WO2013068771A1

WO2013068771A1 - Composition and method

Info

Publication number: WO2013068771A1
Application number: PCT/GB2012/052810
Authority: WO
Inventors: Stephen Cliffe; Peter A Williams; Ian Ratcliffe; Musarrat MOHAMMED; Hugh Christopher GREENWELL; Richard Lloyd ANDERSON; James SUTER; Peter COVENEY; Karen PADMORE; Adrian J FEWINGS; James S MASKERY
Original assignee: M-I Drilling Fluids Uk Limited; Glyndwr University; Durham University; University College London; Centre For Advanced Software Technology Limited
Priority date: 2011-11-10
Filing date: 2012-11-12
Publication date: 2013-05-16
Also published as: GB2509877B; GB2509877A; GB201408236D0; GB201119367D0; NO20140723A1; GB2509877A8

Abstract

The present invention relates to a water-based wellbore fluid comprising water, a hydration inhibitor including a hydroxyalkyl polysaccharide with a weight average molar mass of less than 5,000 and at least one other component selected from the group comprising a rheology modifier, fluid loss control agent, inorganic or organic salt, dispersant and weighting agent. The invention further relates to a method of drilling a subterranean hole and to a method of forming a water-based wellbore fluid.

Description

COMPOSITION AND METHOD

Field of the invention The present invention relates to a shale hydration inhibitor and method of use thereof, and particularly relates to a water-based wellbore fluid comprising water and a hydration inhibitor including a hydroxyalkyl polysaccharide, a method of drilling a subterranean hole with the water-based wellbore fluid and a method of forming the water-based wellbore fluid.

Background art

A drilling fluid or mud is commonly used when drilling a wellbore for the production of oil and/or gas from a hydrocarbon reservoir. The drilling fluid is circulated through the wellbore to remove drill cuttings from the area around the drill bit, stabilise the wellbore walls prior to casing of the wellbore and reduce friction between components of the drill pipe or drill string and the wellbore walls. The drilling fluid may be used to form a filter cake on the walls of the wellbore to reduce the flow of fluids from the surrounding rock formation into the wellbore before drilling is finished and production starts.

There are different types of drilling fluid, including water-based fluids, non- water-based such as oil-based fluids and gaseous (or pneumatic) fluids. Formation solids, including drill cuttings and material that falls into the wellbore from the surrounding rock formation, are dispersed in the drilling fluid during its use downhole. These formation solids are characterised by the rock formation through which the wellbore is drilled. It is common for the rock formation to include a layer of clay. Clay formation solids are dispersed in the drilling fluid like any other type of formation solid, but due to the inherent characteristics of clay, the clay formation solids may swell on contact with the drilling fluid. This is especially a problem if the drilling fluid is water-based. When the clay formation solids swell they increase in volume and thereby, amongst other things, reduce the ability of the drilling fluid to remove drill cuttings from the area around the drill bit and prevent formation of a filter cake, which should be thick enough to reduce the flow of fluids from the surrounding rock formation into the wellbore but thin enough not to impede the flow of fluids or movement of the drill string/drill pipe in the wellbore.

Clays swell when water molecules enter spaces between the unit layers of the clay macrostructure. Various shale/clay swelling and/or hydration inhibitors are known and it is known to add these swelling and/or hydration inhibitors to drilling fluid or mud used when drilling a wellbore for the production of oil and/or gas from a hydrocarbon reservoir.

The particular problem when adding a swelling and/or hydration inhibitor to a drilling fluid is that the inhibitor must be compatible with other components, including additives, in the drilling fluid. Compatibility is important to maintain the required performance of the drilling fluid. For this reason previous water- based drilling fluids have been formulated including polyalkyleneglycol and potassium chloride or polyetheramine. The polyalkyleneglycol can include polyethylene glycols (PEG) and polypropylene glycols (PPG). The potassium chloride is used as an electrolyte and can instead be sodium chloride.

US 2010/0144561 in the name M-l L.L.C describes a water-based drilling fluid including an aqueous based continuous phase, a weighting agent, and a shale hydration inhibition agent, the shale hydration inhibition agent having the eneral formula:

wherein R is independently selected from alkyl and hydroxyl alkyl groups comprising 1 to 15 carbon atoms, and X is an anion. The agent is a bis- quaternary amine shale hydration inhibitor. The shale hydration inhibitor is present in sufficient concentration to reduce the swelling of shale drilling cuttings upon contact with the drilling fluid.

US 5,097,904 describes a method of treating a clay-containing subterranean formation with an aqueous fluid. Quaternary ammonium cationic compounds are described as additives to control formation damage caused by contacting the formation with the aqueous fluid.

US 2008/0277620 describes a depolymerised-carboxyalkyi polysaccharide formed by depolymerising a polysaccharide having between 0.5 and 3.0 degrees of substitution and reducing the molecular weight of said

polysaccharide before or after said depolymerising. This provides a biodegradable scale inhibitor that can be used to prevent the deposition of scale comprising, for example, calcium, barium, sulphate and salts thereof. The depolymerised-carboxyalkyi polysaccharide can be useful in offshore oil production squeeze treatments and in the treatment of scale formed in industrial water treatment.

US 6,455,661 describes an amino-saccharide used in the manufacture of pulp and paper. The amino-saccharide provides limited clay swelling inhibition.

With increasing environmental awareness a need exists to provide an effective swelling and/or hydration inhibitor that has a low environmental impact and is preferably biodegradable. Summary of Invention

According to a first aspect of the present invention there is provided a water- based wellbore fluid comprising water, a hydration inhibitor including a hydroxyalkyl polysaccharide with a weight average molar mass of less than 5,000 and at least one other component selected from the group comprising a rheology modifier, fluid loss control agent, inorganic or organic salt, dispersant and weighting agent. Optionally the hydroxyalkyl polysaccharide has a weight average molar mass in the range of 4,999 to 800, typically in the range of 3,000 to 800 and preferably in the range of 1 ,500 to 800. Typically the polysaccharide is an oligosaccharide. The polysaccharide is formed from the polymerisation of less than 22 monosaccharide monomers, preferably between 5 and 12 monosaccharide monomers. When the polysaccharide is an oligosaccharide, it may be formed from between 2 and 10 monosaccharide monomers. Those monomers which do not terminate the polysaccharide may or may not be identical. Typically the mononers which do not terminate the

polysaccharide are identical. The two monomers which terminate the polysaccharide may be the same or different from each other and may be the same or different to those monomers which do not terminate the

polysaccharide.

