This invention relates to drilling fluid.

Drilling fluids (or "mud") are essential to the completion of a wellbore and are circulated through a central drill string, pass through a drill bit into a wellbore and return via an annular space to the surface.

Treatment reprocessing and maintenance is carried out using surface equipment and the fluid is recirculated for continued drilling.

Physical properties of the drilling fluid such as viscosity, density, salinity and filtration may be modified by chemical addition as necessary.

Commonly used drilling fluids are based upon aqueous solutions and salt and are known as water based drilling fluids. Such fluids contain water as the continuous phase in the system and a quantity of a dissolved salt such as sodium chloride, potassium chloride, calcium chloride or other material at any

concentration up to saturation. Other additives such as bentonite and water soluble polymers may be added to give the desired rheological properties and to provide filtration control during drilling. Further compounds such as barite, dense solids, dispersants and inhibitors may be added to provide a fully formulated system.

Water based drilling fluids are relatively cheap to produce and have a broad application. However, the presentation of water to the drilled wellbore rocks can lead to reaction and hydration of native shales, clays and salts which may disperse or dissolve in the drilling fluid. These reactions lead to instability of the wellbore with enlargement of the drilled hole, change in the drilling fluid properties, excessive torque and general problems which can lead to collapse and loss of the well.

In order to reduce reaction of the water based drilling fluid with the drilled formation steps may be taken to control the type of dissolved salt used, the concentration applied and to add inhibiting polymers and compounds which reduce clay hydration. For example, salt sections may be drilled using fluids in which the aqueous phase is saturated with the salt type being drilled to prevent any more dissolution of the solid. Shale hydration may be reduced by the use of potassium or calcium brines and protective polymers such as polyacrylamide.

However, even with the use of inhibitive water based drilling fluids complete well control cannot be achieved and a limited thermal stability results in only partially successful drilling through hot,

reactive wells.

To overcome these difficulties the use of oil based drilling fluid or "inverts" has seen considerable growth over the past 10 years.

These fluids are produced using mineral oil as the continuous phase of an emulsion containing an internal dispersed brine phase. The brine phase normally consists of calcium chloride in water which is tightly emulsified into the base oil by the use of specific surfactants. The invert fluid then presents an inert oil phase to the drilled section during hole making operations. This is non-reactive to clay, shale and salt sections which results in stable wellbores, rapid drilling and improved well productivity.

The emulsification of brine into the base oil has the ability to cheapen the overall formulation, improve system rheology and importantly control filtration properties of the drilling fluid system.

Oil based fluids are also effective at elevated temperatures above 100°C and typically as high as 200°C.

For these reasons oil based drilling fluid is extensively used in operations where water based fluids would be problematic.

However, drilled cuttings separated from the mud for disposal are coated with a film of the protective oil based fluid which is difficult to remove, and in offshore operations where drilled cuttings are disposed of by overboard dumping this leads to contamination of

the sea bed by an oil coated cuttings pile. This is resistant to degradation and swamps the immediate drilling area reducing marine life and represents a harmful, toxic environmental pollutant.

For this reason increasing legislative moves are directed by eliminating the use of oil based drilling fluids in offshore drilling and replacement by a more environmentally acceptable substitute.

One means by which oil based fluid performance may be attained using water soluble compounds exists in the production of an emulsion using specific glycols, glycol ethers, hydroxy compounds and oxygenated solvents which are soluble in low salinity media such as fresh water or sea water but which may be caused to precipitate as an insoluble phase when contacted with aqueous brines. The separated solvent phase may then be emulsified using suitable surfactants to act as the continuous phase of the emulsion. Upon disposal the glycol invert then resolubilises on dilution to wash away and degrade without leaving any problematic residue.

Such glycol inverts reproduce the inhibitive properties of oil based fluid in presenting a non-reactive phase to drilled shale, clay and salt reactions. They also possess good thermal stability in maintaining rheology after exposure to high temperatures.

