GB2525689A - Drilling fluids - Google Patents

Abstract

Use of a microfibrillated cellulose containing one or more alkali metals (e.g. alkali metal ions) as, or as a component of, a drilling or completion fluid. Microfibrillated cellulose materials for use in the invention include compounds of formula I: [C6H7O2(OH)3-x ∙ (Q)x]n (I) wherein: C6H7O2(OH)3-x represents a microfibrillated cellulose unit; (Q)x either represents a modified hydroxyl group, or a group which replaces a hydroxyl group present in the cellulose unit, and which contains at least one alkali metal; x is an integer from 1 to 3 which indicates the number of hydroxyl groups in each cellulose unit which are either modified or replaced, and which may vary between different cellulose units; and n is an integer which corresponds to the total number of cellulose units. Also included are a drilling and completion fluid of the type described by formula 1, a method of drilling a well in the presence of said fluid and a method of making microfibrillated cellulose material. [No fig]

Description

Drilling Fluids The present invention relates to drilling fluids for use in drilling operations, especially for use in the drilling of wells for the recovery of hydrocarbons such as oil and/or gas.

In particular, it relates to the use of modified microfibrillated cellulose materials as viscosifiers and/or weighting agents in water or oil-based drilling fluids.

In drilling operations, for example in drilling of oil and gas wells, drilling fluids (also known as drilling muds) are used; generally these are either oil-based muds (OBMs) or water-based muds BMs), although emulsions containing both oil and water may also be used. Drilling fluids perform several functions. In addition to cooling and lubrication of the drill bit, and carrying out cuttings (i.e. particles of crushed or cut formation or rock produced by drilling) to the surface, an important function of a drilling fluid is to provide the necessary hydrostatic pressure within the well to prevent formation fluids from entering the well bore. Traditionally this has been achieved using solid (i.e. particulate) weighting materials, such as clays (e.g. bentonite), which confer sufficient density to the drilling fluid that this is able to produce a pressure greater than the surrounding pressure in the well bore. When pumped, such fluids are typically free-flowing, but when pumping stops the static fluid forms a gel-like structure that resists flow.

Other chemicals have been proposed for use in controlling viscosity in drilling fluids and thereby achieve well control during oil and gas production. These include the alkali metal formates which are added to water-based drilling fluids, particularly those for use in treating high pressure! high temperature (HPHT) wells in which a high degree of well control is required; these materials function as both viscosifiers and weighting agents. One such material which finds widespread use in the oil and gas industry is caesium formate.

Aqueous solutions of caesium formate, produced by reaction of caesium hydroxide and formic acid, are used both as oil well drilling and completion fluids. The high density of caesium formate brines (up to 2.3 g.cm3), together with their low toxicity, makes them particularly suited to this purpose. Caesium formate is generally considered to be environmentally friendly; for example, it is biodegradable and may be recycled. Its main drawback is that it is very expensive, thus increasing the overall cost of oil and gas production.

Caesium formate-based drilling fluids may also contain various polymers which serve as rheology and fluid loss control agents. However, these do not impact to any significant extent on the amount of caesium formate which is required and thus the overall cost of the drilling fluid remains high.

One example of a polymer which has been suggested for use in water-based drilling fluids is microfibrillated cellulose. This has interesting mechanical properties, notably its shear-thinning behaviour whereby its viscosity is reduced on application of shear forces. This makes it particularly suitable for use as a rheology agent or viscosifier, as well as a weighting agent. As with other polymer materials used in drilling fluids, however, there is still a need for this to be used together with other weighting agents, such as caesium formate, in order to ensure that a specific weight is achieved which is higher than the equivalent circulation density and which can thus effectively secure control of the well. This is particularly the case for HPHT wells which require a greater degree of control than non-HPHT wells.

A further problem associated with the use of caesium formate brines is their lack of suitability for use in HPHT downhole conditions where increasingly it is becoming necessary to employ oil-based drilling fluids, rather than those which are water-based. The friction coefficient of water-based fluids which contain caesium formate can increase under the drilling conditions encountered in HPHT wells, particularly at elevated temperatures.

Thus there remains a need to restrict the use of expensive caesium formate brines in drilling operations and/or to provide alternative weighting materials which can be used in a wider range of drilling operations, particularly those which to date have necessitated the use of oil-based drilling fluids. Alternative materials which are capable of performing more than one function, for example a range of different functions, including for example acting as a weighting agent, a viscosifier and/or a bridging agent, are also desirable.