The polysaccharide may be formed from one or more of the monomers:

glucose, glucosamine, N-acetyl-D-glucosamine and fructose. The

polysaccharide may be a depolymerised derivative of one or more of the polymers: cellulose, chitin, chitosan, maltodextrin, starch, inulin, glycogen, amylopectin, pullulan, dextran, cyclodextrin and alkyl derivatives thereof. These polymers may be depolymerised using alkali hydrolysis, enzyme degradation and/or oxidative degradation to produce polymers having sufficiently low molecular weight. The polymer may be converted into a cation, as discussed in further detail below.

In one embodiment, the hydroxyalkyi polysaccharide is a polysaccharide that has had hydroxyalkyi substituents added to at least some of the hydroxy groups of a polysaccharide through ester or ether bonding. In a further embodiment, if the polysaccharide comprises amino groups, or substituted amino groups, the hydroxyalkyi substituents may be added to at least some of the amino groups of a polysaccharide through, for example, nucleophilic substitution and subsequent treatment of the ammonium salt, typically with a base.

The water-based wellbore fluid may be selected from the group comprising a drilling fluid, a completion fluid, a packer fluid, a viscous plug fluid and a workover fluid. The hydration inhibitor may also function as one or more of a rheology modifier, fluid loss control agent, dispersant and weighting agent.

The water-based wellbore fluid as described herein requires the presence of at least one other component. In certain embodiments the water-based wellbore fluid comprises at least two, sometimes at least three, optionally at least four and may be all five of the other components selected from the group comprising a rheology modifier, fluid loss control agent, inorganic or organic salt, dispersant and weighting agent. The hydroxyalkyi polysaccharide may at least partially inhibit shale swelling and/or hydration. The hydroxyalkyi polysaccharide can reduce clay mineral hydration and/or swelling by intercalating into the shales. The hydroxyalkyi polysaccharide may further provide the wellbore fluid with anticorrosion and/or antimicrobial characteristics. Corrosion inhibition may be achieved by migration of a hydroxyalkyi polysaccharide in cationic form to a metal/solution interface to form a mono-atomic film at one or more anodic sites. Adsorption of the hydroxyalkyi polysaccharide in cationic form on one or more cell walls may disrupt the cell metabolic processes to provide the antimicrobial effect.

The polysaccharide forming the hydroxyalkyi polysaccharide may be naturally occurring or semi-synthetic being based on a modified naturally occurring material. For example, inulin may be used as the naturally occurring polysaccharide. The inventors of the present invention have appreciated that these polysaccharides are inherently biodegradable. In contrast,

polyetheramine-based shale hydration inhibitors are not inherently

biodegradable.

The hydroxyalkyi polysaccharide of the present invention may make use of an amine or quaternary ammonium group attached to the polysaccharide, preferably as a substituent on the hydroxyalkyi moiety. In nature

polysaccharides and oligosaccharides do not tend to exist as naturally occurring materials since the parent polymer structures are essentially used as skeletal or supporting tissue, or as an energy store in biological systems. Amino acids might be considered for use as hydration inhibitors and occur widely in nature, although these materials are unsuitable as they can easily be denatured and are subsequently more difficult to separate and are less suitable for secondary synthesis reactions.

A bis-quaternary amine shale hydration inhibitor generally has less cationic groups available to bind to clay surfaces compared to the hydroxyalkyi polysaccharide of the present invention and consequently, the level of shale hydration inhibition provided is not as great. Also, problems with accretion and cuttings agglomeration can be linked to the early-time hydration of clay cuttings. Compounds that are more effective shale hydration inhibitors, such as those of the invention, would therefore help to improve performance of the wellbore fluid.

While not wishing to be bound by theory, it is hypothesised that the proximity of the hydroxyl group (-OH) of a hydroxyalkyi substituent to the ether group (-O-) in the hydroxyalkyi polysaccharide may be responsible for the degree of biodegradability of the hydroxyalkyi polysaccharide.

The hydroxyalkyi polysaccharide may be selected from the group comprising a hydroxyalkyi cellulose, a hydroxypropylalkyl polyglucosamine, and a hydroxypropylalkyl polyglucose. A polysaccharide, oligosaccharide or saccharide can also be referred to as a carbohydrate.

The hydroxyalkyi moiety of the hydroxyalkyi polysaccharide may be a hydroxyethyl, a substituted hydroxyethyl, a hydroxypropyl, a substituted hydroxypropyl, a di hydroxypropyl and a substituted dihydroxypropyl. The polysaccharide forming the hydroxyalkyi polysaccharide may be an acid digested (depolymerised) cellulose derivative such as

ethylhydroxyethylcellulose.

Glucosamine and N-acetyl-D-glucosamine may be obtained by chemical and/or enzyme extraction and depolymerisation of chitin and/or chitosan.

The hydroxyalkyi polysaccharide may have a branched or straight chain structure. The hydroxyalkyi polysaccharide may be water insoluble, partially soluble or completely water soluble. Solubility of the hydroxyalkyi polysaccharide may increase with derivatisation. The polysacchande may be depolymehsed. As used herein, the term

"depolymerised" means that the polysaccharide has undergone acid digestion. The polysaccharide may be converted into cationic form or aminated. A polysaccharide in cationic form has a permanent positive charge, whereas an aminated polysaccharide may develop a positive charge under acid conditions. The reagent 3-chloro-2-hydroxypropyl

trimethylammonium chloride:

may be used in the cationic modification of starch and cellulose.

The hydroxyalkyi polysaccharide may include, and preferably comprises, a backbone having one or more of the repeat units shown below, wherein "n" is a number between 1 and 20.

CH₂OR

R¹, R² and R³ can independently be selected from the group comprising: hydrogen; C1 to C6 alkyl; R⁸N⁺R⁹R¹⁰R¹¹X^" wherein R⁸ may be absent or is a C1 to C6 alkylene group, R⁹ to R¹¹ are independently selected from hydrogen or C1 to C6 alkyl groups and X^" is an organic or inorganic anion having a single negative charge, such as a halide, sulphonate, carboxylate or alkylene carboxylate; and substituted or unsubstituted hydroxy C1 -C6 alkyl moiety. At least one of R¹, R² and R³ should be a substituted or unsubstituted hydroxy C1 -C6 alkyl moiety.

