WO2008050138A1

WO2008050138A1 - Method of remediating well bore drilling cuttings

Info

Publication number: WO2008050138A1
Application number: PCT/GB2007/004089
Authority: WO
Inventors: Simon Wait; Robert Preston Schlemmer
Original assignee: Kmc Oiltools B.V.
Priority date: 2006-10-27
Filing date: 2007-10-26
Publication date: 2008-05-02

Abstract

The present invention relates to the field of well bore drilling for access to subterranean stores of resources such as petroleum oil and natural gas. In one aspect the invention seeks to provide a method of treating drill cuttings and spoils which renders them suitable for safe disposal or suitable for use as a fertilizer in agriculture. According to one aspect of the present invention there is provided a method of treating drill cuttings comprising drilling fluid and spoil, which drilling fluid comprises water, silicate ions, hydroxide ions, potassium ions and an optional activity control agent and an optional density modifier, the method comprising treating the cuttings with a precipitation agent which causes the silicate anions to from a substantially insoluble solid silicate which precipitates out of solution in the drilling fluid. The precipitation agent may comprise a calcium salt which reacts to form an insoluble or substantially insoluble calcium silicate which precipitates from the solution. The calcium salt preferably comprises an anion which is suitable for use in forming a fertilizer, such as calcium nitrate or calcium ammonium nitrate. The activity control agent preferably includes anions of carbonate acetate (and any other appropriate organic anions), or sulphate. The method makes is possible to avoid the presence of harmful amounts of chloride and sodium in the treated drill cuttings.

Description

Method of remediating well bore drilling cuttings

The present invention relates to the field of well bore drilling for access to subterranean stores of resources such as petroleum oil and natural gas. In particular, the present invention concerns a well bore drilling fluid, a method of drilling using the fluid and drill cuttings obtained by the method. The present invention involves the use of a water-based rather than oil based drilling fluid. The cuttings may be suitable for use in agriculture.

Oil based (oleaginous) drilling fluids are well known in the art and are used, essentially, to lubricate well bores during drilling, and to provide a carrier for the removal of drilling spoil or drill cuttings back to the surface during drilling. The drill fluid contaminates and makes up a significant portion of the drill cuttings carried to the surface. Such cuttings are considered to be an environmentally harmful waste product, as they may be contaminated with oil and chemicals added to the drilling fluid so as to optimize drilling performance in the particular location.

The treatment and disposal of drilling wastes produced both onshore and offshore presents many challenges. In particular, the treatment of wastes onshore raises issues relating to the current lack of recycling options, and the potential long term liability associated with disposal.

Where possible, the preference is to provide a system whereby the drilling fluid is tailor made to minimise the onshore disposal issues relating to the drilling waste generated. Drill cuttings in particular present large volumes of wastes which are typically treated and disposed of onshore into licensed disposal sites. Contaminants on drill cuttings normally arise from the drilling fluid associated with them, rather than the rock itself. A number of contaminants and properties associated with drilling fluids are of particular concern including:

• Hydrocarbons - which have hazardous properties • Organic surfactants

• Chlorides and sodium cations - which may leach into the environment

• Alkalinity - potentially corrosive

• Heavy metals - environmental poisons In recent years work has been conducted to make drill cuttings more environmentally friendly in response to regulatory conditions. On the one hand efforts have been made to provide treatments of oil-containing based drill cuttings which render them less toxic and suitable for disposal or use as topsoil. On the other hand attempts have been made to use less toxic drilling fluids such as water-based silicate fluids. Specific examples of relevant disclosures are discussed below.

US-A-6,838,082 (Growcock) discloses a biodegradable oil-based well bore fluid. The oil part includes a C(_u.₁₈) paraffin which is emulsified. The waste may be mixed with a filler such as sawdust to provide a vermiculture compost. In preferred embodiments the use of halide salts is said to be avoided. US 2005/0090405 (Growcock) is a related document and has claims directed to a drilling fluid rather than the compost. This document also has method claims directed to a method of treating drill cuttings to provide a remediated soil. The fluid is free of halogen ions and has instead salts of biodegradable anions.