We now propose a range of alternative materials for use in drilling fluid systems, particularly those for use in FIFE-IT wells. Such materials are also suitable for use in completion fluids, for example those for use in drilling operations. These are based on microfibrillated cellulose which is modified to incorporate one or more alkali metals (especially caesium) within the polymer structure thereby increasing ts density. Use of such materials effectively eliminates the need for the separate use of any caesium formate brine thus significantly reducing the cost of drilling operations.

These materials also have the advantage that these can be used in both water and oil-based drilling fluids making them suitable for use in a wide range of drilling operations.

In one aspect the invention thus provides the use of a microfibrillated cellulose containing one or more alkali metals (e.g. alkali metal ions) as, or as a component of, a drilling or completion fluid.

Drilling and completion fluids comprising these modified cellulose materials, together with at least one additional material conventionally used in such fluids, form a further aspect of the invention. Additional materials which may be present in drilling and/or completion fluids include, for example, bridging materials, weighting agents, brines, viscosifiers, fluid loss control additives, pH adjusting agents, lubricants, defoamers, agents which act to minimise bacterial growth, bactericides, or any combination thereof. Such materials will typically be present in the drilling or completion fluid in conventional amounts or in an amount which could readily be determined by those skilled in the art having in mind factors such as the intended use of the fluid, the amount of modified cellulose material which is present, etc. For example, it may be appropriate that some or all of these additional materials can be used in concentrations which are lower than those conventionally used depending on the nature and amount of modified cellulose material present; this is particularly so in the case of additional weighting agents, viscosifiers and/or bridging materials, for example. Any lowered concentrations may readily be determined by those skilled in the art.

In a further aspect the invention also provides a method of drilling a well in the presence of a drilling fluid as herein described. Also provided is a method of completing a well in the presence of a completion fluid as herein described. Such methods will generally be performed in the context of a drilling operation intended to recover oil and/or gas.

Microfibrillated cellulose materials which may be employed in the invention include those of formula I: [C5H7O2(OH)3 (Q)J (I) In this formula, C6H7O2(OH)3 represents a microfibrillated cellulose unit and (Q) either represents a modified hydroxyl group, or a group which replaces a hydroxyl group present in the cellulose unit. The integer x represents the number of hydroxyl groups in each cellulose unit which are either modified or replaced. The value of x may vary between different cellulose units. Similarly, the nature of Q may vary between different cellulose units within the molecule. Where xis greater than 1 the nature of moiety 0 may also vary within a given cellulose unit. The integer n corresponds to the total number of modified cellulose units present in the material and, as will be understood, will be dependent on the nature of the microfibrillated cellulose material and the method by which this is produced. Typical values for n will range from 90 to 10,000, e.g. 600 to 6,000.

More specifically, in formula I: n is an integer in the range from 90 to 10,000; each xis an integer independently selected from 1, 2 and 3, preferably I or 2, e.g. 1; each moiety Q is independently selected from: -a modified hydroxyl group of formula -aM in which M represents an alkali metal ion; and -an organic group containing at least one (e.g. one or two, preferably one) alkali metal, preferably at least one alkali metal ion.

Each moiety 0 may vary between the different cellulose units, but typically each moiety 0 in formula I will be identical.

As will be understood, not all hydroxyl groups in the original microfibrillated cellulose structure are identical. In formula I above, and in all other formulae presented herein, no limitation is intended on the specific hydroxyl group (or groups) within the cellulose unit which are modified and/or replaced. Dependent on the process used to produce the modified cellulose materials it will be understood that in certain cases mixed cellulose units may arise in a given molecule in which different hydroxyl groups are modified or replaced in the various cellulose units. All possible forms and variations of the resulting cellulose compounds are intended to form part of the invention.

In one embodiment the microfibrillated cellulose materials for use in the invention comprise one or more modified hydroxyl groups. Such materials may be represented by formula II: [C6H702(OH)3.. (OM)], (II) wherein M, x and n are as hereinbefore defined. Although each group M in formula II may vary between the different cellulose units, typically these will be identical. In the case where xis greater than 1 each M within a given cellulose unit need not be identical. Preferably, however, they will be the same. In a preferred embodiment each group M in formula II represents a caesium ion. In formula II, xis preferably 1.