However, current compositions, unlike oil based fluid inverts, do not possess an inherent fluid loss control. That is, the liquid will readily penetrate a porous substrate during drilling and lead to severe mud losses and hole instability. Glycol inverts must therefore be

conditioned in the same way as water based fluid and a separate fluid loss additive provided to control filtration.

We have found that conventional oil and water based fluid loss control additives are not effective in glycol invert fluids, and this is a serious limitation in the application of such drilling fluids since filtration control is an essential requirement for any practical drilling fluid.

To enable application of such glycol based emulsions there exists a need for a material which may be included to provide filtration control and allow successful application of the drilling process.

According to the present invention there is provided drilling fluid containing a phosphate ester of a hydroxy polymer

Preferably the polymer is a phosphate polyalkylene oxide polymer.

The phosphate ester is preferably based upon ethoxylated or propoxylated compounds such as phenols, alcohols, alkyl phenols, glycerol, glycol ethers, ethylene/propylene oxide co-polymers, polyalkylene oxides, associated polyglycol ethers.

These hydroxy compounds may be reacted with a phosphorus chloride such as phosphorus oxychloride or phosphorus trichloride to provide the reacted phosphate or phosphate esters.

The structure of the hydroxy compounds suitable for

phosphatisation to yield fluid loss additives may advantageously be:

R[ (-OC.H ₄ ) _m (-OC ₃ H ₆ ) _n (-OC ₄ 0H ₈ ) _p -OH] _r

where R may be an alkyl grouping, alkylaromatic, alkoxide group, substituted alkyl, substituted alkylaromatic or substituted aromatic function, may take values of 5-500, n and p may be integers from 0-500 and r will be an integer from 1-3.

Preferably the hydroxy polymer is a polyalkylene oxide polymer.

Preferably also the phosphate ester also acts as an emulsifier.

The phosphate ester of the hydroxy polymer is typically present in an amount of from 0.1 to 10% by weight.

In general the ethylene oxide content is adjusted to give good solvation in the glycol emulsion base but poor solubility in the emulsion brine phase.

Preferably the hydroxy compound is based upon ethylene oxide/propylene oxide (EO/PO) co-polymers to give good thermal stability when phosphated.

Preferably phosphatisation is conducted using phosphorus oxychloride to yield the acid phosphate which may be added to the glycol emulsion to control fluid loss.

Other hydroxy compounds such as alcohols and ethoxylated alcohols may be used to provide fluid loss

control after phosphatisation, but they have limited thermal stability.

The drilling fluid of this invention is preferably in the form of an emulsion of an organic fluid which is at least 40% soluble in water and a salt-containing aqueous solution wherein the organic fluid is insolubilised in the salt-containing aqueous solution.

The organic fluid may be selected from ethers, esters, alkanolamines, glycols, glycol ethers and alcohols.

Preferably the polymer is present in an amount of from 0.1 to 10% by weight.

Illustrations of drilling fluids which are not in accordance with the invention will now be described in the following Comparative Examples.

COMPARATIVE EXAMPLE 1

A glycol ether invert emulsion was prepared using ethoxypropoxypropanol as a base fluid (EDP) and an EDP/water ratio of 70/30 using a laboratory Silverson mixer.

The mud was weighted to a density of 1.43 (12 ppg) by barite addition and varying emulsifiers added to the system.

Formulation: 197 mis βthoxy propoxy propanoi 0.7 g PB82 polybasic acid ex Union Camp Chemicals 1.3 g tall oil fatty acid Unitol AC ex Union Camp Chemicals 3 g iime 12 g Perchem 97 organoclay ex Akzo Chemicals 82 ml water 35 g 82-85% calcium chloride

180 g baπte

7 g emulsifiers

PB82, tall oil fatty acid and lime are provided as supplemental emulsifiers. Perchem 97 is a gelling agent designed to provide system rheology.

After mixing the above each produced emulsion was hot rolled at 95°C (200°F) for 16 hours and the emulsion properties measured before and after exposure to thermal ageing (BHR/AHR) .