WO 2005/0082553 (Adams) discloses a chemical-biological stabilization process for repairing soils and cuttings contaminated with oils and petroleum derivatives. Calcium oxides and organic conditioners are applied to dilute the total petroleum hydrocarbon concentration to a non toxic level.

US 6711830 (Gary Hensley et al) discloses a drill cuttings treatment system. This provides a method for treating cuttings containing oil-based drilling fluid. A non-toxic additive is used to reduce oil content to less than -1%. The mixture additive and cuttings mixture is disposed of in the immediate vicinity of the well.

US 7059805 (Addison) discloses a method for the environmental pacification of drill cuttings. Oil or water based drill cuttings are treated by adding limestone coke ash. This entraps the wet drilling fluid by binding to the ash. The resulting material may be used as a road base.

WO 2006/032931 (Strong et al) discloses a method for remediating drill cuttings which involves heat treating the cuttings to produce solids and a by-product oil/water liquid stream. A weir tank is used to separate remaining solids from the liquid stream. WO 03/072903 (Cordova) discloses a drill cutting treatment method for safe disposal of oil- contaminated clay cuttings obtained from an oil-based drilling fluid. Oil is removed by sulphuric acid treatment and alkaline (lime) treatment to leave a soil which may be safely disposed of.

WO 2004/013455 (Killham et al) discloses a method for. the decontamination of drill cuttings and other waste material. A steam generator is used to drive off oil from the cuttings. The ratio of C:N:P is monitored to ensure that the cuttings are compatible with micro-organisms added to the cuttings. Activated carbon and desalination stages may also be included.

US 2001/0039887 (Reddoch) discloses a drilling fluid recovery and cuttings processing system. This system uses a cuttings press to remove drilling fluid for re-use and a dryer to dry the pressed cuttings. Lime and fly ash may be added to the solids before disposal of the spoil in the environment.

US-A-5,725,771 (Aliphat et al) discloses a process for enzyme pre-treatment of drill cuttings. A lipase is used to hydrolyse synthetic (ester based) drilling mud. This pre- treatment helps render the cuttings suitable for disposal into the sea (or elsewhere).

Silicates in their most basic form were first used in the 1930' s to drill wells, see Vietti, W. V. and Garrison, A.D.: "Method of drilling wells," U.S. patent 2,165,824 (1939). See also Baker, CL. and Garrison, CL. : 'The chemical control of heaving shale," The Oil Weekly (February 6, 1939). A number of technical problems were encountered and the use of silicates petered out. However, silicate mud systems were revisited in the 1990's and since then many successful wells have been drilled both onshore and offshore.

Silicates systems in their present form are characterised by their high pH from the continual addition of hydroxide ions. Silicate is normally added in the sodium salt form which at high pH forms. Sodium silicate is typically a mixture of sodium , monosilicate, disilicate, trisilicate, and/or tetrasilicate species characterized by a net ratio "n" of about 1.5 to 3.3 represented by the general formula Na₂O^»(SiO₂)_n. A typical formulation is displayed in Table 1 below:

WO97/05212 (Urquhart) discloses an aqueous drilling fluid based upon potassium silicate and which is suitable for drilling through reactive clays, shale and chalk.

DE-A- 19915090 discloses a drilling fluid comprising sodium and/or potassium silicate so as to prevent swelling of clay during drilling.

The Society of Petroleum Engineers (SPE) publication SPE 86700 (29-31 March 2004) discloses a water-based drilling fluid in which potassium silicate is used as the base silicate rather than sodium silicate. This was found to be a viable alternative to oil based drilling fluids. This system improves the overall environmental performance of the system in relation to waste disposal. Furthermore, the author describes the success of adding a calcium sulphate and an acid to treat the drilling fluid. However, the system still contains leachable sodium and chloride ions which have potentially harmful environmental impact.