The substituted hydroxy C1 -C6 alkyl moiety may comprise a linking group substituent connecting the C1 -C6 alkyl portion to the hydroxyalkyl

polysaccharide backbone, such as an alkylene ether, such as -CH₂CH₂O- or CH₂CH(CH₃)O-, or a polyalkylene oxide, such as polyethylene oxide or linear or branched polypropylene oxide. The hydroxyalkyi moiety may be substituted with a R⁸N⁺R⁹R¹⁰R¹¹ X^" substituent, as defined above. The hydroxyalkyi moiety may be substituted by one or more of sulphonate, carboxylate and alkylene carboxylate in order to provide the hydroxyalkyi polysaccharide in anionic form.

The R¹, R² and R³ groups can independently be selected from the group comprising hydrogen; CH₂CH₃; CH₂N⁺(CH₃)₃Cr;

R⁵ [CH₂— CH 0] CH₂CHCH₂— I⁺— R ₆ ⁶*X^"

R⁴ OH R⁷

wherein at least one of R¹, R² and R³ is independently selected from the group comprising:

wherein R⁴ can independently be selected from the group comprising a hydrogen atom and a methyl group;

wherein "a" is a whole number between 1 and 6;

wherein R⁵, R⁶, R⁷ can independently be selected from the group comprising hydrogen and a C1 to C6 alkyl group; and

wherein X^" is a halide, such as chloride or bromide.

One or more of the R¹, R² and R³ groups can comprise a C1 -C6 alkyl quaternary ammonium group, for instance as a substituent on the

hydroxyalkyi moiety or as the group R⁸N⁺R⁹R¹⁰R^{1 1}X^" discussed above.

Thus, for the structure:

the portion:

is an alkylene ether or a polyalkylene oxide linking group;

is the hydroxyalkyi group; and

is a quaternary ammonium substituent group on the hydroxyalkyi.

If the hydroxyalkyi polysaccharide is formed from the monomer chitosan or glucosamine and R³ is hydrogen, it may have a low molecular weight compared to hydroxyalkyi polysaccharide based on the other monomers listed above. A hydroxyalkyi polysaccharide based on less than 20 monomers of glucosamine and preferably less than 10 monomers of glucosamine is a particularity effective hydration inhibitor.

The hydroxyalkyi moiety may be attached to the polysaccharide backbone in the repeat unit:

via the nitrogen group. The quaternary ammonium derivative of the hydroxyalkyi polysaccharide including a nitrogen atom may be an effective shale hydration inhibitor over a wide range of salinity including freshwater and a saturated salt solution. The nitrogen atom in the amine or quaternary ammonium group is important for the strong interaction and therefore binding of the hydroxyalkyi

polysaccharide to the clay. If the nitrogen atom is part of a hydrazide group, it may also be important for the strong interaction and therefore binding of the hydroxyalkyi polysaccharide to the clay.

The hydroxyalkyi polysaccharide repeat unit (i.e. the backbone repeat unit) and R¹, R² and R³ groups may be selected such that there is on average between 0.1 and 3 quaternary ammonium groups per repeat unit, wherein at least one of the R¹, R² and R³ groups in at least one of the repeat units is a hydroxyalkyi moiety.

Preferably the substituted or unsubstituted C1 to C6 alkyl group is a methyl group, ethyl or propyl group. The hydroxyalkyi polysaccharide may be obtainable by the reaction of one or more of ethylene oxide, propylene oxide, glycidol, and 3-chloro-1 ,2- propanediol with a polysaccharide to produce the hydroxyalkyi

polysaccharide. A dihydroxypropyl polysaccharide may be produced by the reaction of glycidyltrimethylammonium chloride (GTAC):

with the hydroxyalkyi polysaccharide. The concentration of the hydroxyalkyl polysaccharide in the water-based wellbore fluid is optionally between 0.1 kg/m³ and 200 kg/m³; typically between 3 kg/m³ and 50 kg/m³; and normally between 15 kg/m³ and 30 kg/m³.

The polysaccharide may be a salt and include, or have associated with it, metal ions. The associated metal ions may be alkaline earth metals, which may include magnesium (Mg) and calcium (Ca), or alkali metals, which could include lithium (Li), sodium (Na), potassium (K), and caesium (Cs) ions, or transition metal or other metal ions, which may include iron (Fe) and aluminium (Al). To form the salt, the polysaccharide may also have a negatively charged substituent, that may be one or more of a sulphonate and carboxylate such as -C(=O)O^" and alkylene carboxylate, typically as -CH₂- C(=O)O^". The substituent used is selected depending on the

depolymerisation programme used.

According to another aspect of the present invention there is provided a method of drilling a subterranean hole with a water-based wellbore fluid, the method comprising at least the steps of:

- forming a water-based wellbore fluid by mixing water and a hydration inhibitor including a hydroxyalkyl polysaccharide with a weight average molar mass of less than 5,000 and at least one other component selected from the group comprising a rheology modifier, fluid loss control agent, inorganic or organic salt, dispersant and weighting agent; and

- drilling a subterranean hole using the water-based wellbore fluid.

The wellbore fluid may be pumped down to the bottom of the wellbore through a drill pipe, where for example the fluid emerges through ports in the drilling bit. In one embodiment the fluid may be used in conjunction with any drilling operation, which may include, for example, vertical drilling, extended reach drilling, and directional drilling. Specific formulations may depend on the state of drilling a wellbore at a particular time, for example, depending on the depth and/or the composition of the formation.

The method of drilling a subterranean hole preferably uses the hydroxyalkyl polysaccharide according to the first aspect of the present invention.

The water-based wellbore fluid with hydration inhibitor may be used in drilling, cementing, workover, fracturing and abandonment of subterranean wells through a formation containing a shale or a clay which swells in the presence of water.

In a third aspect of the present invention, there is provided a method of drilling a subterranean hole with a water-based wellbore fluid, the method comprising at least the step of drilling a subterranean hole using the water-based wellbore fluid of the first aspect of the present invention.

The method of drilling a subterranean hole may further comprise the step of providing the water-based wellbore fluid of the first aspect of the present invention prior to drilling the subterranean hole.

The method of the third aspect may comprise one or more of the preferable features of the previous aspect.

According to a fourth aspect of the present invention, there is provided a method of forming a water-based wellbore fluid comprising at least the steps of mixing water and a hydration inhibitor including a hydroxyalkyl

polysaccharide with a weight average molar mass of less than 5,000 and at least one other component selected from the group comprising a rheology modifier, fluid loss control agent, inorganic or organic salt, dispersant and weighting agent. The method of forming a water-based wellbore fluid preferably uses the hydroxyalkyi polysaccharide according to the first aspect of the present invention. By this it will be understood that the hydroxalkyl polysaccharide used in the method may be as described above with reference to the first aspect of the present invention. Accordingly, the water-based wellbore fluid formed may be in accordance with the first aspect of the present invention.

