oil-wetting molecules.

A low molecular weight silane can be used at low concentration as a cross-linking agent for a CLOWA having at least one group capable of reacting with said low molecular weight silane cross- linking agent. This contrasts with US Patent 5,376,629 in which the weighting agent is coated with a hydrophobic layer that is entirely a silane or siloxane that has been polymerised or cured by heat treatment.

The present invention overcomes the disadvantages of US 5,376,629 of high price, acidic or flammable emissions during manufacture, and (especially) the relative fragility of the hydrophobic coating. The coating provided in the present invention is more robust owing to the multiple anchor sites onto the particle surface wherein the anchor groups are chosen for their strong adsorption onto the inorganic substance in question. Generally the quality of the dispersion in an oily phase (exhibiting minimal particle to particle interactions) is better when the exposed groups protruding from the particle are predominantly hydrocarbon in nature and hence similar to the oily phase itself. Although hydrophobic, the polysiloxane coating of 5,376,629 is not as lipophilic as the oil-wettable coating of the present invention.

The CLOWA can be added to the inorganic particles during or after any grinding that is required. Intense mixing is desirable to promote optimal distribution and adsorption of the CLOWA. The coating can be onto dry powder, or by mixing the CLOWA in an aqueous suspension of the inorganic particles. Alternatively the process is performed in a suspension in an oleaginous liquid that is compatible with the non-aqueous liquid that is the base fluid of the wellbore fluid. Dry or aqueous processes are preferred if an oil-soluble reactive co-monomer is used because this ensures that the co-monomer (which may not by itself be strongly adsorbed) is present at high concentration in the adsorbed layer prior to cross-linking.

In the case of a coating process performed in a water phase, after adsorbing and immobilising the coating, the water must be removed from the product prior to use. When the inorganic particles become coated with the hydrophobic layer they will flocculate. This facilitates the separation of the coated material by filtration. The wet filter cake may then be dried and milled to give a dry powder product.

As will be apparent to those skilled in the art there are a huge number of potential combinations of cross-linkable groups and cross-linking agents or processes.

It is believed that the wellbore fluids of this invention are novel for any combination of cross-linking groups, cross-linking agents, or cross-linking processes. The strongly adsorbed, amphiphilic, oil-wetting agents bearing hydrocarbyl groups of this invention differ from the silane or siloxane hydrophobising materials described in US 5,376,629.

Wellbore Fluids of this Invention

Wellbore fluids such as drilling fluids with advantageous properties can be formulated using the coated inorganic powder as a weighting agent, bridging agent, or fluid loss control agent. The non-aqueous phase of the wellbore fluid can be any oleaginous liquid with suitable physical, safety and environmental properties such as are well known to those in the field of well construction and repair. For instance it could be chosen from mineral oils, certain esters, n-alkanes, olefins, polyalphaolefins, and diesel oil.

The material chosen to be a powdered weighting agent suitable for coating as per this invention can be any suitable inorganic compound or element that is of sufficient density for the purpose. Suitable materials include barium sulphate (barite) , calcium carbonate, magnesium carbonate, dolomite, ilmenite, synthetic manganese tetroxide (Mn₃θ₄ - synthetic hausmannite) , synthetic or natural hematite or other iron ores, powdered iron or stainless steel, olivine, siderite, strontium carbonate, strontium sulphate, titanium dioxide, or any mixture thereof.

A preferred embodiment of this invention uses coated inorganic particles having a much smaller than usual particle size. For example, a product whose particles have a weight average particle diameter (D50) of less than 2 micron, as taught by PCT/EP97/03802, is preferred. This results in a product that is more resistant to attrition by the force of inter-particle collisions than one based on conventional API Barite with a D50 of around 25 microns. It also allows the use of finer screens or more intense centrifugation to remove drilled solids even more efficiently, without the substantial co-removal of the weighting agent that would occur with conventional, much larger, particle sizes .