In a traditional silicate fluid formulation, a number of additives are present that are not beneficial for waste recycling including: • Sodium - sodium silicate and sodium hydroxide are key ingredients of the system. However, sodium is known for its cation exchange capacity in soils, reducing the quality of the soil it contaminates.

• Chloride - potassium chloride is also a key ingredient. Chlorides per se are not a problem in soil and are found naturally in most soils with an average level or approximately lOOppm. However excessive levels, in combination with the metal cation, can cause a negative osmotic effect where plants are unable to take up water into the roots.

• Alkalinity - silicate requires an unusually high pH relative to other well bore mud systems to ensure the silicate is of the form required. This is typically achieved through the use of a metal hydroxide. Such a high pH would not comply with legislation in many parts of the world, or would increase the hazard category of the waste, if left untreated.

• Heavy Metals - these are often associated with barite and can be minimised through the use of low heavy metal ore.

Thus, a typical silicate drilling fluid system appears to have many or most of the contaminants which one tries to avoid.

In one aspect the invention seeks to provide a method of treating drill cuttings and spoils which renders them suitable for safe disposal or suitable for use as a fertilizer in agriculture.

In a preferred aspect the drill cuttings have a plant growth promoting effect and thus may be used as a fertilizer to improve the fertility of top soil in agriculture.

According to one aspect of the present invention there is provided a method of treating drill cuttings comprising drilling fluid and spoil, which drilling fluid comprises water, silicate ions, hydroxide ions, and potassium ions, the method comprising treating the cuttings with a precipitation agent which causes the silicate anions to from a substantially insoluble silicate which precipitates out of solution in the drilling fluid.

The precipitation agent may comprise a calcium salt which reacts to form an insoluble or substantially insoluble calcium silicate which precipitates from the solution. The calcium salt preferably comprises an anion which is suitable for use in forming a fertilizer, such as calcium nitrate or calcium ammonium nitrate. A preferred precipitation agent is CAN which is a commercially available fertilizer stock.

The drilling fluid may comprises sulphate ions which react with the precipitation agent to form a low solubility sulphate which precipitates, for example calcium sulphate.

According to another aspect of the invention the method further comprises treating the cuttings with a pH modifying agent so as to achieve a target pH in the cuttings. The pH modifying agent may be one or more mineral acid or acid anhydride capable of neutralising hydroxide anions. Preferably the modifying agent comprises a phosphorus-containing or nitrogen-containing acid or acid anhydride. The modifying agent may be an oxoacid of phosphorus, such as phosphoric acid or phosphorus anhydride. The modifying agent may

V comprise an oxoacid of nitrogen, such as nitric acid.

A target pH may depend upon the intended use of the cuttings in agriculture and the nature of the ecology of the area. However, a typical target pH is between 6 and 8.5.

In a preferred composition the drilling fluid comprises potassium silicate, potassium sulphate, potassium hydroxide, a density modifier - such as a mineral filler - and water. This drilling fluid may further comprise one or more soluble salts selected from: potassium sulphate, potassium nitrate, potassium acetate (or other potassium salt of organic acids), potassium carbonate, or mixtures thereof.

In this method, the initial drilling fluid composition should preferably be free of added chlorine and sodium ions, although trace amounts may be present. Certain levels of chloride and sodium ions are however acceptable provided that the final treated level in the cuttings is low enough to avoid contamination of land. Preferably the levels should be consistent with good plant growth. Thus typically chloride levels should be below 140 ppm and preferably below 70 ppm. Levels of chloride above 5 to 6ppm are typically required in soil. The amount of sodium present in the cuttings should be low enough so that less than 10% of the ion exchange capacity of the soil is occupied by any sodium present, and preferably less than this.