The molecular weight of the hydroxyalkyi polysaccharide may be reduced by reacting the polysaccharide with one or more of an oxidising agent, an enzyme reducing agent and an acid reducing agent, and/or by exposing the polysaccharide to ionising radiation and/or electronic beam radiation.

The molecular weight of the hydroxyalkyi polysaccharide may be reduced before the hydroxyalkyi polysaccharide is used in the wellbore.

The polysaccharide may be depolymerised. A depolymerised quaternary ammonium hydroxypropylalkyi polysaccharide may be produced by reaction with glycidyltrimethylammonium chloride and one or more of methyl cellulose, methylhydroxyethylcellulose, ethylhydroxyethylcellulose, glucosamine, glucose syrup, matodextrin, inulin, konjac glucomannan, glucosamine, chitosan, and chitin.

The polysaccharide may be cationic form. A depolymerised

hydroxypropylalkyi polysaccharide in cationic form may be produced by reaction with one or more of carboxymethyl trimethylammonium hydrazide and glycidyltrialkylammonium halides including glycidyltrimethylammonium chloride, glycidyltriethylammonium chloride and glycidyltrimethylammonium bromide. A depolymerised hydroxypropylalkyi polysaccharide may be produced by the additional reaction with a suitable alkyl halide to produce an alkyl ether derivative. The hydroxypropylalkyi polysaccharide may be in cationic form. Adding additional alkyl ether groups to the polysaccharide reduces the hydroxyl number, and in the case of the longer alkyl ether groups, makes the material more hydrophobic. The alkyl halide may be selected from the group comprising ethyl chloride, propyl chloride and butyl chloride.

The hydroxyalkyl polysaccharide may have a biodegradation in sea water (OECD 306) of between 20 and 100% elimination after 28 days. The inventors of the present invention have appreciated that the degree of cationic derivatisation does not adversely increase aquatic toxicity. The skeletonema algae toxicity of the hydroxyalkyl polysaccharide may be greater than

100mg/L.

In a further aspect of the present invention, there is provided use of the water- based wellbore fluid of the first aspect of the present invention as a drilling fluid, a completion fluid, a packer fluid, a viscous plug fluid or a workover fluid.

The linear swelling may be reduced with the quaternary ammonium

hydroxypropyl ethylhydroxyethyl cellulose polymer compared to a

polyetherdiamine. In linear swelling experiments the bulk volume and axial expansion of outcrop shale cores or hydraulically compacted clay pellets were measured when contacted with a test fluid. There is a direct correlation between linear swelling, bulk volume change and the amount of water adsorbed by the exposed outcrop shale cores or hydraulically compacted clay pellets. A reduction in linear swelling is an indication that quaternary ammonium hydroxypropyl ethylhydroxyethyl cellulose polymer additive may be particularly effective in reducing clay hydration and swelling of reactive shales when employed in a water-based wellbore fluid.

Detailed description

Embodiments of the present invention will now be described by way of example only. The term 'wellbore fluid' is used synonymously with the term 'wellbore mud', such that they are intended to have the same meaning. Similarly, the terms 'drilling fluid' and 'drilling mud' are also used synonymously. The term 'shale' is used synonymously with the term 'clay'.

The water-based wellbore fluid is principally composed of an aqueous solution as the continuous phase. The aqueous-based continuous phase is generally selected from the group comprising fresh water, seawater, natural brine solutions, brines formed by dissolving suitable salts in water, mixtures of water and water-soluble organic compounds, and combinations of these and similar compounds that should be known to one of skill in the art. Suitable inorganic salts include chloride and bromide salts of potassium, sodium, calcium, magnesium, zinc, and caesium. Suitable organic salts include acetate and formate salts of potassium, sodium, calcium, magnesium, zinc, and caesium.

The amount of the aqueous-based continuous phase should be sufficient to form a water-based wellbore fluid, such as a drilling fluid. This amount may range from greater than 30 vol.% of the wellbore fluid to less than 100 vol.% of the wellbore fluid. Preferably, the amount of the aqueous-based

continuous phase is in the range from about 40 vol.% to about 90 vol.% of the wellbore fluid. The hydroxyalkyl polysaccharides described herein can intercalate into the interlamellar spacing in clay dispersed in the wellbore fluid. This is not the same as surface intercalation or cation exchange in the clay.

There has previously been little interest in relatively low molecular weight polymers with an average molecular mass of less than 5,000 because only now have the inventors of the present invention recognised that there is a critical molecular weight range for optimum binding efficiency to shales. The inventors of the present invention have also recognised that the degree of cationic substitution may also affect the binding efficiency to shales. The synthesis of these materials can involve multiple reaction steps. The presence of pendant alkyl ether and hydroxyl groups may contribute to the inherent lower toxicity of these cationic materials to aquatic organisms and make these materials inherently more biodegradable than alkyl amines, alkyletheramines and quaternary ammonium compounds of alkyl amines and alkylether amines. The incorporation of charge screening hydroxyl sites offers a less toxic and effective alternative to conventional highly charged copolymers. Charge screening hydroxyl sites are electronegative sites that can effectively shield the positive charge of the cationised hydroxyalkyl polysaccharide.

The results in Table 1 show the percentage recovery and percentage water content of native Oxford clay after treatment with various shale hydration inhibitors. High percentage recovery and low percentage water content indicate good shale hydration inhibition, for example TMA-HP-maltodextrin (twice reacted), 85.3% (Ref. IR/060910A); TMA-HP-HP-maltodextrin (HP groups added first), 82.4% (Ref. IR/1 10810); TMA-HP-Glucosamine, higher DS, 86.8% (Ref. IR/100510); and TMA-HP-EHEC (acid digest), 44.9% (Ref. IR/150310).

