GB2287049A - Drilling fluid - Google Patents

Abstract

There is provided a drilling fluid comprising an emulsion in which the continuous phase comprises an alkylated cycloalkyl compound. A preferred formula for the cycloalkyl compound is R-CpH2p-2-CnH2n+1 where n is an integer of from 4 to 40; p is 4, 5, 6, 7 or 8; R is a hydrogen atom or a lower alkyl group. The drilling fluid has good biodegradability and limits the formation of toxic compounds which occur using aromatic ring compounds.

Description

"Drillina Fluid' This invention relates to drilling fluid for use in the drilling of wells.

Drilling fluids are circulated down a wellbore during well drilling operations. The fluid is usually pumped down a central drillstring, passes through the drill bit into the wellbore and then returns to the surface.

The fluid is then recovered, solid materials extracted, processed and reused.

Drilling fluids are required to remove rock cuttings generated during the boring process, to lubricate and cool the drill bit and maintain the integrity of the hole. Physical properties of the drilling fluid such as viscosity, density, salinity and filtrate loss may be modified by chemical addition as necessary.

One major problem which occurs in the use of water based drilling fluids is the hydration of rock being drilled; this is particularly acute when the interval contains clays and shales. These materials exhibit a great affinity for water and adsorption leads to swelling of the rock with resultant stresses leading to collapse of the borehole or loss of structure.

Such failures lead to wellbore expansion, stuck pipe, excessive rheology, and general drilling problems.

A second problem with water based drilling fluids which is particularly prevalent in the North Sea is the drilling of so called "salt stringers". These intervals comprise regions of high concentrations of water soluble salts such as sodium, magnesium and potassium chloride which will dissolve in the drilling fluid and lead to hole enlargement, washout and general failure of the wellbore.

One solution to the above problem has been the use of so called "salt saturated" solutions in which a soluble salt, usually sodium chloride, is dissolved at maximum concentration in the aqueous medium and used as the drilling fluid base. Such solutions limit shale hydration and prevent further dissolution of drilled salts into the fluid.

However, salt saturated solutions are expensive, have limitations on the density range which may be used and limit the number of additives which may be used to control the properties of the drilling fluid.

A second and more widely applied solution involves the use of oil based drilling fluids which are usually formulated with mineral oils. These fluids comprise a salt-containing aqueous phase which is tightly emulsified into an external oil phase by the use of suitable surfactants.

Oil based drilling fluids therefore present to the surface of drilled rocks an inert oil phase which will not hydrate shale nor dissolve salt. Further, cuttings recovered from oil based fluids are covered with a thin film of oil which prevent hydration and breakage.

Oil based drilling fluids have a much wider range of density, rheology, thermal stability and application than salt saturated or water based fluids and are widely used.

However, disposal of rock cuttings which contain a significant proportion of water insoluble oil, especially by disposal through marine dumping at the drill site, is becoming environmentally unacceptable.

In attempts to upgrade the performance of water based fluids further additives have been used to attempt to control shale hydration, for example potassium chloride, polyacrylamide, polyglycerols, carboxymethyl derivatives, gilsonite, calcium chloride and sodium silicate. However, none of these systems have proved to match the performance of oil based fluids and importantly have minimal effect in preventing solution of salt sections.

There exists a need for an environmentally acceptable alternative to oil based drilling fluid which exhibits control of both shale hydration and salt dissolution and which may be used over the density range covered by oil based fluids.

Currently-used oil based drilling fluids are described as "low toxicity" by virtue of the highly refined nature of the base oils which contain only a small percentage of aromatic compounds which can be harmful to marine life or to the product handler. However, such fluids are very poorly degraded and will remain as a persistent contaminant at disposal sites for many years.

"Low toxicity" oils are produced by a series of fractionation and occasionally solvent extraction/precipitation processes from crude oils and hence contain a broad range of molecular structures only a small number of which are biodegradable.

However, hydrocarbons having similar structures to mineral oil may be prepared synthetically by polymerisation of ethylene or other unsaturated gases and liquids in manufacturing processes such as the Shell higher olefins process (SHOP). The resultant polyalphaolefins (PAO) are high purity compounds which because of the linear structure are highly biodegradable. Such a property would make a highly desirable alternative fluid to conventional mineral oil based drilling fluids.