After the manufacture of the coated lipophilic powder a simple test can demonstrate how successful is the immobilisation of the oil-wettable layer. A slurry of the coated lipophilic powder dispersed in mineral oil can be mixed at high shear rate with water with no sign of emulsification or water wetting of the barite. The barite-in-oil dispersion and substantially clear water rapidly separate as two distinct liquid phases after mixing is ceased. In contrast conventional OBM barite dispersions form viscous gelled emulsions that are very difficult to separate when mixed at high shear with water.

Using the coated weighting material in an oil-based fluid, there is no need to add conventional emulsifiers or wetting agents . A fluid that is substantially water-free may be readily formulated. Any contamination of the fluid by surface or subterranean water will not be strongly emulsified into the fluid, in contrast to the slops that are generated by conventional oil-based mud contaminated by a high concentration of dispersed water. Hence water contamination may be readily removed from the fluids of this invention, for example by centrifugation. The wellbore fluid is normally denser than water so it can be recovered by centrifugation as the bottom or underflow phase whilst the water is removed as the top or overflow phase.

An alternative means of removing water is now made possible by the absence of mobile emulsifiers. In such wellbore fluids that have been viscosified by organically modified clays water contamination can be loosely physically emulsified by the gel structure. There may be sufficient structure in the fluid to make centrifugal separation more difficult. However the simple addition and mixing of sufficient water-loving untreated clay like bentonite will "find" the water contamination, causing it to gel and agglomerate. The occluded lumps of gelled water are simple to remove by sedimentation or screening.

When no free emulsifier or wetting agent is present, rock cuttings such as water-wet shale remain water-wet. This results in reduced dispersion in the oil phase of the cuttings into finely divided drill solids. The solids are more efficiently removed by screening and centrifugation because they stay larger and, if anything, tend to aggregate together through polar attractions.

Another benefit is that the oily residues adhere less to the cuttings, and little, if any, oil penetrates into the cuttings (in contrast to the oil imbibition induced by conventional emulsifiers and wetting agents) . Hence the efficiency of cuttings cleaning by processes like washing or centrifugation is improved.

The fluids are therefore well adapted for use in an environmentally sound manner. Because contamination by water or solids is more easily removed, the fluids may be re-used many times. Difficult-to-treat effluents such as slops are avoided. Decreased quantities of (oily) drilling fluid are associated with the cuttings that need disposal, and disposal is facilitated.

Other benefits of avoiding mobile oil-wetting agents or emulsifiers include improved bonding of cement to rock and steel tubulars, and avoiding production losses caused by a wettability change of the producing rock formation, or caused by emulsion blocks .

In conventional OBM the performance of organoclays is adversely affected by mobile surfactants that have anionic functional groups. They tend to deflocculate the structure formed by the clay platelets, especially after exposure to high temperatures. The fluids of this invention are preferably free of such conventional additives, and the immobilised oil-wetting agents cannot relocate to deflocculate the organoclay. Therefore the rheological profile is much improved compared to conventional OBM of otherwise similar properties. Generally the viscosity at low shear rate is much higher and the Plastic Viscosity at higher shear rate is lower. These improvements contribute to better hole-cleaning, weight material suspension and reduced Equivalent Circulating Density (reduced frequency of opening fractures and losing fluid to the formation) . The viscous properties are also much more stable to the effects of high temperature exposure.

Examples and Experimental Procedures

Dose Curve Procedure

A base slurry of 277g barite in 217g oil was prepared by shearing on a Silverson mixer at high shear (7000 rpm) . The dispersant was titrated into the mixture, typically in 0.25 ml increments . After each addition the sample was sheared for 5 to 10 minutes, until the slurry thinned back visibly. Usually during the addition of dispersant a sample would go from extremely viscous, to the consistency of double cream, to appearing very "thin" while shearing but still having a visible gel (like single cream in appearance) to not having a visible gel at all. After this final stage the rheology of the slurry is measured.

Sample Preparation - Procedure

A base slurry was prepared using 217g Clairsol 350M HF base oil and 277g Microbar 4C Barite.