In general the sodium ion and chloride content of all components (whether cuttings or treated drilling fluid) should be limited to concentrations which are physiologically benign to potential plant development and growth. Thus, the cumulative sodium ion and chloride content of all components of a drilling fluid composition, whether added in the original mix, or by means of maintenance additions to the fluid, should be limited to concentrations which are physiologically benign to potential plant growth. Preferably the concentrations should be well below this level so that any chloride or sodium content of the spoil does not cause the cumulative concentration of the cuttings to rise above benign levels. Similarly, the calculated cumulative sodium ion and chloride content of the treated cuttings should be at a level or below that which will not cause such damage to plants that they cannot grow.

According to another aspect of the invention there is provided a well drilling method which comprises:

(i) providing an aqueous, non-oleaginous drilling fluid, which drilling fluid comprises metal ions, silicate ions and hydroxide ions, along with an optional density modifier, (ii) drilling a subterranean well bore using the drilling fluid, thereby to obtain water based, oil free drill cuttings comprising drill spoil and used drilling fluid, and (ii) treating the drill cuttings using a method as hereinbefore described.

In a preferred aspect there is provided a well drilling method which comprises: (i) providing an aqueous, non-oleaginous drilling fluid, which drilling fluid is free of sodium and chloride atoms and comprises metal ions, silicate ions and hydroxide ions, along with an optional density modifier,

(ii) drilling a subterranean well bore using the drilling fluid, thereby to obtain water based, oil free drill cuttings comprising drill spoil and used drilling fluid, and (iii) treating the drill cuttings- by adding one or more precipitation agents to the cuttings, which agent dissociates to donate nitrate ions to the cuttings and counter ions which combine with the silicate ions to form a stable insoluble silicate precipitate. The nitrate ions may be ammonium nitrates or nitrates.

The treating step (iii) may further comprise adding a pH modifier to modify the pH of the cuttings to a target value, such as nitric acid or an oxoacid of phosphorus such as phosphoric ' acid, or the anhydrides thereof. The metal ions in the drilling fluid in step (i) preferably comprise potassium ions. The drilling fluid in step (i) may further comprise a soluble potassium salt selected from potassium sulphate, potassium acetate (or other potassium salts of organic acids), potassium carbonate and mixtures thereof. The nitrate donating agent in step (iii) is preferably calcium nitrate or calcium ammonium nitrate, which reacts with silicate ions to form calcium silicate and potassium nitrate, or potassium ammonium nitrate.

The drilling fluid of step (i) preferably comprises potassium hydroxide, potassium sulphate and potassium silicate and the treatment at step (iii) preferably comprises adding nitric acid or phosphoric acid (or anhydrides) and calcium nitrate or calcium ammonium nitrate to the drill cuttings, whereby calcium sulphate and calcium silicate are precipitated as insoluble stable compounds and potassium nitrate or potassium ammonium nitrate is left in the cuttings.

In yet another aspect of the invention there is provided remediated drill cuttings obtainable by the methods hereinbefore described.

In yet another aspect of the invention, there is provided a non-oleaginous drilling fluid comprising water, silicate ions,- hydroxide ions, potassium ions and optionally a density modifier and which is free, or essentially free, of added chlorine and sodium ions.

An activity modifying agent may be present to control osmotic pressure during drilling, e.g. carbonate or sulphate. Thus the drilling fluid may further comprise ionic moieties selected from sulphate, nitrate, acetate (or other potassium salt of organic acids), carbonate, or mixtures thereof. The silicate is preferably provided as potassium silicate, similarly the hydroxide may be provided as potassium hydroxide, and the further ionic moiety may be provided as one or more of potassium sulphate, potassium acetate (or other potassium salt of organic acids), potassium carbonate, or mixtures thereof.

In a further aspect of the invention there is provided a method of subterranean drilling using the drilling fluid composition as hereinbefore described. The drilling is preferably conducted through soil thereby producing drill cuttings comprising drilling fluid and soil.

In a preferred aspect of the invention there is provided a drilling fluid comprising:

The density modifier is added to give the fluid sufficient density to perform its role. Suitable materials include finely powdered ores. A typical density modifier is barium sulphate or barite. The ore should be selected to have a low content of heavy metals which are generally regarded as contaminants to be avoided.