Alkali metal ions which may be present in the cellulose materials herein described will generally be selected from sodium, potassium, and caesium ions. Most preferably these will be caesium ions. A combination of different alkali metal ions may be present in any given cellulose material, for example a combination of potassium and caesium ions, but most preferably these will all be caesium.

Any organic group which contains at least one alkali metal, preferably at least one alkali metal ion, may be present in the microfibrillated cellulose materials herein described. A preferred alkali metal is caesium. As will be understood the nature of such groups may be varied provided that these fulfil the requirement that these should contain the desired alkali metal or metals (e.g. at least one alkali metal ion).

Suitable organic groups may readily be determined by those skilled in the art, but typically these will contain from ito 20 carbon atoms, for example ito 10 carbon atoms, e.g. i to 6 carbons. Preferably these will also include one or more oxygen atoms either as substituent groups (e.g. carbonyl groups) or as interrupting groups (e.g. oxo groups). Organic groups containing one or more carboxylic acid derivatives (e.g. -COOM groups), are particularly preferred.

Examples of suitable organic groups which may be present in the cellulose compounds for use in the invention include C1.10 straight-chain or branched alkyl groups (e.g. C1.6 alkyl) substituted by one or two, preferably one, -COOM group (where M is an alkali metal ion, e.g. caesium).

Non-limiting examples of suitable organic groups, Q, include the following: -COOM, -CH2COOM, -C(CH3)2COOM, -CH2C(CH3)2COOM, -O-C(CH2M)2(OC(O)M), and -O-C(OC(O)M)2-CH2M in which M is an alkali metal, preferably an alkali metal ion, e.g. caesium. Where these groups contain more than one metal, these may be the same or different. In one embodiment these will be the same, preferably caesium.

Microfibrillated cellulose materials as herein described, in particular those of formulae I and II, are in themselves novel and thus form a further aspect of the invention.

Microfibrillated cellulose (MFC), also known as nanocellulose, is a form of cellulose in which the individual microfibrils have been partly or completely detached from one another. MFC is normally very thin (about 5-20 nm) and the length ranges from tens of nanometers to several micrometers. Typically it is made from wood cellulose fibres, but may also be made from other sources such as microbial sources, agricultural fibres, etc. The fibrils may be isolated from the cellulose-containing source by methods which involve mechanical treatment, e.g. high pressure, high temperature and/or high velocity impact homogenisation. Homogenisers are used to delaminate the cell walls of the fibres thereby liberating the nanosized fibrils.

Alternatively, it is possible to produce microfibrils from cellulose using various chemicals which serve to break up or dissolve the cellulose fibres. Such methods may involve treatment with an acid (i.e. hydrolysis) or an enzyme (e.g. a cellulase).

Microfibrillated cellulose may be modified to produce the materials herein described using methods known in the art. For example, in the case where group Q represents a modified hydroxyl group these may be prepared by reaction of a microfibrillated cellulose and an alkali metal hydroxide (e.g. caesium hydroxide). Reaction conditions may be readily determined by those skilled in the art.

In the case where the microfibrillated cellulose is modified to include one or more organic groups which include an alkali metal (e.g. an alkali metal ion), free radical reactions are generally used to prepare these materials. Examples of such reactions include the following: Reaction 1: [C5H7O2(OH)3] + x HCOOM -, [C6H7O2(OH)3(COOM),.j + x I12O Reaction 2: [C5H7O2(OH)3] + x CH3COOM -* [C5H7O2(OH)3(CH2COOM)] + x H20 Reaction 3: [C5H7O2(OH)3], + x CH3-C(CH3)2COOM -* [C6H702(OH)3X(C(CH3)2COOM)J. + x CH3OH or [C5H7O2(OH)3] + x CH3-C(CH3)2000M -* [C6H702(OH)3x(CHzC(CH3)COOM)yJri + x CH3OH + x H20 Reaction 4: [C5H702(OH)3]. + x MCOO-CO-CH2M -. [(C5H702(OH)3x(O-C(CH2M)2(OC(O)M)))(]n or [C6H7O2(OH)3] + X MCOOtOCH2M -* F(C5H702(OH)3x(0C(CH2M)(OC(O)M)2)x]n In each of the above reactions [C6H702(OH)3],1 represents a microfibrillated cellulose starting material, and M is an alkali metal, preferably an alkali metal ion, e.g. caesium. Typical reaction conditions, for example nature of catalysts, free radical initiators, solvents, etc., may be readily determined by those skilled in the art. To enhance the reaction of the cellulose, this may if required be subjected to a pre-treatment step in order to open up the hydrogen bonded structure. Methods to achieve this are known in the art, for example in Saito et a!., Colloids and Surfaces -Physicochemical and Engineering Aspects 289 (1-3): 21 9-225, 2006, which describes TEMPO-catalysed oxidation using 3.8 mmol NaCIO per gram of cellulose.

