TECHNICAL FIELD

The present invention relates to subterranean treatment fluids with improved stability and environmental compatibility based on high internal phase ratio (HIPR) invert emulsions, to their use in subterranean applications. More particularly, the treatment fluids comprise the combination of an ester and an oil soluble acrylic copolymer as emulsion stabilizer.

The present invention further relates to a method for subterranean treatments comprising :

I) providing an invert emulsion subterranean treatment fluid based on high internal phase ratio invert emulsions and comprising the combination of an ester and an oil soluble acrylic copolymer as emulsion stabilizer and II) introducing said fluid into at least a portion of a subterranean formation. PRIOR ART

Emulsions usually comprise two immiscible phases : a continuous (or external) phase and a discontinuous (or internal) phase, the discontinuous phase usually being a liquid dispersed in droplets in the continuous phase.

Oil-in-water emulsions usually include a fluid that is at least partially immiscible in oil (an aqueous-based fluid) as the continuous phase and an oil phase as the discontinuous phase.

Water- in -oil emulsions are the opposite, having the oil phase as the continuous phase and a fluid that is at least partially immiscible in the oil phase (usually an aqueous-based fluid) as the discontinuous phase.

Water- in -oil emulsions are also referred to as invert emulsions. Both kinds of emulsions have been used widely in oil and gas applications, for instance, for drilling and other subterranean treatment applications.

Oil based drilling fluids or muds (OBM) are generally used in the form of invert emulsions, that are preferred when the formation is remarkably sensitive to contact with water and they usually guarantee better lubrication of the drill strings and downhole tools, thinner filter cake formation and better thermal resistance and hole stability, especially for water sensitive formations.

The oil phase/aqueous phase ratio of invert emulsion fluids is traditionally in the range of 55/45 to 85/ 15 v/v. Inverse high internal phase ratio emulsions, i .e. systems possessing a larger volume of internal aqueous phase (> 50% in volume), are highly preferred because of the significant reduction of the oil phase, with its associated costs and possible environmental concern for waste and disposal .

When used in subterranean applications, emulsions undergo exceptional mechanical and thermal stress, and therefore stability is an especially critical aspect of their formulation. For HIPR invert emulsions the stabilization is even more critical .

Emulsions are generally stabilized by addition of one or more emulsion stabilizing agents.

The main stabilizing agents, also referred to as emulsifiers, decrease the interfacial tension between water and oil phases, usually forming an interracial film, thus preventing the droplets coalescence, phase separation and the compromising of the emulsion performance.

To further improve the stability of the emulsion several other additives may be used, like co-emulsifiers, salts, viscosifiers or rheology modifiers.

The emulsifiers that are traditionally used in OBMs have surfactant-character, comprising a hydrophobic portion and a hydrophilic portion, while amine modified bentonites are often used to thicken the oil phase, thus stabilizing the emulsion. However these solid rheology modifiers and the commonly used emulsifiers are under observation in some areas due to environmental concerns. Moreover, drilling fluids lacking organophilic clays are considered to contribute to superior performances of the fluids.

US 4,670,501 and its continuation-in-part US 4,777,200 describe acrylic copolymers that can be used to thicken the oil phase of water in oil emulsion based drilling muds. The copolymers a re substantially uncross-linked copolymers formed from 80- 100% by weight of hydrophobic monomers, 0-20 % by weight (wt) of hydrophilic monomers and in which at least 25 % wt of the monomers a re both polar and hydrophobic and not more than 30% wt of the monomers a re hydrophobic aromatic hydrocarbon monomers. The presence of 7% wt of an emulsifier together with 1.5% wt active copolymer is reported in the exemplified 25/75 v/v water-in-oil emulsion drilling fluid, but nothing is said about the importance or the chemical nature of this emulsifier, that in general it is said that it may be anionic, non-ionic or cationic. Moreover, nothing is said about the possibility of using this polymer in HIPR invert emulsion drilling fluids.