However, another desirable property of the oil component of an oil based drilling fluid is that the oil should have a high flash point to ensure safety in use and a low freezing point to enable liquid handling under the low temperatures experienced during winter use or in low temperature regions of the world.

The flash point of a polyalphaolefin increases as the molecular weight increases but unfortunately the freezing point also rapidly increases such that liquid handling becomes difficult.

In addition polyalphaolefins contain a reactive unsaturated terminal grouping which is prone to oxidation, polymerisation and undesirable reactions which can lead to a change in the physical properties of the fluid and could cause problems during the drilling process.

Other highly refined mineral oils such as liquid paraffins or polyalphaolefins stabilised by hydrogenation to yield liquid paraffins also suffer from the problem of high freezing point in high flash point fractions.

In WO-A-94/12589 there is disclosed the use of linear alkyl benzene derivatives as the continuous phase of an emulsion suitable for drilling purposes. It has now been found that further advantages are achieved if a similar compound type is used in saturated form.

According to the present invention there is provided drilling fluid comprising an emulsion in which the continuous phase comprises an alkyl a ted cycloalkyl compound.

The term "cycloalkyl compound" includes, inter alia, cyclopentyl, cyclohexyl and cycloheptyl compounds. The cycloalkyl compound of the invention may have one or two alkyl groups attached to the cyclic ring.

According to the present invention an alkylated cyclohexyl compound may be prepared by reaction of polyalphaolefin with benzene or a benzene derivative (such as toluene or alkyl benzene, eg ethyl benzene) in the presence of a suitable catalyst followed further by hydrogenation effected by standard methods. Thus in one sense, the cyclohexyl compounds of the present invention may be considered as hydrogenated alkyl benzene compounds. Compounds of the invention exhibit a high flash point, low freezing point, stable liquids of good biodegradability which can be advantageously used to replace mineral oil in drilling mud.

The drilling fluid of the invention may be used to replace conventional "clean oil" drilling muds but is inherently biodegradable and may be treated or disposed of safely to the surrounding environment.

In addition the replacement of paraffinic "clean oil" by an alkylated cycloalkyl compound considerably increases the polarity of the drilling fluid oil phase such that improved surfactant, emulsion and gellant characteristics are obtained from mud additives designed to effect the mud emulsion and convey suitable rheology to the system.

A preferred structure of the alkylated cycloalkyl compound used as the hydrocarbon phase of the drilling fluid emulsion is given by the formula: R'CpH2p-2'CnH2n+l where n is an integer having a value of 4 to 40, preferably 4 to 30 and most preferably 4 to 20; p is 4, 5, 6, 7 or 8, preferably 5 or 6; and R may be a hydrogen atom or a lower alkyl group. Preferably R may be a hydrogen atom or R may be a -CH3, -CzH5 or -C3H7 group.

A more preferred structure of the compounds of the invention is given by the formula: R'C6HIO'CnH2n+l wherein R is as defined above, but is preferably H; and n is an integer having a value of 4 to 20.

The minimisation of branched alkyl side-chains attached to the cycloalkyl ring may be desirable to maximise biodegradability of the fluid.

Suitable compounds may for example be produced by the reaction of chlorinated paraffins or olefins with benzene in the presence of Friedel-Crafts catalyst, or the direct reaction of polyalphaolefin with benzene in the presence of hydrogen fluoride.

The resultant alkylbenzene may then be hydrogenated by the direct addition of hydrogen in the presence of a catalyst such as nickel or by the other methods known to the industry.

The compounds of the invention may then be used as the external phase of an oil based emulsion at preferable oil/water ratios varying from 25/75 up to (but not including) 100/0, for example from 25/75 to 99/1, in a similar manner to conventional oil muds.

Other additives known to those skilled in the art of mud formation may be included in the composition such as organoclay viscosifiers, fluid loss additives, weighting agents (such as barite and haematite), and speciality polymers.

Gelling agents, viscosity-controlling agents and water-soluble salts may also be present. Likewise a hydrocarbon oil and/or oil-soluble ester may optionally be included in the continuous phase of the emulsion.

The emulsified water content of the drilling fluid may contain dissolved salts such as sodium chloride, potassium chloride, calcium chloride, potassium acetate or any other soluble material added to adjust the resultant salt solution and drilling fluid density or to change the brine properties to enhance drilling.