Pre-treatment of the cellulose in this way aids in reaction of more than one hydroxyl group per cellulose unit.

Starting materials for use in the above reactions are either commercially available or may be produced using methods known in the art. For example, caesium acetate is available from PriChem, and caesium methylate is available from Haihang Industry Co. The following methods are illustrative of reactions which may be used to produce suitable starting materials in the case where the alkali metal ion is caesium; these may readily be adapted for cases where the alkali metal ion is other than caesium, e.g. sodium or potassium: a. reaction of formic acid and CsOH (caesium hydroxide) to produce caesium formate brine: HCOOH + CsOH -* HCOOCs i-H20 b. reaction of acetic acid and CsOH to produce caesium acetate: CH3COOH + CsOH -* CH3COOCs + H20 c. reaction of a carboxylic acid (or salt of a carboxylic acid) to produce caesium methylate or caesium propionate: CH3COOH (or Na) + CH3OCs -s CH3COOCs + CH3OH (or Na) + NaOH + H20 CH3CH2COONa + CsOH -CH3CH2COOCs + NaOH d. reaction of an iso-carboxylic acid or salt thereof and caesium hydroxide: CH3C(CH3)2COOH (or Na) + CsOH -* CH3C(CH3)2COOC5 ÷ H20 (or NaOH) H000H (or Na) + CH3COOH (or Na) + CsOH -* Cs000-CO-CH2Cs + H20 (or NaOH) In a further aspect the invention provides a modified cellulose material obtained or obtainable by any of the reactions herein described. Methods of producing the modified cellulose materials according to any of the processes herein described form a further aspect of the invention.

As a result of the methods by which the microfibrillated cellulose starting material is produced and the methods used to modify this to incorporate the desired alkali metal ions, it will be appreciated that the materials that are produced will generally be a mixture of different modified MFC materials, such as a mixture of compounds represented by formula (I), e.g. having different degrees of polymerisation (represented by n), etc. The use of mixtures of modified MFC materials as herein described is encompassed by the invention.

The modified cellulose materials herein described are typically suspended in water or other aqueous solutions, optionally together with other materials, to form a drilling fluid. Whilst it is envisaged that waterwill generally form the base of the drilling fluid, i.e. the fluid will be an aqueous suspension of the materials herein described, it is also possible that an oil-based or oil-emulsion based drilling fluid may be employed.

In the case of an oil-emulsion both oil and water are present. Water-based drilling fluids are, however, preferred since these are most cost effective and environmentally friendly.

The amount of modified cellulose material which may be provided in the drilling and completion fluids herein described may be varied according to their intended use, drilling conditions, the nature and amount of other components in the fluid, nature of the base fluid, etc., but may be readily determined by those skilled in the art.

Generally, the amount may be up to 10 wt.% (based on the total weight of the drilling or completion fluid), preferably up to 5 wt.%, e.g. ito 3 wt.%.

Conventional components used in drilling fluids may also be present. Other materials include other weighting agents such as barite, haematite, and hausmannite ores (e.g. available as MICROMAXTM from Halliburton), bridging agents such as calcium carbonate, filtration control agents, agents for lubrication, fluid loss agents, pH adjusting agents, shale inhibitors, anti-bacterial agents. These may be used in conventional amounts known to those skilled in the art. Where additional weighting agents are present, however, these will generally be used in reduced amounts due to the properties of the modified cellulose materials herein described. In one embodiment no additional viscosity polymers will be present due to the rheology I viscosity properties of the modified cellulose materials.

Sealing or fluid loss agents for pore bridging, e.g. polysaccharides and other polymers, may be provided in the drilling fluids in order to form a filter cake on the formation surface of the well bore. During a drilling operation and afterwards, the filter cake effectively seals the surface of the formation within the well bore. In this way, the well bore can be formed without leakage of the contents of the bore hole (e.g. the drilling fluid) into the formation surface and without leakage from the formation surface into the well bore. Due to the ability of the modified cellulose materials to act as bridging agents and as preventative LCMs (lost circulation materials), the need for any sealing or fluid loss agents may be reduced.