US 2004/0043905 describes the use of a polymer from 2- ethylhexylacrylate/acrylic (99/1), prepared as described in US 4,670,501, to enhance the suspension characteristics of a drilling fluid devoid of organophilic clays. The drilling fluid may comprise one or more emulsifier, but nothing is said about the importance or the chemical nature of this emulsifier and nothing is said about the possibility of using this polymer in HIPR invert emulsion drilling fluid . US 2011/0257051 describes a consolidating fluid comprising a tackifying agent that can also be prepared according to US 4,670,501 and an emulsifying agent that comprises at least one cationic or amphoteric surfactant.

WO 2011/037954 discloses an HIPR invert emulsion treatment fluid characterized by the presence of an alkoxylated ether acid as emulsifier. Although the presence of viscosifiers and/or emulsion stabilizer like organophilic clays or oil soluble polymers is described, conventional emulsifiers (SUREMUL® available from M -I L. L.C. Houston, Texas) are used to demonstrate the impossibility to form stable and manageable HIPR invert emulsion drilling fluids.

For these reasons, there is still the need of emulsifying system for the preparation of high internal phase ratio invert emulsion treatment fluids havingbetter resistance to separation even at high temperature, that do not require the use of solid particles to further stabilize the emulsion and that can be obtai ned more economically and with a lower environmental impact, with the same performance.

It has now been found that the combination of an ester obtained by reaction of a monocarboxylic acid and a polyol (acting as emulsifier), with a specific acrylic, oil soluble polymer (acting as rheology modifier) has excellent stabilizing properties when used in HIPR invert emulsion subterranean treatment fluids, being able to guarantee long term stability of the fluids, even at high temperature. Moreover, this emulsifying system, or emulsion stabilizer, allows an easy prepa ration of the emulsion also in difficult conditions, such as those encountered in on -field operations.

DESCRIPTION OF THE INVENTION

It is, therefore, an object of the present invention an invert emulsion subterranean treatment fluid comprising :

a) an oil phase;

b) an aqueous phase;

c) as emulsifier, from 0.5 to 10 % by weight of an ester obtained by reaction of a linear or branched, saturated or unsaturated, C ₆ -C ₂ 4 a liphatic monocarboxylic acid with a linear or branched, C ₄ -Ci ₂ aliphatic polyol having from 4 to 7 hydroxyls (n), with a degree of esterification comprised between 1 and n-n/4; d) as rheology modifier, from 0. 1 to 5 % by weight of an oil soluble acrylic copolymer obtained by reaction of from 90 to 99.5% by weight of a C ₄ -Ci ₂ alkyl (meth)acrylate and from 0.5 to 10% by weight of an ethylenically unsaturated acid ;

wherein the oil phase/aqueous phase ratio by volume is in the range of from 50/50 to 20/80, preferably from 45/55 to 30/70.

In another embodiment, the present invention relates to a method for subterranean treatments comprising :

I) providing the above described invert emulsion treatment fluid and

II) introducing said fluid into at least one portion of a subterranean formation .

The features and advantages of the present invention will be read ily apparent to those skilled in the art upon reading of the description of the preferred embodiments, which follow.

DETAILED DESCRIPTION OF TH E INVENTION

With the expression "degree of esterification" of the ester of the invention we mean the molar ratio between the monoca rboxylic acid and the polyol .

Preferably the HIPR invert emulsion treatment fluid comprises from 2 to 6 % by weight of said ester c) and from 0.2 to 3 % by weight of said oil soluble acrylic copolymer d) .

In a preferred embodiment, the ester c) has a degree of esterification comprised between 1 and 3.

The esters of the invention are well known and can be prepared according to any of the procedure described in the art and known to the experts.

Examples of C ₆ -C ₂₄ aliphatic unsaturated monocarboxylic acids suitable for the present invention include both unsaturated and polyunsaturated aliphatic carboxylic acids with from 6 to 24 ca rbon atoms. Examples of these acids are palmitoleic acid, oleic acid, linoleic acid, linolenic acid , arachidonic acid, and the like.

Examples of C ₆ -C ₂₄ aliphatic saturated monocarboxylic acids include decanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, and the like.