All fluids were formulated with 2.5ppb encapsulating polymer which is a relatively high molecular weight polymer, typically 200,000- 1 ,000,000 g/mol, which provides a degree of physical shale stabilisation by surface adsorption onto shale surfaces. The Oxford clay cuttings were sized between 2 and 4mm and were hot rolled in each fluid for 16hrs at 150°F/65°C. The cuttings were recovered on a 2mm screen. The abbreviation "SW" has been used to indicate Sea Water and the abbreviation "FW" has been used to indicate Fresh Water. Table 1

TMA-HP-HP-maltodextrin (HP IR/1 10810 12.7 SW 9.0 100 18.7 groups added first), 82.4%

TMA-HP-maltodextrin, 87.4% IR/180810 12 SW 9.0 97.1 18.6

TMA-HP-maltodextrin (twice IR/060910A 12.3 SW 9.0 100 18.5 reacted), 85.3%

TMA-HP-HP-maltodextrin (HP IR/060910B 13.8 SW 9.0 100 18.9 groups added last), 76%

Polyetheramine n/a 10.5 SW 9.0 99.9 18.7

TMA-HP-glucosamine, 75% IR/190310 14 SW 9.0 98.1 19.4

TMA-HP-glucosamine, higher IR/100510 12.1 SW 9.0 97.7 18.9 DS, 86.8%

TMA-HP-maltodextrin (twice IR/060910A 12.3 SW 9.0 100 19.0 reacted), 85.3%

Polyetheramine n/a 10.5 SW 9.0 100 18.6

TMA-HP-maltodextrin (twice IR/060910A 12.3 SW 9.5 100 19.1 reacted), 85.3%

TMA-HP-glucosamine, higher IR/100510 12.1 SW 9.0 100 18.8 DS, 86.8%

Example 1

This example describes the preparation of N,N,N-trimethyl-3-amino, 2- hydroxypropyl glucosamine in alkali conditions.

A reaction vessel was charged with deionised water, 1000g and glucosamine hydrochloride, 98wt%, Alfa Aesar A15532, 200g added with mixing. Sodium hydroxide solution, 50wt%, approx 70g was then added to bring the pH to 1 1 .0. A nitrogen purge was applied to the vessel and maintained. The vessel was heated to 60°C and mixing commenced. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride (GTAC), 90wt%, Aldrich 50053, 120.77g. After 15 minutes GTAC addition was commenced in a dropwise fashion, over approximately 20 minutes. Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after commencement of the first addition. Two hours after commencement of the final addition, the vessel heating was turned off and the batch cooled with mixing to ambient temperature.

Hydrochloric acid, 32wt% was added to the batch with mixing until neutral (pH 7.0). The product was reduced by rotary evaporation to a syrup and purified by solvent washing. In this instance 1 volume of syrup was mixed with 2 volumes of isopropanol, IPA. On standing, the product and solvent layers separated and the upper IPA layer was removed by decanting. This process was twice repeated and traces of IPA removed from the syrup by rotary evaporation. The resultant product was thus a concentrated solution of N,N,N-trimethyl-3-amino, 2-hydroxypropyl glucosamine. Example 2

This example describes the preparation of N,N,N-trimethyl-3-amino, 2- hydroxypropyl glucosamine in pH neutral conditions.

A reaction vessel was charged with deionised water, 1000g and glucosamine hydrochloride, 98wt%, Alfa Aesar A15532, 200g added with mixing. Sodium hydroxide solution, 50wt%, 10g was then added. A nitrogen purge was applied to the vessel and maintained. The vessel was heated to 60°C and mixing commenced. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride (GTAC), 90wt%, Aldrich 50053, 120.77g. After 15 minutes GTAC addition was commenced in a dropwise fashion, over approximately 20 minutes. Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after

commencement of the first addition. Two hours after commencement of the final addition, the vessel heating was turned off and the batch cooled with mixing to ambient temperature. Hydrochloric acid, 32wt% was added to the batch with mixing until neutral (pH 7.0). The product was reduced by rotary evaporation to a syrup and purified by solvent washing. In this instance 1 volume of syrup was mixed with 2 volumes of isopropanol, IPA. On standing, the product and solvent layers separated and the upper IPA layer was removed by decanting. This process was twice repeated and traces of IPA removed from the syrup by rotary evaporation. The resultant product was thus a concentrated solution of N,N,N-trimethyl-3-amino, 2-hydroxypropyl glucosamine.

Example 3

This example describes the preparation of N,N,N,-trimethyl-3-amino, 2- hydroxypropyl ethyl hydroxyethyl cellulose. This and other examples use an already partially derivatised polysaccharide and further derivatise the polysaccharide using hydroxyalkyl.

A reaction vessel was carefully charged with deionised water, 500g and hydrochloric acid, 32wt%, 500g and the vessel heated to 90°C. Ethyl hydroxyethylcellulose, EHEC, Akzo Nobel Bermocoll E 230X, 200g was added to the vortex whilst mixing and mixing continued at this temperature for one hour. Cooling was applied to the vessel and slow addition of sodium hydroxide (pellet) 203.2g was made, such that 'boiling over' was avoided. Following completion of sodium hydroxide addition the digest was cooled to ambient temperature and, if necessary, corrected to pH 7.0 by appropriate addition of hydrochloric acid, 32wt%, or sodium hydroxide solution, 50wt%. Water, deionised was added to the 1 L mark on the reactor. Sodium

hydroxide solution, 50wt%, 10g was then added. A nitrogen purge was applied to the vessel and maintained. The vessel was heated to 60°C and mixing commenced. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride (GTAC), 90wt%, Aldrich 50053, 120.77g. After 15 minutes GTAC addition was commenced in a dropwise fashion, over approximately 20 minutes. Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after

commencement of the first addition. Two hours after commencement of the final addition, the vessel heating was turned off and the batch cooled with mixing to ambient temperature. Hydrochloric acid, 32wt% was added to the batch with mixing until neutral (pH 7.0). The product was reduced by rotary evaporation to a syrup and purified by solvent washing. In this instance 1 volume of syrup was mixed with 2 volumes of acetone. On standing, the product and solvent layers separated and the upper acetone layer was removed by decanting. This process was twice repeated and traces of acetone removed from the syrup by rotary evaporation. The syrup was allowed to stand to facilitate precipitation of salt. The product was separated from excess salt by decanting off the upper product layer and setting aside. Further product was recovered from the salt precipitate by filtration under reduced pressure on a sinter, assisted by the use of a limited quantity of deionised water. The decanted liquid and filtrate were combined and mixed. The resultant product was thus a concentrated solution of a Ν,Ν,Ν,-trimethyl- 3-amino, 2-hydroxypropyl ethyl hydroxyethyl cellulose digest.