When used in drilling, the drilling fluid may also contain drilling fines such as shale and sandstone fines.

The drilling and completion fluids can be prepared by mixing all of the components together. In the case of a water-based drilling fluid the continuous water phase will generally comprise about 90% by volume of the fluid.

The density of the drilling fluid can be varied in order to provide different density fluids for use in different drilling situations. For example, drilling depth or other variables in the drilling process will impact on the desired density. Typically the density of the drilling fluid may be in the range 1.90 g.cm3to 2.05 g.cm3, more preferably 2.00 g.cm3to 2.05 g.cm3, e.g. 2.03 g.cm3 to 2.05 g.cm3. Similar densities are applicable to completion fluids.

The drilling fluids of the invention can be introduced into the well bore using any conventional technique such as being pumped into the drill pipe. Conventional techniques may also be used to recover the drilling fluids and, if desired, to separate the modified cellulose material such that this may be recycled.

During the course of a drilling operation it may be desirable to use the drilling fluid in combination with other agents, e.g. a foam-suppressing agent. Examples of such agents include n-butyl alcohol, 1-butanol propyl carbinol, etc. Once drilling operations are completed the well is prepared for the completion process, i.e. made ready to be put into production. During the completion process the drilling fluid remaining in the well bore may be displaced by a completion fluid prior to running a production screen into the well. Completion fluids are typically water-based and formulated to have the same density as the mud used for drilling thereby maintaining the hydraulic pressure on the well bore. The materials herein described are equally suited to use as completion fluids.

The materials according to the invention have been described primarily for use in the drilling or completion of a well for the recovery of hydrocarbons such as oil and/or gas. However, these may be used in other drilling operations where drilling fluids are used, such as in perforation, plug and abandonment operations, slot recovery, killing of wells, etc. The fluids are particularly suitable for use in drilling under high temperature and pressure conditions. High pressure is generally considered to be any pressure greater than atmospheric pressure, e.g. reservoir shut-in pressures are generally at least 552 bars (8,000 psi), typically in excess of 690 bars (10,000 psi). High temperatures such as those encountered at the base of a formation (Bottom Hole Temperature) are typically in the range of 149°C to 200°C.

An important advantage of the materials herein described is the ability to adjust the density of the drilling fluid according to the drilling conditions, e.g. drilling depth, type of formation, etc. Density can be adjusted by varying the amount of the alkali metal ion (e.g. caesium) which is present. Density of the drilling fluid can be expected to range from 1.85 glcm3 to 2.00 glcm3. For lower densities, caesium can be replaced by other alkali metals such as potassium and sodium.

The drilling fluids according to the invention enable a variety of densities to be achieved, in turn minimising or eliminating the need for any additional weighting material that may be used in conventional drilling fluids. Preferably the amount of other weighting materials, such as caesium formate, is less than 5 wt.%, preferably less than 1 wt.%, e.g. 0 wt.%.

The materials herein described may further act as sealing or fluid loss agents for pore bridging (also referred to as bridging agents). This may minimise the need for other bridging agents to be present. For example, these may be present in an amount of less than 5 wt.%. Generally the drilling fluids according to the invention will include calcium carbonate in an amount ranging from 40-60 g/litre independent of the final density of the drilling fluid (i.e. the proportion of cellulose material present).

Claims (18)