Mixtures of C ₆ -C ₂₄ saturated and unsaturated aliphatic monocarboxylic acids can be also used

Mixtures of monocarboxylic acid derived from natural oils, such as coconut oil, palm oil, olein, soy oil, canola oil and tall oil, are particularly preferred . Preferably the C ₆ -C ₂ 4 aliphatic monocarboxylic acid is a mixture of C ₆ -C ₂ 4 saturated and unsaturated linear monocarboxylic acids comprising at least 55 % by weight, preferably at least 70 % by weight, of oleic acid .

The C4-C ₁₂ aliphatic polyol may be, for example, pentaerythritol, dipentaerythritol, di-trimethylolpropane, diglycerol, triglycerol, tetraglycerol, sorbitan, sorbitol and the like.

Preferably, said polyol has from 4 to 6 hydroxyl groups and more preferably it is sorbitan, triglycerol, pentaerythritol or mixture thereof.

According to a preferred aspect of the present invention, the invert emulsion treatment fluid does not comprise any additional emulsifier.

The preferred oil soluble copolymer d) is obtained by reaction of from 95% to 99.5% by weight of a C ₄ -C ₁₂ alkyl (meth)acrylate and from 0.5% to 5 % by weight an ethylenically unsaturated acid .

The alkyl group of the alkyl (meth)acrylate contains of the from 4 to 12 and preferably from 6 to 10 carbon atoms. A particularly preferred C4-C ₁₂ alkyl (meth)acrylate is 2-ethyl hexyl acrylate.

The ethylenically unsaturated acid can be a sulphonic acid such as vinyl sulphonic acid or an unsaturated carboxylic acid such as methacrylic acid or acrylic acid . The acid is generally in protonated form, but it may also be present as salt.

Particularly preferred are copolymers obtained by reaction of from 90 to 99.5 % by weight, in particular from 95 to 99.5 % by weight, of C ₆ -Ci ₀ alkyl (meth)acrylate with from 0.5 to 10% by weight, in particular from 0.5 to 5 % by weight, of (meth)acrylic acid .

Usually, the copolymer of the invention is provided as an aqueous emulsion, prepared, for example, according to US 4,670,501.

The oil phase b) used in the water-in-oil subterranean treatment fluids of the present invention may comprise any oil-based fluid suitable for use in emulsions. The oil phase may derive from a natural or synthetic source. Examples of suitable oil phase include, without limitation, diesel oils, paraffin oils, mineral oils, low toxicity mineral oils, olefins, esters, amides, amines, synthetic oils such as polyolefins, ethers, acetals, dialkyl carbonates, hydrocarbons and combinations thereof. The preferred oil phases are paraffin oils, low toxicity mineral oils, mi neral oils, polyolefins, olefins, esters and mixtures thereof. Factors determining which oil phase will be used in a particular application, include but are not limited to, its cost and performance cha racteristics, environmental compatibility, toxicological profile and availability. The HIPR invert emulsion subterranean treatment fluid of the present invention also comprises an aqueous phase a) that is at least partially insoluble in the oil phase.

Suitable examples of aqueous phase include fresh water, sea water, salt water, and brines (e.g., saturated salt waters), glycerine, glycols, polyglycol amines, polyols and derivatives thereof, that are partially insoluble in the oleaginous fluid, and combinations thereof.

Suitable brines include heavy brines. Heavy brines, for the purposes of this application, include brines with various salts at variable concentrations, that may be used to weight up a fluid; generally the use of weighting agents is required to provide the desired density of the fluid.

Brines generally comprise water soluble salts. Suitable water soluble salts are sodium chloride, calcium chloride, calcium bromide, zinc bromide, sodium formate, potassium formate, sodium acetate, potassium acetate, calcium acetate, ammonium acetate, ammonium chloride, ammonium bromide, sodium nitrate, potassium nitrate, ammonium nitrate, calcium nitrate, sodium carbonate, potassium carbonate, and mixtures thereof.

The aqueous phase is chosen taking into account several factors including cost, environmental and health safety profile, density, availability, and which oil phase has been chosen. Another factor that may be considered is the kind of application of the emulsion. For example, if the application needs an emulsion with a heavy weight, a zinc bromide brine (for example) may be chosen. Preferred brines are those having a density above 1.2 g/ml, more preferably above 1.6 g/ml.