The incorporation of differing emulsifiers gave the following results:

Emulsifier Z-94C M92-120 Kleemui 50

Producer Union Camp Union Camp BW Muds

Type Alkanolimidazoline Bis-imadazoiine Proprietary Amide BHR AHR BHR AHR BHR AHR

Apparent Viscosity/cP 36.5 52.5 30 34.5 52.5 50 Yield Point/Pa 14.9 22.6 5.8 10.1 23.5 19.2 Gel Strength/Pa 8.6/12 13/13.4 2.9/7.2 3.8/5.8 1 1 /12.4 10.6/1 1 .5 Fluid Loss * 89 mis 1 18 mis 86 ml

*Fluid loss measured at 95°C and 500 psi pressure over 30 minutes.

The results show that although stable emulsions may be produced with a variety of emulsifiers fluid loss cannot be adequately controlled and is not an inherent property of the emulsion.

COMPARATIVE EXAMPLE 2

A simplified mud system was used to evaluate a series of non-ionic polymers.

The composition was:

Blends were produced using a laboratory mixer and rheology and filtration properties immediately measured.

Emulsifier PE6100 Dowfax 30C05

Producer BASF Dow Chemicals

Type EO/PO Block Polymer EO/PO Block Polymer

Apparent Viscosity/cP 24.5 29 Yield Point/Pa 5.3 7.2 Gel Strength/Pa 1 .4/4.8 2.0/3.4 Ruid loss: 100 psi/25°C 73 ml 85 ml 500 psi/95 ^β C No control No control

Again although fluid emulsions are produced no fluid loss control is obtained in the system.

Embodiments of the present invention will now be described in the following Examples.

EXAMPLE 1

A series of phosphate emulsifiers were evaluated in a system comprising:

184 g EDP 3 g lime 82 g water 35 g 82-85% calcium chloride 180 g barite 10 g emulsifier

Blends were prepared using a laboratory mixer and rheologies and fluid loss control measured.

Emulsifier Crodafos 03A Crodafos 05A Crodafos 010 A Crodafos N20A Type * 3 mole EO 5 mole EO 10 mole EO 20 mole E0 AV/cp 30.5 25 20 16.5 YP/Pa 7.2 3.8 2.9 0.5 Gels/Pa 2.4/2.9 1 .4/2.9 0.5/1 .4 Fluid Loss: 100 psi/25°C - 66 ml 61 ml 500 psi/95° 140 ml - - 7 ml

*A11 emulsifiers based upon ethoxylated oleyl alcohol as the free phosphate acid.

It can be seen that as the ethylene oxide content of the phosphate ester is increased system rheology drops but fluid loss improves.

At a 20 mole ethylene oxide content particularly good fluid loss control is achieved, although even a 5 mole ethylene oxide the fluid loss control is superior to that in the Comparative Examples.

EXAMPLE 2

The additive Crodafos N20A was formulated into a mud system and properties measured before and after hot rolling at 95°C (200°F) .

Formulation: 184 g EDP 10 g Crodafos N20A oieylaicohoi ethoxy phosphate 6 g lime 7 g Pβrchem 97 gellant 82 g water 35 g 82-85% calcium chloride 180 g barite

After mixing the mud properties were found to be:

BHR AHR AV/cP 26 20.5 YP Pa 3.8 0.5 Gels/Pa 1.0/1.4 0.5/0.5 Fluid loss * 7 ml 48 ml

*Fluid loss at 500 psi and 95°C.

It can be seen that after exposure to elevated temperature fluid loss properties of the simple phosphate ester are reduced.

Examination of other esters demonstrated that for all commercially examined products not forming part of the invention fluid loss could not be controlled after exposure to temperatures of 60°C and above. Such limitations would severely limit glycol emulsion mud usage.

EXAMPLE 3

A series of ethylene oxide/propylene oxide co-polymers of high molecular weight were phosphated by reaction with phosphorous oxychloride to produce phosphate esters.