As with conventional aqueous drilling fluids, further additives may be present such as viscosity modifiers and filtration controllers. Such additives will be known to the person skilled in the art.

The drilling may be well bore drilling. This may be on land or offshore. The composition of the drilling fluid will typically be tailored to the nature of the rock formations through which drilling will take place.

The metal salt added as a precipitation agent may be a calcium salt which reacts to form calcium silicate, which is a stable insoluble precipitate. Alternatively, the calcium salt may be calcium sulphate, calcium ammonium nitrate or calcium nitrate. The calcium sulphate may be provided as gypsum.

Suitable pH modifiers include mineral acids. A preferred acid is nitric acid. In this way nitrate ions may be donated to the cuttings. Nitrates are known to have a fertilizing effect when added to soils.

Following is a description by way of example only of one method of putting the various aspects of the present invention into effect. The drilling fluid

In the preferred embodiment, the drilling fluid comprises the following essential mineral and aqueous components:

The further additives/modifiers such as filtration controllers and viscosity modifiers may be added in the amounts required to obtain desired rheological or chemical properties.

Well bore drilling This fluid is used, subject to the addition of further optimising agents common in the art, as a drilling fluid in a subterranean drilling process. The drilling is conducted through soil, rock or the like and drill cuttings are returned to the surface as an aqueous slurry comprising drilling spoils and the used drilling fluid.

The actual composition of the spoils will depend upon the composition of the earth through which the drilling is conducted. For example, the spoils may comprise rock particles, shale, sand or soil.

Drill cuttings treatment The drill cuttings are treated at the well site so as to permit disposal without adverse environmental effects. The treatment may be continuous or batch wise, such as in storage reservoirs. Silicate is removed from solution through the addition of a calcium salt to produce insoluble calcium silicate, which is a benign mineral found in nature. The calcium source is conveniently calcium sulphate (waste gypsum or equivalent). An alternative source is calcium nitrate. The addition of calcium nitrate has the benefit of removing soluble sulphate ions from the drill cuttings system whilst having the beneficial result of producing potassium nitrate. The alkalinity (high pH number) of the cuttings is reduced using by a mineral acid such as nitric acid. This reacts with the drilling fluid composition to produce potassium nitrate and water. Sufficient acid is added to reduce the pH of the cuttings to neutral levels, or to a level suited to a particular intended agricultural use, which will depend upon the plant and/or environment.

Taking a simplistic view of the mechanism, and ignoring side reaction involving the drilling spoils and other contaminants, it may be seen that several parallel reactions take place during treatment, as represented in the equations below:

K₂SO₄ + Ca(NOs)₂ ► CaSO₄(D + ² KN⁰S (sulphate precipitation)

Ca(NO₃)₂ + K₂SiO₃ ► CaSiO_3(i) + 2 KNO₃ (calcium silicate precipitation)

KOH + HNO₃ ► H₂O + KNO₃ (potassium hydroxide neutralisation)

Thus in this embodiment, the potassium silicate and the potassium sulphate derived from the drilling fluid can be stabilised in the drilling cuttings into an insoluble mineral (calcium silicate) and a relatively insoluble salt (calcium sulphate) by the addition of a calcium salt such as calcium nitrate. Final neutralisation can be achieved by the addition of an acid. In the case of nitric acid, the product is potassium nitrate.

Potassium nitrate, which is produced by every reaction step mentioned above, is a well known fertiliser. Thus in contrast to cuttings derived from drilling fluids which contain harmful sodium and chlorides, the drilling cuttings treated in accordance with the present invention may be used as fertilizing modifier to be added to soils. Thus the cuttings may be used in agriculture and have a potential commercial value, rather than being a harmful waste product and regulatory liability.