The HIPR invert emulsion subterranean treatment fluid of the invention may further comprise conventional additives including weighting agents, wetting agents, fluid loss reducers, thickeners, thinning agents, lubricants, anti-oxidants, corrosion inhibitors, scale inhibitors, defoamers, biocides, pH modifiers, and the like.

The subterranean treatment fluids, in particular, also contain at least one filtrate reducer, preferably chosen among gilsonite, organophilic lignite, organophilic tannins, synthetic polymers, polycarboxylic fatty acids.

When used in certain applications, the HIPR fluids may include particulates such as proppant or gravel.

The HIPR invert emulsion subterranean treatment fluids of the present invention are suitable for use in any treatment of subterranean formations wherein invert emulsions can be used. As used herein, the term "treatment," or "treating," refers to any subterra nean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The fluids disclosed herein are especially useful in the drilling, completion and working-over of subterranean oil and gas wells and also in stimulation operations (such as fracturing), sand control treatments such as installing a gravel pack, cementing, maintenance and reactivation.

The following examples are included to illustrate the preferred embodiments of the invention .

EXAMPLES

In the examples, the following compounds have been used as emulsifiers :

• Emulam FP and Emulam HT (Lamberti S. p.A. ) are emulsifiers commonly used in the oil field and are produced by reaction of polyamines with ma leic anhydride and fatty acids.

• Phosphate 1 and 2 are monophosphoric esters (acid) of ethoxylated cetyl- oleic alcohol (4 EO) and ethoxylated cetyl-stearyl alcohol (4 EO), respectively.

• Emulsogen COL 020 (Clariant) is a fatty alcohol polyoxyethylene carboxylic acid (2 EO) .

• Triglycerol monooleate and sorbitan sesquioleate were obtained by esterification of triglycerol or sorbitan with a n olein having an oleic acid content of about 85 % wt. These esters have a degree of esterification of 1.02 and 1.43, respectively.

• Sorbitan monooleate, sorbitan trioleate, triethanolamine (TEA) trioleate and pentaeritrithol trioleate were obtained by esterification of sorbitan, TEA or pentaeritrithol with an olein containing about 78 % wt of oleic acid . These esters have a degree of esterification of 1.05, 2.91, 2.95 and 2.7, respectively.

• Sorbitan monolaurate was produced by esterification of sorbitan with the carboxylic acids obtained from coconut oil, comprisi ng about 50 % by weight of lauric acid . This ester has a degree of esterification of 1.10.

An emulsion of a copolymer (RM 1) obtained by reaction of 98 % by weight of 2- ethylhexyl acrylate and 2 % by weight of methacrylic acid , prepared according to Example 1 of US 4,670,501, was used as rheology modifier of the invention .

As comparative rheology modifiers, a copolymer containing 81 % by weight of N- oleyl acrylamide and 19 % of acrylic acid (RM2) and an ethoxylated (5EO) C ₅₄ trimer fatty acid (RM3) were used . As oil phase, a mineral oil synthetic paraffin base (specific gravity = 0.767, Flash Point > 100 °C) was used .

The performances of the emulsifying system assessed by the invention were evaluated by determining the stability of the obtained HIPR invert emulsions, measuring the separation of the oil phase or the aqueous phase after aging at a temperature of 120 °C. An emulsion is considered stable when it shows, after 72 hours of thermal treatment at 120°C, a sepa ration of oil phase below 5 % by volume (v/v) and is considered very stable when it shows, after 336 hours (2 weeks) of thermal treatment at 120°C, a separation of oil phase below 10 % by volume (v/v) . An emulsion showing after 72 hours of thermal treatment at 120°C a separation of the aqueous (brine) phase is not considered stable.