The phosphatisation was carried out by the combination

of POCl ₃ and the EO/PO polymer with stirring on a hot plate at 80-90°C for 1 hour. Approximately 25% deionised water was then added to scavenge unreacted P0C1 ₃ and the resultant blend tested for fluid loss and emulsification properties.

Four alkylene oxides were initially reacted as follows:

Chemical Manufacturer Formula Weight % Ethylene Oxide PE 3100 BASF 1 100 10 PE 6100 BASF 2000 10 PE 6400 BASF 3000 40 Dowfax 30CO5 Dow 3000 5

Phosphorous oxychloride was added in a theoretical amount to react in a 1:1 ratio with the hydroxy1 content of the polymers and produce the monoester phosphate.

Muds were produced to the formulation:

184 g EDP 10 g phosphate polymer 6 g lime 82 g water 35 g 82-85% calcium chloride 180 g barite - Perchem 97 organoclay

Organoclay content was varied to evaluate influence on rheology. All muds were hot rolled for 16 hours at 95° with the following results:

Polymer Phosphate PE 3100 PE 6100 30C05 Perchem 97 5g 5g 10g

BHR AHR BHR AHR BHR AHR

AV/cP 1 6 21 21.5 21 35 26 YP/Pa 1 .9 2.9 2.4 1 .0 6.7 2.9 Gels/Pa 0.5 0.5 0.5 0.5 1.0 0.5 Fluid loss: 100 psi/25°C 1 ml 500 psi/95 ^β C 8ml 16ml 8ml 8ml 7ml

The mud based upon phosphate PE 6400 proved to be unstable to hot rolling. However, all other examples give good fluid loss control before and after ageing at 95°C.

In comparison with the muds of example 2 where unreacted PE 6100 and 30CO5 are used the benefit of phosphatisation in controlling fluid loss is clearly seen.

EXAMPLE 4

The product Dowfax 30C05 was further reacted to produce a monoester of the phosphate under the following conditions:

100 g of 30C05 and 15.6 g POCl ₃ are charged to a stirred reactor at 25°C stirring is continued for 3 hours after which 2 ml of pyridine is added to scavenge produce hydrochloric acid.

The reactor is left for 48 hours after which 10 ml of water is added to produce a clear yellow solution of the acid phosphate.

A mud formulation was prepared as:

184 g ethoxypropoxypropanol 6 g lime 82 g water 35 g 82% calcium chloride 7 g Perchem 97 organoclay 180 g barite

4 g of the phosphate additive and varying emulsifiers were added at varying levels. Muds were hot rolled at 95°C for 16 hours and properties measured.

Phosphate Ester 4 g 4 g Emulsifier (8 g) 30C05' Z-940 ² BHR AHR BHR AHR AV/cP 26 21.5 23.5 21.5 YP/Pa 2.9 1 .4 4.3 1.4 Gels/Pa 1 .4/3.8 0.5/0.5 1.0/4.3 -/0.5 Fluid loss: 500 psi/95°C - 4 ml - 4.6 ml

' 30CO5 unreacted polymer ~ Z-940 amidoamine emulsifier from Union Camp Chemicals

The results show that in different emulsifier systems the phosphate ester provides equal fluid loss control and therefore acts as a separate fluid loss additive.

EXAMPLE 5

The phosphate fluid additive of Example 6 was compared with Lecethin - a natural phosphate ester used as a drilling fluid emulsifier in a mud formulation consisting of:

Hot rolling was again conducted at 200°F when the following results were obtained:

Additive 4 g Phosphate 5 g Monoester Lecethin BHR AHR BHR AHR

AV/cP 36.5 40 25 unstable

YP/Pa 10.1 3.8 2.9

Gels/Pa 3.8/1 1.5 1.0/1.4 1.0/1.9

Fluid loss:

500 psi/95°C 12.0 mis No control

500 psi/125° 12.4 mis

The use of Lecethin as a glycol invert fluid loss additive is therefore unsuitable whereas the phosphate ester of a polyalkylene glycol gives control up to 125°C.