The dispersant was added in the quantity determined in the dose curve procedure and the mixture sheared at 5000 RPM using a Silverson mixer for 20 minutes. The cross-linking agent (where required) was then added and the mixture sheared for 5 minutes at 2000 RPM. If necessary a catalyst was then added and mixed in at 2000 RPM for 1 minute. The samples were then hot rolled in pressurised cells at 100 ⁰C for 16 hours before testing (on occasion other cure temperatures were used as indicated) .

Emulsion Test To ensure that the dispersion contained little or no free dispersant and there was no desorption of the dispersant from the barite over time, a simple emulsion test was performed. 10 ml of water was placed in a glass bottle and shaken to ensure the glass surface was water wet; then 10 ml of the sample slurry was placed in the bottle and shaken vigorously for 30 seconds. A sample passes the test if the water phase and the slurry separate readily, there is no visual increase in viscosity, and the glass remains water-wet. Conversely the sample fails the test if the phases emulsify forming a gel and the glass becomes wet by the oil-based slurry. If the sample passes initially progressively more aggressive shaking is applied to determine whether the oil-wet coating on the particles is susceptible to gradual breakdown or desorption.

The emulsion test can be regarded as a test of the stability and resilience of the coating.

Examples using Surface Active Polymers Containing Maleic Anhydride (MA)

The Ricon resins as ordered from Sartomer Company, Inc.

502 Thomas Jones Way Exton, PA 19341 are anhydride grafted polybutadienes of molecular weight about 5000. It is thought that trace moisture on the barite ring opens the anhydride to produce dicarboxylic acid groups which in turn act as anchors for the dispersant as shown in FIG. 3. Further classes of male±c anhydride copolymers identified as good dispersants at the 1-4 ppb dosage are:

• maleic anhydride grafted polyethylenes

• poly (maleic anhydride co alpha-olefins)

• maleic anhydride grafted polyisoprene

Typical Rheology data for 12ppg slurries are given in Table 1.

Table 1: Rheology of barite slurries with MA adducts .

VoI Rheology

Dispersant ml/ppb PV YP

None (Base slurry) 0 116 51

Poly maleic anhydride-alt-1-octadecene 2.5 10 2

Poly maleic anhydride-alt-alpha olefin

2.5 7 3

C₂4^~ C₂8

Maleic Anhydride Grafted Polyethylene

2.25 7 6 Acid No. 160

Polyisoprene-graft-Maleic Anhydride 4 9 1 Ricon 131MA17 (50%w/w solution) 10 9 2

It should be noted that the plastic viscosity (PV) and yield point (YP) values of the base slurry are 116 and 51.

The alpha-olefin derivatives are cross-linkable using commercial crosslinking agents Epikures 3055, 3115, which are oligomeric polyamides formed from a polyamine and a dimer acid (as used herein dimer acid is a term for the product of dimerisation of oleic or linoleic acid using a clay catalyst) as the MA groups are capable of reacting with the amino functions to give amide or imide links. Samples of these compounds pass the above emulsion after the barite is oven dried to remove residual moisture. The polyisoprene maleic anhydride adducts (PI/MA) are cross- linkable using elevated temperatures (130-160⁰C) and peroxides with a higher decomposition temperature. Dicumyl peroxide is a more effective crosslinker than benzoyl peroxide.

Examples using Surface Active Polymers Containing Polyunsaturated Fatty Acids

Triglyceride drying oils such as linseed oil, soybean oil and tung oil are produced in large quantities and are relatively inexpensive. The products useful in this invention are polyunsaturated fatty acids with low levels of monounsaturated and saturated fatty acids. These unreactive or inert components can be removed by adsorption using an agent for scavenging free fatty acids into a non-surface-active form. Scavenging through adsorption by polyvalent metal hydroxides, or addition of lime are possible mechanisms.