The emulsified brine may also contain natural brines such as sea water, aquifer fluids or may be fresh water of minimal dissolved salt content.

A component of the drilling fluid composition is preferably a surfactant which emulsifies the aqueous phase into the external phase comprising a compound of the invention and may typically be an organic acid, amide, ethoxylate, amine, phosphate, propoxylate or combination thereof.

The use of the alkylated cycloalkyl compounds reduces the toxicity to marine organisms which can arise under certain circumstances by the metabolism or degradation of standard linear alkyl benezene fractions, such as those disclosed in WO-A-94/12589.

Use of a compound according to the invention prevents the undesirable formation of toxic components such as toluene, xylene, benzene, phenol, etc by converting the aromatic crude to a non-toxic cycloalkyl ring.

Although the full mechanism of the metabolism of such components is not fully understood there is clear evidence that use of muds having a fully saturated external phase as described herein reduces the toxicity of drilling to specific organisms.

Further, the present invention provides the use of the drilling fluids as described above in drilling operations.

In addition the present invention provides a method of drilling in which a drilling fluid in accordance with the invention is used.

Embodiments of the invention will be described by way of illustration in the following Examples.

EXAMPLES 1. Reaction to Oilfield Elastomers It is important that any fluid used as a drilling mud base should show minimal reaction to elastomeric compounds used in drilling operations in order to reduce the incidence of material failure.

Hydrogenation of the linear alkyl benzene reduces polarity of the compound and advantageously improves compatibility with elastomeric compounds as illustrated below.

Elastomer tests conducted by a 2 week soak test at 900C immersed in the test fluid.

Base Fluid Nitrile Rubber Neoprene Rubber Wt Increase Hardness Wt Increase Hardness None - 75 - 65 P400 16% 60 95% 19 P500 18% 65 138% 5 P600 13% 65 114% 10 Shellsol DMA 2% 74 41% 30 P400H 2% 78 49% 30 P585H 1% 75 50% 25 Desirable properties are a low increase in weight and high hardness (minimal change) value.

In the above Shellsol DMA is a low aromatics drilling fluid base (prior art) used in conventional oil based muds and supplied by Shell Chemicals.

All other products are supplied by Petresa, Petroquimica Espanola S.A., Madrid as: P400 Conventional LAB of 232 molecular weight P500 Conventional LAB of 236 molecular weight P600 Conventional LAB of 262 molecular weight P400H Hydrogenated version of P400 P585H Hydrogenated version of P500/P600.

The advantage of hydrogenation in producing a fluid of much improved elastomer compatibility equivalcnt to a conventional mineral oil is clearly seen.

2. Physical Properties In order to be suitable as an oilfield drilling fluid base it is important that the hydrocarbon base retains a high flash point, low pour point and low viscosity to optimise on mud properties.

Conventional saturated linear paraffins which are sold as biodegradable oils cannot achieve these requirements whereas linear alkyl benzenes fully fulfil these needs.

Hydrogenation raises viscosity but still produces highly acceptable pour point and flash point oils.

To be suitable as a drilling fluid the hydrocarbon fluid for safety and handling purposes should have a pour point of -5 C or below and flash point above +100 C.

Base oil Pour point/ C Flash Point/ C Viscosity 100 F/cSt P400 < -50 126 3.99 P500 < . -50 130 4.40 P600 < -50 156 5.80 Petrepar n-clO < -25 50 1.00 Petrepar 120 -23 74 1.40 Petrepar n-c14 +4 104 2.20 Petrepar 147 +5 105 2.40 P400H < -25 131 5.80 P585H < -25 156 8.00 In the above Petrepar products are a range of linear paraffins produced by Petresa as:: Petrepar n-cX0 Linear paraffin of 142 molecular weight Petrepar 120 Linear paraffin of 164 molecular weight Petrepar n-cl4 Linear paraffin of 198 molecular weight Petrepar 147 Linear paraffin of 205 molecular weight.

As can be seen, increasing the linear paraffin molecular weight to raise flash point to acceptable values results in a rise in pour point to an unacceptable level.

However the hydrogenated LAB's , P4OOH and P585H retain very high flash point and low pour point making them highly acceptable base fluids for drilling use.

The increase in viscosity may be obviated by blending with linear paraffin without loss of flash point at pour point requirements.