Examples 1 -20

The HIPR invert emulsions of Examples 1- 12 were prepared va rying the chemical nature of the emulsifier and keeping constant the other parameters, such as the oil phase/aqueous phase volume ratio (60/40) and the amount of emulsifier or rheology modifier. Emulsifiers commonly utilized in the field were used for comparison . 120 ml of oil and 10 g of emulsifier (3.6 % by weight of the final invert emulsions) were added in a plastic beaker and stirred for 3 min with a Silverson® mixer mod . L4R at 3000 rpm . After this period 3.5 g of 2-ethylhexyl acrylate/acrylic acid copolymer ( 1.3 % by weight of the final invert emulsions) were added and the mixture was homogenized for 5 mi n. 180 ml of CaBr ₂ saturated brine were added under stirring. The mixture was emulsified by stirring for other 10 min . The invert emulsions were transferred in graduated glass jars which were subsequently sealed and stored in a static oven for 72 hours at 120 °C. After this thermal treatment the separation of the oil phase and/or aqueous phase was visually evaluated .

The chemical nature of the emulsifiers and the separation data (% v/v) are reported in Table 1.

No sepa ration of the aqueous phase and a minimal separation of the oil phase was observed with the emulsifying systems of the invention.

The same experiments were performed using some of the emulsifiers described in Table 1 at a concentration of 1.8% by weight. Table 2 reports the separation of the oil phase (% v/v) after 72 and 336 hours of thermal treatment at 120 °C. No separation of the aqueous phase was observed . Table 1

* Comparative

Table 2

Comparative

The emulsifying systems of the invention exhibit also a very good long-term stability.

Examples 21 -23

HIPR invert emulsions were prepared by varying the chemical nature of the rheology modifier and keeping constant the other parameters, such as the oil phase/aqueous phase volume ratio (60/40) and the amount of emulsifier and rheology modifier. 120 mL of oil and 10 g of sorbitan trioleate were added in a plastic beaker and stirred for 3 min with a Silverson® mixer at 3000 rpm. After this period the rheology modifiers were added and the mixture was homogenized for 5 min. Finally 180 mL of CaBr ₂ saturated brine were added under stirring. The mixture was emulsified by stirring for other 10 min

The invert emulsions were transferred in sealed glass jars which were subsequently closed and stored in a static oven for 72 hours at 120 °C. After this thermal treatment the separation of the oil phase and/or aqueous phase was visually evaluated.

The chemical nature of the rheology modifiers and the separation data (% v/v) are reported in Table 3.

Table 3

Comparative

Examples 24-27

The effect of the concentration of rheology modifier was determined keeping constant the oil/aqueous phase volume ratio (40/60) and the concentration of emulsifier (sorbitan trioleate).

Different HIPR invert emulsions were prepared using the procedure of Example 10 and adding different amounts (% wt of emulsion) of the 2-ethyl-hexyl acrylate copolymer (RM 1).

The amount of rheology modifier and the separation of the oil phase observed after the thermal treatment of 72 hours at 120 °C are reported in Table 4. No separation of the aqueous phase was observed.

Table 4

Rheology % Oil

Example Modifier (% Separation

wt)

24 3.8 4.3

25 1.9 3.9

26 1.3 4.0

27 0.65 4.8 Examples 28-31

The effect of the chemical naure of the brine on the stability of the HIPR invert emulsions was determined keeping constant the oil/aqueous phase volume ratio (40/60), the concentration of emulsifier (sorbitan trioleate) and of the 2-ethyl- hexyl acrylate copolymer (RM 1) .

The emulsions were prepared using the procedure of Example 10.

The kind of brines and the separation of the oil phase observed after the thermal treatment of 72 hours at 120 °C are reported in Table 5. No separation of the aqueous phase was observed .

Table 5

Examples 32-35

The stability of HIPR invert emulsions with different oil phase/aqueous phase volume ratio was determined maintaining in all the Examples the same component ratios aqueous phase/emulsifier (v/w) and oil/rheology modifier (v/w) as in Example 10.

The HIPR invert emulsions were prepared with the same procedure of Exa mple 10.

The amounts of oil and brine and the separation of the oil phase after 72 hours of thermal treatment are reported in Table 6.

All the tests demonstrate that the emulsifying system of the invention allow the preparation of very stable HIPR invert emulsions both with various oil phase/aqueous phase ratio and with different brines.

Table 6

Oil Brine Oil/Brine % Oil

Example

(ml) (ml) (% v/v) Separation

32 152 152 50/50 3.2

33 122 185 40/60 4.0

34 93 217 30/70 3.8

35 62 248 20/80 5.0