Example 4

This example describes the preparation of N,N,N,-trimethyl-3-amino, 2- hydroxypropyl methyl cellulose. A reaction vessel was charged with deionised water, 160g and isopropanol, IPA, 1580 ml. Methylcellulose, MC, Ashland Culminal MC 3000P, 200g was added to the vortex whilst mixing. A nitrogen purge was applied to the vessel and maintained. The vessel was heated to 60°C and after 15 minutes, sodium hydroxide solution, 50wt%, 9.24g was added. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride

(GTAC), 90wt%, Aldrich 50053, 73.56g. GTAC addition was commenced in a dropwise fashion, over approximately 30 minutes. Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after commencement of the first addition. Two hours after

commencement of the final addition, the vessel heating was turned off and the batch cooled with mixing to ambient temperature. Acetic acid, glacial, was added to the batch with mixing until neutral (pH 7.0). The crude product was purified by precipitation by the addition of excess acetone. The supernatant was removed by decanting and two volumes of acetone poured onto the solid. The solid was washed twice with acetone by mixing, standing and decanting. The solid was air dried and then oven dried overnight at 40°C. A reaction vessel was carefully charged with deionised water, 1375g and hydrochloric acid, 32wt%, 625g and the vessel heated to 90°C. The dried solid, 200g was added to the vortex whilst mixing and mixing continued at this temperature for one hour. Cooling was applied to the vessel and slow addition of sodium hydroxide (pellet) 254.2g was made, such that 'boiling over' was avoided. Following completion of sodium hydroxide addition the digest was cooled to ambient temperature and, if necessary, corrected to pH 7.0 by appropriate addition of hydrochloric acid, 32wt%, or sodium hydroxide solution, 50wt%. Concentration of the solution was undertaken using a rotary evaporator. The syrup was allowed to stand to facilitate precipitation of salt. The product was separated from excess salt by decanting off the upper product layer and setting aside. Further product was recovered from the salt precipitate by filtration under reduced pressure on a sinter, assisted by the use of a limited quantity of deionised water. The decanted liquid and filtrate were combined and mixed. The resultant product was thus a concentrated solution of a N,N,N,-trimethyl-3-amino, 2-hydroxypropyl methyl cellulose digest.

Example 5

This example describes the preparation of N,N,N,-trimethyl-3-amino,2- hydroxypropyl methylethyl hydroxyethylcellulose.

A reaction vessel was charged with deionised water, 160g and isopropanol, IPA, 1580 ml. Methylethyl hydroxyethylcellulose , MEHEC, Akzo Nobel Bermocoll M10, 200g was added to the vortex whilst mixing. A nitrogen purge was applied to the vessel and maintained. The vessel was heated to 60°C and after 15 minutes, sodium hydroxide solution, 50wt%, 9.24g was added. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride (GTAC), 90wt%, Aldrich 50053, 73.56g. GTAC addition was commenced in a dropwise fashion, over approximately 30 minutes. Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after commencement of the first addition. Two hours after commencement of the final addition, the vessel heating was turned off and the batch cooled with mixing to ambient

temperature. Acetic acid, glacial was added to the batch with mixing until neutral (pH 7.0). The crude product was purified by precipitation by the addition of excess acetone. The supernatant was removed by decanting and two volumes of acetone poured onto the solid. The solid was washed twice with acetone by mixing, standing and decanting. The solid was air dried. A reaction vessel was carefully charged with deionised water, 1375g and hydrochloric acid, 32wt%, 625g and the vessel heated to 90°C. The dried solid, 200g was added to the vortex whilst mixing and mixing continued at this temperature for one hour. Cooling was applied to the vessel and slow addition of sodium hydroxide (pellet) 254.2g was made, such that 'boiling over' was avoided. Following completion of sodium hydroxide addition the digest was cooled to ambient temperature and, if necessary, corrected to pH 7.0 by appropriate addition of hydrochloric acid, 32wt%, or sodium hydroxide solution, 50wt%. Concentration of the solution was undertaken using a rotary evaporator. The syrup was allowed to stand to facilitate precipitation of salt. The product was separated from excess salt by decanting off the upper product layer and setting aside. Further product was recovered from the salt precipitate by filtration under reduced pressure on a sinter, assisted by the use of a limited quantity of deionised water. The decanted liquid and filtrate were combined and mixed. The resultant product was thus a concentrated solution of a N,N,N,-trimethyl-3-amino,2-hydroxypropyl methylethyl

hydroxyethylcellulose digest. Example 6

This example describes the preparation of N,N,N,-trimethyl-3-amino, 2- hydroxypropyl ethyl hydroxyethyl cellulose. A reaction vessel was carefully charged with deionised water, 500g and hydrochloric acid, 32wt%, 500g and the vessel heated to 90°C. A

hydrophobically modified Ethyl hydroxyethylcellulose, HM-EHEC, Akzo Nobel Bermocoll EHM200, 200g was added to the vortex whilst mixing and mixing continued at this temperature for one hour. Cooling was applied to the vessel and slow addition of sodium hydroxide (pellet) 203.2g was made, such that 'boiling over' was avoided. Following completion of sodium hydroxide addition the digest was cooled to ambient temperature and, if necessary, corrected to pH 7.0 by appropriate addition of hydrochloric acid, 32wt%, or sodium hydroxide solution, 50wt%. Water, deionised was added to the 1 L mark on the reactor. Sodium hydroxide solution, 50wt%, 10g was then added. A nitrogen purge was applied to the vessel and maintained. The vessel was heated to 60°C and mixing commenced. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride

(GTAC), 90wt%, Aldrich 50053, 120.77g. After 15 minutes GTAC addition was commenced in a dropwise fashion, over approximately 20 minutes.

Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after commencement of the first addition. Two hours after commencement of the final addition, the vessel heating was turned off and the batch cooled with mixing to ambient temperature.

Hydrochloric acid, 32wt% was added to the batch with mixing until neutral (pH 7.0). The product was reduced by rotary evaporation to a syrup and purified by solvent washing. In this instance 1 volume of syrup was mixed with 2 volumes of acetone. On standing, the product and solvent layers separated and the upper acetone layer was removed by decanting. This process was twice repeated and traces of acetone removed from the syrup by rotary evaporation. The syrup was allowed to stand to facilitate precipitation of salt. The product was separated from excess salt by decanting off the upper product layer and setting aside. Further product was recovered from the salt precipitate by filtration under reduced pressure on a sinter, assisted by the use of a limited quantity of deionised water. The decanted liquid and filtrate were combined and mixed. The resultant product was thus a concentrated solution of a hydrophobically modified N,N,N,-trimethyl-3-amino, 2- hydroxypropyl ethyl hydroxyethyl cellulose digest. Example 7

This example describes the preparation of N,N,N-trimethyl-3-amino, 2- hydroxypropyl maltodextrin.