Mixture of Pour Point/ C Flash point/ C Viscosity P585H/Petrepar 147 100 F/cSt 80/20 -17 132 5.39 70/30 -11 125 4 5.0X 60/40 -6 118 4.36 Such blends may be advantageously used to reduce viscosity where this is a particular concern.

3. Drilling Fluid Preparation The properties of a standard drilling fluid formulation based upon standard mineral oil (Shellsol DMA), current use LAB (P400) and the objects of the invention P400H and P585H are given below.

In all cases it can be seen that acceptable invert emulsion drilling fluids are produced.

Mud formation: 180 ml of base oil 3 g of Unitol AC (fatty acid from Union Camp Chemicals) 7 g of Unimul M92-47 (fatty amide emulsifier from Union Camp Chemicals) 1.5 g of organoclay viscosifier 51.9 g of 82-85Uo purity calcium chloride 118.1 ml of water 78.2 g of barium sulphate.

The above was mixed at low speed on a Hamilton beach mixer for 1 hour to give a 60/40 oil/water ratio emulsion of 1.2 g/ml density.

Mud properties were then measured using standard test equipment to API specification.

Shellsol DMA P400 P400H P585H Electrical Stability/volts 123 161 215 280 Apparent Viscosity/cP 18.5 24 31 42 Plastic Viscosity/cP 14 21 26 33 Yield Viscosity/Pa 4.32 2.88 4.8 8.64 Gel Strengths/Pa 2.4/2.4 1.92/2.4 2.4/2.X8 8.36/3.84 The hydrogenated LAB systems based on P400H and P585H thereof produce drilling fluids of acceptable quality and stability for field use.

4. Biodegradability The biodegradability of a number of hydrocarbons was studied in a simple closed bottle test.

100 mg/litre of the test oil was added to North Sea sea water in a darkened bottle fitted with magnetic stirrer.

0.1% of a non degradable ethylene oxide/propylene oxide block copolymer surfactant was added to aid oil dispersion and the bottle sealed and attached to a mercury manometer.

It should be noted that no additional nutrients or microbes were added to the system.

Test bottles were then agitated and oxygen consumption noted against a blank reference bottle by the mercury manometer.

The difference in oxygen consumption between test jars and reference jar is indicative of biodegradation of hydrocarbons to yield carbon dioxide and water.

Knowing molecular weight a percentage biodegradation may then be calculated.

Fluid % Degradation Reference Blank 0 Sodium Acetate (control) 72 in 14 days P400 14 in 33 days P600 16 in 33 days P400H 28 in 16 days P585H 30 in 18 days P400H 38 in 18 days Good biodegradability is seen with hydrogenation of LAB appearing to improve the degradation rate which is advantageous in offshore drilling applications.

Claims (10)

CLAINS

1. A drilling fluid comprising an emulsion in which the continuous phase comprises an alkylated cycloalkyl compound.

2. A drilling fluid as claimed in Claim 1 wherein said cycloalkyl compound has the formula R~CpH2p 2~CnH2n+1 wherein n is an integer of from 4 to 40; p is 4, 5, 6, 7 or 8; R is a hydrogen atom or a lower alkyl group.

3. A drilling fluid as claimed in either one of Claims 1 and 2 wherein said alkylated cycloalkyl compound is a cyclopentyl, cyclohexyl or cycloheptyl moiety.

4. A drilling fluid as claimed in any one of Claims 1 to 3 wherein said cycloalkyl compound has the formula R'C6H1O'CnHZn+l wherein group R is as defined in Claim 2 and n is an integer of from 4 to 20.

5. A drilling fluid as claimed in either one of Claims 2 and 4 wherein R is a hydrogen atom.

6. A drilling fluid emulsion according to any one of the preceding Claims having an oil/water ratio of from 25/75 up to (but not including) 100/0.

7. A drilling fluid emulsion according to any one of the preceding Claims comprising gelling agents, viscosity controlling agents, water-soluble salts, hydrocarbon oil or oil-soluble ester.

8. A drilling fluid as claimed in any one of Claims 1 to 7 further including a surfactant selected from an organic acid, amide, ethoxylate, amine, phosphate, propoxylate or any combination thereof.

9. Use of a drilling fluid as claimed in any one of Claims 1 to 8 in drilling operations.

10. A method of drilling, wherein a drilling fluid as claimed in any one of Claims 1 to 8 is used.