Even when the drill cuttings are simply discarded, they are environmentally friendly and are thus capable of being dispersed in landfill sites. Re-growth of vegetation is encouraged by the nitrates present in the cuttings. Leaching of harmful elements into the soil or water supplies is prevented as these are absent. Comparative example

A typical known water-base, non oleaginous drilling fluid composition is set out below, alongside an invention composition:

It is evident that drilling cuttings from the prior art fluid will contain chloride and sodium ions which may be harmful leachants which can contaminate land fill, water supplies or soil over which the cuttings are spread during disposal. The present invention effectively substitutes benign potassium for sodium, and nitrates for chlorides. Assuming that the drill spoils are not themselves contaminated with additional harmful agents such as heavy metals or radioactivity, the invention composition permits cuttings to be easily treated so as to be rendered environmentally neutral, or indeed beneficial. The treated cuttings may be disposed of without regulatory problems, or indeed sold as a commercially desirable fertilising product.

To summarise, in one aspect the invention seeks to provide a method of treating drill cuttings and spoils which renders them suitable for safe disposal or suitable for use as a fertilizer in agriculture. According to one aspect of the present invention there is provided a method of treating drill cuttings comprising drilling fluid and spoil, which drilling fluid comprises . water, silicate ions, hydroxide ions, potassium ions and an optional activity control agent and an optional density modifier, the method comprising treating the cuttings with a precipitation agent which causes the silicate anions to from a substantially insoluble solid silicate which precipitates out of solution in the drilling fluid. The precipitation agent may comprise a calcium salt which reacts to form an insoluble or substantially insoluble calcium silicate which precipitates from the solution. The calcium salt preferably comprises an anion which is suitable for use in forming a fertilizer, such as calcium nitrate or calcium ammonium nitrate. The activity control agent preferably includes anions of carbonate, acetate (and any other appropriate organic anions), or sulphate. The method makes is possible to avoid the presence of harmful amounts of chloride and sodium in the treated drill cuttings.

Claims

1. A method of treating drill cuttings comprising drilling fluid and spoil, which drilling fluid comprises water, silicate ions, hydroxide ions, and potassium ions, the method comprising treating the cuttings with a precipitation agent which causes the silicate anions to from a substantially insoluble silicate which precipitates out of solution in the drilling fluid.

2. A method as claimed in any of the preceding claims wherein the precipitation agent comprises a calcium salt which reacts to form an insoluble or substantially insoluble calcium silicate which precipitates from the solution.

3. A method as claimed in claim 2 wherein the calcium salt comprises an anion which remains in solution after the calcium silicate precipitates and promotes use of the cuttings as a fertilizer.

4. A method as claimed in claim 2 or claim 3 wherein the precipitation agent comprises calcium nitrate, calcium ammonium nitrate, calcium sulphate, or any mixture thereof.

5. A method as claimed in any preceding claim wherein the drilling fluid further comprises sulphate ions or carbonate ions which react with the cations of the precipitation agent to form a low solubility sulphate or carbonate which precipitates.

6. A method as claimed in any preceding claim further comprising treating the cuttings with a pH modifying agent so as to achieve a target pH in the cuttings.

7. A method as claimed in claim 6 wherein the pH modifying agent is one or more mineral acid or acid anhydride or organic acid capable of neutralising hydroxide anions.

8. A method as claimed in claim 7 wherein the modifying agent comprises a phosphorus- containing or nitrogen-containing acid or acid anhydride.

9. A method as claimed in claim 8 wherein the modifying agent comprises an oxoacid of phosphorus, such as phosphoric acid or phosphorus anhydride.

10. A method as claimed in claim 8 wherein the modifying agent comprises an oxoacid of nitrogen, such as nitric acid.

11. A method as claimed in any preceding claim wherein the drilling fluid comprises: potassium silicate, potassium hydroxide, potassium sulphate and/or potassium carbonate, and a density modifier - such as a mineral filler - and water.

12. A method as claimed in claim 1 wherein the drilling fluid further comprises one or more soluble salts selected from: potassium sulphate, potassium nitrate, potassium acetate, a potassium salt of an organic acid, potassium carbonate, or mixtures thereof.

13. A method as claimed in any preceding claim wherein the concentration of chloride and sodium ions in the treated drill cuttings is low enough for the cuttings to be benign for plant growth.