A reaction vessel was charged with deionised water, 200g and maltodextrin, Cargill C^* Dry MD 01905, 200g added with mixing. A nitrogen purge was applied to the vessel and maintained. The vessel was heated to 45°C with mixing. After 30 minutes sodium hydroxide solution, 10wt%, 35.5g was added. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride (GTAC), 90wt%, Aldrich 50053, 120.77g. GTAC addition was commenced in a dropwise fashion, over 30-90 minutes. Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after commencement of the first addition.

Eighteen hours after commencement of the final addition, the vessel heating was turned off and the batch cooled with mixing to ambient temperature. Acetic acid, glacial, was added to the batch with mixing until neutral (pH 7.0). The product was reduced by rotary evaporation to a syrup and purified by solvent washing. In this instance 1 volume of syrup was mixed with 2 volumes of isopropanol, IPA. On standing, the product and solvent layers separated and the upper IPA layer was removed by decanting. This process was twice repeated and traces of IPA removed from the syrup by rotary evaporation. The resultant product was thus a concentrated solution of N,N,N-trimethyl-3-amino, 2-hydroxypropyl maltodextrin.

Example 8

This example describes the preparation of N,N,N,-trimethyl-3-amino, 2- hydroxypropyl ethyl cellulose. A reaction vessel was charged with deionised water, 160g and isopropanol, IPA, 1580 ml. Ethylcellulose, EC, Ashland Aqualon EC-N10, 200g was added to the vortex whilst mixing. A nitrogen purge was applied to the vessel and maintained. The vessel was heated to 60°C and after 15 minutes sodium hydroxide solution, 50wt%, 9.24g was added. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride (GTAC), 90wt%, Aldrich 50053, 73.56g. GTAC addition was commenced in a dropwise fashion, over approximately 20-25 minutes. Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after commencement of the first addition. Two hours after commencement of the final addition, the vessel heating was turned off and the batch cooled with mixing for 45 minutes. Acetic acid, glacial, was added to the batch with mixing until neutral (pH 7.0). The crude product was purified by precipitation by the addition of excess water. The solid was recovered on a glass sinter under reduced pressure and washed with copious amounts of water prior to air drying. The resultant product was thus N,N,N,-trimethyl-3- amino, 2-hydroxypropyl ethyl cellulose.

Example 9

This example describes the preparation of N,N,N-trimethyl-3-amino, 2- hydroxypropyl sucrose.

A reaction vessel was charged with deionised water, 200g and sucrose 200g added with mixing. A nitrogen purge was applied to the vessel and

maintained. The vessel was heated to 45°C with mixing and after 15 minutes, sodium hydroxide solution, 50wt%, 7.1g was added. A pressure equalised dropping funnel was charged with glycidyl trimethylammonium chloride (GTAC), 90wt%, Aldrich 50053, 120.77g. GTAC addition was commenced in a dropwise fashion, typically over 30 minutes. Second and third additions of the same quantity of GTAC were added in the same manner two and four hours after commencement of the first addition. Eighteen hours after commencement of the final addition, the vessel heating was turned off and the batch cooled to ambient temperature. Acetic acid, glacial, was added to the batch with mixing until neutral (pH 7.0). The product was reduced by rotary evaporation to a syrup and purified by solvent washing. In this instance 1 volume of syrup was mixed with 1 volume of isopropanol, IPA. On standing, the product and solvent layers separated and the upper IPA layer was removed by decanting. This process was twice repeated and traces of IPA removed from the syrup by rotary evaporation. The resultant product was thus a concentrated solution of N,N,N-trimethyl-3-amino, 2-hydroxypropyl sucrose.

Example 10.

This example describes the preparation of 3-amino, 2-hydroxypropyl ethyl hydroxyethyl cellulose. A reaction vessel was carefully charged with deionised water, 1375g and hydrochloric acid, 32wt%, 625g and the vessel heated to 90°C. Ethyl hydroxyethylcellulose, EHEC, Akzo Nobel Bermocoll E 230X, 200g was added to the vortex whilst mixing and mixing continued at this temperature for one hour. Cooling was applied to the vessel and slow addition of sodium hydroxide (pellet) 203.2g was made, such that 'boiling over' was avoided. Following completion of sodium hydroxide addition the digest was cooled to ambient temperature and, if necessary, corrected to pH 7.0 by appropriate addition of hydrochloric acid, 32wt%, or sodium hydroxide solution, 50wt%. The product was reduced by rotary evaporation to a paste and transferred to a reaction vessel. A nitrogen purge was applied to the vessel and the contents heated to 85°C. Glacial acetic acid, 100ml_ was added and subsequently epichlorohydrin, 400ml_. After mixing for four hours at 85°C the batch was cooled by mixing at ambient temperature for 30 minutes. After standing overnight, the batch was diluted by addition of deionised water, 750ml_ and heated to 70°C. Ammonium hydroxide, 35wt%, 1429ml_ was added and the batch was mixed for 8 hours. The batch was then allowed to cool without mixing. The product was reduced by rotary evaporation to a syrup and purified by solvent washing. In this instance 1 volume of syrup was mixed with 1 volumes of acetone. On standing, the product and solvent layers separated and the upper acetone layer was removed by decanting. This process was twice repeated and traces of acetone removed from the syrup by rotary evaporation. The resultant product was thus a concentrated solution of a 3-amino, 2-hydroxypropyl ethyl hydroxyethyl cellulose digest.

Improvements and modifications may be made without departing from the scope of the invention.

Claims

1 . A water-based wellbore fluid comprising water, a hydration inhibitor including a hydroxyalkyi polysaccharide with a weight average molar mass of less than 5,000 and at least one other component selected from the group comprising a rheology modifier, fluid loss control agent, inorganic or organic salt, dispersant and weighting agent.

2. A water-based wellbore fluid according to claim 1 , wherein the hydroxyalkyi polysaccharide has a weight average molar mass of between 3,000 and 800.

3. A water-based wellbore fluid according to either claim 1 or claim 2, wherein the hydroxyalkyi polysaccharide is formed from one or more of the monomers: glucose, glucosamine, N-acetyl-D-glucosamine and fructose.

4. A water-based wellbore fluid according to either claim 1 or claim 2, wherein the hydroxyalkyi polysaccharide is a depolymerised derivative of one or more of the polymers: cellulose, chitin, chitosan, maltodextrin, starch, inulin, glycogen, amylopectin, pullulan, dextran, cyclodextrin and alkyl derivatives thereof.