The examples tested are 99% pure linoleic acid commercially available through for example Sigma, and a dehydrated castor oil Prifac 5981 commercially available from Uniqema. Prifac 5981 is known as being >90% polyunsaturated. Using the slurry of Microbar 4C barite in Clairsol mineral oil described before, both these materials were found to be good dispersants that reduce the viscosity efficiently as shown in Table 2.

Table 2: Typical results for Polyunsaturated fatty acids

VoI Rheology

Dispersant ml/p

PV YP pb

Linoleic acid 6 8 2

Prifac 5981 3.5 7 1

Prifac 5981 Air cured overnight

3.5 10 4 /w MgO treatment

Linoleic acid Air cured

6 1 3 overnight /w MgO treatment

Curing was achieved by air-blowing - bubbling air gently through the slurry for 24h, using Cobalt actylacetonate (an oil-soluble complex of cobalt) as a catalyst. Adding an antioxidant; 0.5ppb 2, 6,di-t-butyl-4-dimethylaminomethyl)phenol to the slurry after 24h prevents excessive curing and thickening of the slurry.

Residual unreacted and saturated fatty acids can be removed using MgO dessicant beads as an absorbent. After mixing the slurry containing the MgO beads overnight, the beads (together with the adsorbed fatty acids) were removed from the slurry by screening. The "cleaned" slurry then passes the emulsion test.

Examples using Surface Active Polymers Containing Carboxylated Amidoamine

A carboxylated amidoamine dispersant commercially available as EMUL HT (Trademark of M-I Drilling Fluids), has at least one dicarboxylic acid groups available to adsorb onto the barite surface, and up to three amine (-NH) groups that are potentially reactive sites to cross-link.

Cross-linking is performed using formaldehyde which is known to react with amine functions. Alternatively larger difunctional aldehydes, glutaraldehyde and diglycidyl ether of Bisphenol A, can be used as cross-linking agents. Table 3 summarises some of the properties obtained using the same Hicrobar 4C / Clairsol base slurry as before, dispersed with 2.5ppb Emul HT.

Table 3: Typical results for Emul HT Fluids

Rheology Cross-linking agent

PV YP

None (Base Mud (2.5ppb Emul HT) 8 4 Base + 0.25ppb Formaldehyde 8 1

0.75ppb Formaldehyde 8 1 0.625ppb Bisphenol A diglycidyl ether 8 4

1.875ppb Bisphenol A diglycidyl ether 12 6

1.5ppb MA/AO + lOppb MgO 9 3 1.5ppb Glutaraldehyde + lOppb

MgO 30 0

Emul HT is known to be a good oil-wett ing and emulsifying agent, hence its good dispersant quality . But the base slurry fails the emulsion test . Partially success ful emulsion tests can be achieved using dialdehydes , particularly glutaraldehyde .

Examples using Surface Active Polymers Containing Amphiphilic block copolymers

The maleinised multi-block copolymer styrene-

( ethylene , butylene ) -styrene ( SEBS ) used in this example is coiranercially available as FG 1901 from Kraton Polymers . This block copolymer was dissolved at 100 degC in Xylene, then sheared while still hot in 217g Clairsol base oil with 277g Microbar 4C Barite at 5000 RPM in Silverson mixer for 10 minutes . To complete the coating and dispersion , the sample was hot rolled at 150 °C for 48 hours . Another product included as an example is a solvent-soluble AB block copolymer based on polyacrylate containing long alkyl chains commercialised as Nuosperse FX 9086 from Elementis.

Table 4 summarises some of the properties obtained.

Table 4: Typical results for Block copolymers

VoI Rheology

Dispersant ml/p

PV YP pb

40% Maleinised block SEBS in

13 20 2 Xylene

Nuosperse FX 9086 9 11 3

The advantage of this type of system is that no crosslinking step is needed. The emulsion test is directly passed after dispersion.

Claims

1. A wellbore fluid having a continuous non-aqueous phase and inorganic particles wherein said particles are coated with an immobile layer of amphiphilic molecules essentially permanently adsorbed via at least one polar section and having at least one section causing a lipophilic behaviour of said particles under operating conditions .