14. A well drilling method which comprises:

(i) providing an aqueous, non-oleaginous drilling fluid, which drilling fluid comprises metal ions, silicate ions, and hydroxide ions, and optionally sulphate ions or carbonate ions, along with an optional density modifier, (ii) drilling a subterranean well bore using the drilling fluid, thereby to obtain water based, oil free drill cuttings comprising drill spoil and used drilling fluid, and

(ii) treating the drill cuttings using a method according to any of claims 1 to 13.

15. A well drilling method which comprises: (i) providing an aqueous, non-oleaginous drilling fluid, which drilling fluid is free of added chloride or sodium ions, or has a concentration of chloride and sodium ions which is benign to plant growth, and comprises metal ions, silicate ions and hydroxide ions, along with an optional density modifier,

(ii) drilling a subterranean well bore using the drilling fluid, thereby to obtain water based, oil free drill cuttings comprising drill spoil and used drilling fluid, and

(iii) treating the drill cuttings by adding one or more precipitation agents to the cuttings, which agent dissociates to donate nitrate ions to the cuttings and counter ions which combine with the silicate ions to form a stable insoluble silicate precipitate.

16. A method as claimed in claim 15 wherein the precipitation agent comprises a nitrate salt or an ammonium nitrate salt or mixtures thereof.

17. A method as claimed in claim 15 or claim 16 wherein the treating step (iii) further comprises adding a pH modifier to modify the pH of the cuttings to a target value.

18. A method as claimed in claim 17 wherein the pH modifier added in step (Hi), is nitric acid or an oxoacid of phosphorus such as phosphoric acid, or the anhydrides thereof.

19. A method as claimed in any of claims 15 to 18 wherein the metal ions in the drilling fluid in step (i) comprise potassium ions.

20. A method as claimed in any of claims 15 to 19 wherein the drilling fluid in step (i) further comprises a potassium salt selected from potassium sulphate, potassium acetate, a potassium salt of an organic acid, potassium carbonate, and mixtures thereof.

21. A method as claimed in any of claims 15 to 20 wherein the nitrate donating precipitation agent in step (iii) comprises calcium nitrate salt, or mixed calcium and ammonium nitrate salt, which reacts with silicate ions to form calcium silicate and potassium nitrate.

22. A method as claimed in claim 21 wherein the drilling fluid of step (i) comprises potassium hydroxide, potassium sulphate and potassium silicate and wherein the treatment at step (iii) comprises adding nitric acid and calcium nitrate or calcium ammonium nitrate to the drill cuttings, whereby calcium sulphate and calcium silicate are precipitated as insoluble stable compounds and potassium nitrate or ammonium nitrate is left in the cuttings.

23. Nitrate ion or ammonium nitrate-enriched remediated drill cuttings obtainable by the method of any of claims 15 to 22.

24. A non-oleaginous drilling fluid comprising water, silicate ions, hydroxide ions, potassium ions and optionally a density modifier and which is essentially free of added chloride and sodium ions.

25. A drilling fluid as claimed in claim 24 further comprising ionic moieties selected from sulphate, nitrate, acetate or another potassium salt of an organic acids, carbonate, or mixtures thereof.

26. A drilling fluid as claimed in claim 24 or 25 wherein trie silicate is provided as potassium silicate.

27. A drilling fluid is claimed in any of claims 24 to 26 wherein the hydroxide is provided as potassium hydroxide.

28. A drilling fluid as claimed in claims 25 wherein the further ionic moiety is provided as one or more of potassium sulphate, potassium acetate, a potassium salt of an organic acid, potassium carbonate, or mixtures thereof.

29. A method of subterranean drilling using the drilling fluid composition of any of claims 24 to 28.

30. A method as claimed in claim 29 wherein the drilling is conducted through soil thereby producing drill cuttings comprising drilling fluid and soil.

31. Drill cuttings obtainable by the method of any of claims 1 to 14.