5. A water-based wellbore fluid according to any preceding claim, wherein the hydroxyalkyi moiety of the hydroxyalkyi polysaccharide is selected independently from the group comprising a hydroxyethyl, a substituted hydroxyethyl, a hydroxypropyl, a substituted hydroxypropyl, a dihydroxypropyl and a substituted dihydroxypropyl.

6. A water-based wellbore fluid according to any preceding claim, wherein the hydroxyalkyi polysaccharide includes a backbone having one or more of the repeat units:

wherein "n" is any whole number between 1 and 20 and R¹, R² and R³ are independently selected from the group comprising: hydrogen; C1 to C6 alkyl; R⁸N⁺R⁹R¹⁰R^{1 1}X^" wherein R⁸ is absent or is a C1 to C6 alkylene group, R⁹ to R¹¹ are independently selected from hydrogen and C1 to C6 alkyl groups and X^" is an organic or inorganic anion having a single negative charge; and substituted or unsubstituted hydroxy C1 -C6 alkyl moiety; and wherein at least one of R¹, R² and R³ comprises the substituted or

unsubstituted hydroxy C1 -C6 alkyl moiety.

7. A water-based wellbore fluid according to claim 6, wherein the value of "n" is such that the hydroxyalkyl polysaccharide is an oligosaccharide.

8. A water-based wellbore fluid according to either of claims 6 or 7, wherein one or more of the R¹, R² and R³ groups comprise a C1 -C6 alkyl quaternary ammonium group.

9. A water-based wellbore fluid according to claim 8, wherein the backbone repeat unit and R¹, R² and R³ groups are selected such that there is on average between 0.1 and 3 quaternary ammonium groups per repeat unit, wherein at least one of the R¹, R² and R³ groups in at least one of the repeat units is a hydroxyalkyl moiety.

10. A water-based wellbore fluid according to any of claims 6 to 9, wherein R¹, R² and R³ are independently selected from a group comprising: hydrogen,

CH2CH3, CH₂N⁺(CH₃)₃Cr;

R⁵

CH₉CHCH— I⁺— R ₆ ⁶*X^" R⁵

^■ [CH₂— CH 0]i CH₂CHCH₂— +⁺— τ R>6^b.*X

R^ OH R⁷

and at least one of R¹ , R² and R³ is independently selected from the group comprising:

an

R⁵

^■ [CH₂— CH 0]i CH₂CHCH₂— +⁺— τ R>6^b.*X

R⁴ OH R⁷

wherein R⁴ is independently selected from a group comprising a hydrogen atom and a methyl group;

"a" is a whole number between 1 and 6;

R⁵, R⁶, R⁷ are independently selected from the group comprising hydrogen and a C1 to C6 alkyl group; and

X^" is a halide.

1 1 . A water-based wellbore fluid according to any preceding claim, wherein the concentration of the polysaccharide in the water-based wellbore fluid is between 0.1 kg/m³ and 200 kg/m³.

12. A water-based wellbore fluid according to any preceding claim, wherein the hydroxyalkyi polysaccharide is a salt of an alkaline earth metal, the alkaline earth metal selected from the group comprising magnesium and calcium.

13. A water-based wellbore fluid according to any preceding claim, wherein the hydroxyalkyi polysaccharide is a salt of an alkali metal, the alkali metal selected from the group comprising lithium, sodium, potassium, and caesium.

14. A water-based wellbore fluid according to any preceding claim, wherein the hydroxyalkyi polysaccharide is a salt of a transition or other metal, the transition or other metal selected from the group comprising iron and aluminium.

15. A water-based wellbore fluid according to any preceding claim, wherein the hydroxyalkyi polysaccharide is semi-synthetic and based on a modified naturally occurring material.

16. A water-based wellbore fluid according to any preceding claim, wherein the hydroxyalkyi polysaccharide is biodegradable.

17. A water-based wellbore fluid according to claim 16, wherein the hydroxyalkyi polysaccharide is biodegradable in sea water.

18. A method of drilling a subterranean hole with a water-based wellbore fluid, the method comprising at least the step of drilling a subterranean hole using the water-based wellbore fluid of any of claims 1 to 17.

19. The method of drilling a subterranean hole according to claim 18 further comprising the step of providing the water-based wellbore fluid of any of claims 1 to 17 prior to drilling the subterranean hole.

20. A method of forming a water-based wellbore fluid comprising at least the steps of mixing water and a hydration inhibitor including a hydroxyalkyl polysaccharide with a weight average molar mass of less than 5,000 and at least one other component selected from the group comprising a rheology modifier, fluid loss control agent, inorganic or organic salt, dispersant and weighting agent.

21 . The method of forming a water-based wellbore fluid according to claim 20, wherein the hydroxyalkyl polysaccharide is the hydroxyalkyl

polysaccharide as claimed in any of claims 1 to 17.

22. The method of forming a water-based wellbore fluid according to either claim 20 or claim 21 , further including the step of reducing the molecular weight of the hydroxyalkyl polysaccharide by reacting the hydroxyalkyl polysaccharide with one or more of an oxidising agent, an enzyme reducing agent, and an acid reducing agent.

23. The method of forming a water-based wellbore fluid according to any of claims 20 to 22, further including the step of reducing the molecular weight of the hydroxyalkyl polysaccharide by exposing the hydroxyalkyl

polysaccharide to one or both of ionising radiation and electronic beam radiation.

24. The method of forming a water-based wellbore fluid according to any of claims 20 to 23, further including the step of depolymerising the

hydroxyalkyi polysaccharide by reaction of the hydroxyalkyi polysaccharide with glycidyltrimethylammonium chloride and one or more of methyl cellulose, methylhydroxyethylcellulose, ethylhydroxyethylcellulose, glucosamine, glucose syrup, matodextrin, inulin, konjac glucomannan, glucosamine, chitosan, and chitin.

25. The method of forming a water-based wellbore fluid according to any of claims 20 to 24, further including the step of converting the hydroxyalkyi polysaccharide into a cation by reaction of the hydroxyalkyi polysaccharide with one or more of a carboxymethyl trimethylammonium hydrazide and a glycidyltrialkylammonium halide selected from the group comprising glycidyltrimethylammonium chloride, glycidyltnethylammonium chloride and glycidyltrimethylammonium bromide.

26. The method of forming a water-based wellbore fluid according to any of claims 20 to 25, wherein the hydroxyalkyi polysaccharide is biodegradable in sea water.