2. The wellbore fluid of claim 1 wherein the polar group effecting the immobilisation is directly linked to a carbon atom which in turn is linked to the rest of the amphiphilic molecule through further carbon-carbon links .

3. The wellbore fluid of claim 1 wherein the section causing a lipophilic behaviour is hydrocarbyl-based.

4. The wellbore fluid of claim 3 wherein the amphiphilic molecules comprise at least one lipophilic section including an alkyl, aryl, or alkylaryl group.

5. The wellbore fluid of claim 1 wherein the at least one polar section is selected from a group consisting of carboxylate, phosphate ester, phosphonate, sulphate ester, sulphonate groups adsorbing onto alkaline-earth mineral particulate substrates, amine groups including primary, secondary, or tertiary amines, quaternary ammonium groups adsorbing onto acidic hydroxyl-bearing particulate substrates, and polyoxyethylene groups .

6. The wellbore fluid of claim 1 wherein the amphiphilic molecules are block copolymers with at least one hydrophilic block containing polar groups and at least one lipophilic block.

7. The wellbore fluid of claim β wherein the lipophilic block comprises hydrocarbyl lipophilic blocks.

8. The wellbore fluid of claim 6 wherein the lipophilic block comprises a hydrocarbyl lipophilic block having a molecular weight in the range 400 - 40,000.

9. The wellbore fluid of claim 6 wherein the hydrophilic block of the amphiphilic block copolymer is a polymer of an acid- bearing vinylic monomer selected from a group consisting of acrylic acid, maleic acid (or anhydride) , methacrylic acid, itaconic acid, vinylsulphonic acid, vinyl sulphuric acid, styrene sulphonic acid, 2-acrylamido-2-methylpropane sulphonic acid, and vinylic phosphate esters or a hydrophilic salt of said polymer of an acid-bearing vinylic monomer.

10. The wellbore fluid of claim 6 wherein the hydrophilic block of the amphiphilic block copolymer comprises at least one polyethylene oxide segment.

11. The wellbore fluid of claim 6 wherein the hydrophilic block of the amphiphilic block copolymer comprises a non-polar polymeric group with at least one grafted polar hydrophilic group.

12. The wellbore fluid of claim 1 wherein the amphiphilic molecules comprise at least one reactive group capable of forming chemical links to at least one neighbouring amphiphilic molecule.

13. The wellbore fluid of claim 12 wherein the particles are coated with amphiphilic molecules in a cross-linked state.

14. The wellbore fluid of claim 1 wherein the hydrophobic- lipophilic balance of the amphiphilic molecules is less than 12.

15. The wellbore fluid of claim 6 wherein the hydrophobic- lipophilic balance of the amphiphilic block copolymer is less than 12.

16. The wellbore fluid of claim 1 wherein the concentration of free oil-wetting agents or emulsifiers in the fluid is less than 1 per cent by weight as determined by analysis of the oil obtained from filtration of a sample of the wellbore fluid.

17. The wellbore fluid of claim 1 wherein the particles have a weight average particle diameter of less than about 10 microns.

18. The wellbore fluid of claim 1 wherein the inorganic particles are selected from a group consisting of barium sulphate (barite) , calcium carbonate, magnesium carbonate, dolomite, ilmenite, synthetic manganese tetroxide (Mn3O4 - synthetic hausmannite) , synthetic or natural hematite or other iron ores, powdered iron or stainless steel, olivine, siderite, strontium carbonate, strontium sulphate, titanium dioxide, and any mixture thereof.

19. The wellbore fluid of claim 1 wherein the continuous non¬ aqueous phase comprises one or more liquids selected from a group consisting of mineral oils, esters, n-alkanes, olefins, polyalphaolefins, and diesel oil or a mixture thereof.

20. The wellbore fluid of claim 1 further comprising one or more viscosifier or one or more fluid loss control agent, or a mixture thereof.

21. The wellbore fluid of claim 1 further comprising a dispersed aqueous phase.