1. Claims: 1. Use of a microfibrillated cellulose containing one or more alkali metals (e.g. alkali metal ions) as, or as a component of, a drilling or completion fluid.
2. 2. Use as claimed in claim 1, wherein said microfibrillated cellulose is a compound of formula I: [C5H7O2(OH)3. (Q)], (I) wherein: C6H702(OH)3. represents a microfibrillated cellulose unit; (Q) either represents a modified hydroxyl group, or a group which replaces a hydroxyl group present in the cellulose unit, and which contains at least one alkali metal; xis an integer from Ito 3 which indicates the number of hydroxyl groups in each cellulose unit which are either modified or replaced, and which may vary between different cellulose units; and n is an integer which corresponds to the total number of cellulose units.
3. 3. Use as claimed in claim 2, wherein in formula I: n is an integer in the range from 90 to 1 0,000; each xis an integer independently selected from 1, 2 and 3, preferably 1 or 2, e.g. 1; and each moiety Q is independently selected from: -a modified hydroxyl group of formula -aM in which M represents an alkali metal ion; and -an organic group containing at least one (e.g. one or two, preferably one) alkali metal, preferably at least one alkali metal ion.
4. 4. Use as claimed in claim 1, wherein said microfibrillated cellulose is a compound of formula II: [C6H7O2(OH)3. (aM),<] (II) wherein M, x and n are as defined in claim 2 or claim 3.
5. 5. Use as claimed in claim 2 or claim 3, wherein each moiety 0 independently represents an organic group which contains at least one alkali metal, from 1 to 20 carbon atoms, for example 1 to 10 carbon atoms, e.g. 1 to 6 carbons, and, optionally, one or more oxygen atoms either as substituent groups (e.g. carbonyl groups) or as interrupting groups (e.g. oxo groups).
6. 6. Use as claimed in claim 5, wherein each moiety 0 independently represents an organic group containing one or more carboxylic acid derivatives, for example -000M groups in which M is an alkali metal ion.
7. 7. Use as claimed in claim 6, wherein each moiety 0 independently represents a C1.10 straight-chain or branched alkyl group (e.g. C1.5 alkyl) substituted by one or two, preferably one, -COOM group (where M is an alkali metal ion).
8. 8. Use as claimed in claim 2 or claim 3, wherein each moiety 0 is independently selected from the group consisting of-COOM, -CH2COOM, -C(CH3)2COOM, -CH2C(CH3)2COOM, -O-C(CH2M)2(OC(O)M), and -O-C(OC(O)M)2-CH2M in which M is an alkali metal, preferably an alkali metal ion.
9. 9. Use as claimed in any one of claims 2 to 8, wherein xis 1.
10. 10. Use as claimed in any one of claims 2 to 9, wherein each group M represents a caesium ion and/or each alkali metal is caesium.
11. 11. A drilling or completion fluid comprising a microfibrillated cellulose as defined in any one of claims ito 10, together with at least one additional material conventionally used in a drilling or completion fluid, preferably at least one material selected from the group consisting of bridging materials, weighting agents, brines, viscosifiers, fluid loss control additives, ph adjusting agents, lubricants, defoamers, agents which act to minimise bacterial growth, and bactericides.
12. 12. A drilling or completion fluid as claimed in claim ii, wherein said microfibrillated cellulose is present in said fluid in an amount of up to 5 wt.%.
13. 13. A method of drilling a well in the presence of a drilling fluid as claimed in claim 11.
14. 14. A method of completing a well in the presence of a completion fluid as claimed in claim 11.
15. 15. A microfibrillated cellulose material as defined in any one of claims ito 11.
16. 16. A method of making a microfibrillated cellulose material as claimed in claim 15, said method comprising the step of reacting a microfibrillated cellulose and an alkali metal hydroxide (e.g. caesium hydroxide).
17. 17. A method of making a microfibrillated cellulose material as claimed in claim 15, said method comprising at least one of the following reactions: Reaction i: [C6H7O2(OH)3] + x HCOOM [C8H702(OH)3x(COOM),Jri + x H20 Reaction 2: [C5H7O2(OH)3] + x CH3COOM [C8H702(OH)3.x(CH2COOM)x]r, + x H20 Reaction 3: [C5H7O2(OH)3] + x CH3-C(CH3)2COOM - [C5H7O2(OH)3.(C(CH3)2COOM)J + x CH3OH or [C5H7O2(OH)3], + x CH3-C(CH3)2COOM -* [C3H702(OH)3x(CHrC(CH3)000M),.jri + x CH3OH + x H20 Reaction 4: [C5H7O2(OH)3], + x MCOO-CO-CH2M -, [(C5H7O2(OH)3(O-C(CH2M)2(OC(O)M))1],1 or [C5H7O2(OH)3] + x MCOO-CO-CH2M -[(C5H7O2(OH)3.(O-C(CH2M)(OC(O)M)2)J wherein [C6H702(OH)3],, represents a microfibrillated cellulose, and M is an alkali metal, preferably an alkali metal ion, e.g. caesium.
18. 18. A modified microfibrillated cellulose material obtainable by a method as claimed in claim 16 or claim 17.