ITVA20120052A1 - METHOD FOR THE FRACTURE OF UNDERGROUND FORMATIONS - Google Patents

Description

Description of the industrial invention entitled:

METHOD FOR THE FRACTURATION OF UNDERGROUND FORMATIONS

Owner: LAMBERTI SpA - Albizzate (VA)

TECHNICAL FIELD

The present invention describes a method for fracturing a portion of an underground formation by using an aqueous fracturing fluid comprising, dissolved therein, a cross-linked carboxymethyl cellulose.

STATE OF THE ART

The fracturing technique is widely used to stimulate the production and recovery of oil and gas from underground formations. Fracturing involves the injection of a suitable fluid into the well which reaches a formation; the fluid must be injected under pressure in such a way as to extensively fracture the formation and to allow the escape of oil and gas, which are blocked in the pores of the formation, and to help their flow inside the well. Often they are injected into the formations of the appropriate materials (proppants) to prevent the reclosing of the fractures.

Usually to widen fractures effectively and to inhibit filtrate loss, fracturing fluids are gelled with water-soluble polymers, particularly natural, as-such or derivatized polymers. Examples of these polymeric gelling agents are guar, guar derivatives, carob gum, karaya gum, carboxymethyl cellulose (CMC), carboxymethyl hydroxyethyl cellulose and hydroxyethyl cellulose.

In high temperature applications, the use of these polymers as the only thickening agents is rather limited due to the high thermal coefficient of viscosity of the thickened aqueous solutions. As soon as they are injected into an underground formation, these solutions lose viscosity due to the increase in temperature and release the proppants before they can be inserted into the open fractures within the formation. To overcome this negative effect, it is necessary to introduce a crosslinker, for example polyvalent metal ions, which create bonds between the polymer chains increasing the viscosity of the system and its stability at high temperatures.

Guar, guar derivatives and other gums are among the most widely used polymeric gelling agents, but unfortunately they contain sensitive amounts of insoluble substances, from about 1.5 to more than 10% by weight. The presence of such insoluble substances in a billing fluid is highly undesirable as it can clog the pores of the formation or the fracture itself.

Cellulose ethers, for example carboxymethyl cellulose, have been proposed as an alternative to guar, guar derivatives or other gums, due to their low insoluble content.

For example, US 3,845,822 describes fracture treatment methods using mainly carboxymethyl cellulose, a CrF6-based crosslinker and a reducing compound suitable for reducing the valence of chromium to a lower value, for example Cr <+3>.

WO 2011/107759, WO 2011/107758 and WO 2011/107757 describe fracturing fluids comprising a basic aqueous fluid, a suitable cellulosic thickening agent, which may be a carboxymethylcellulose, and a compatible crosslinking agent, usually a metal cation such as ions. of aluminum, iron or zirconium. The pH of the aqueous fracturing fluid is preferably acidic, generally between 3.5 and 6.

EP 104009 concerns a carboxymethyl hydroxyethyl cellulose which, cross-linked with a suitable aluminum ion in aqueous solution, does not lose viscosity even if used in environments at temperatures up to about 200 ° F (93 ° C)

US 4,749,040 discloses a crosslinking composition which can produce delayed crosslinking of an aqueous solution of a crosslinkable organic polymer. The composition comprises an organic complex of titanium and an organic alpha-hydroximonocarboxylic acid, preferably hydroxyacetic acid, and is particularly useful for fracturing underground formations. Aqueous solutions of organic polymers, which can be cross-linked from the composition, include solutions of carboxymethyl hydroxyethyl cellulose.

EP 112102 discloses a fracturing fluid and a method for fracturing underground formations penetrated by a well. The fracturing fluid comprises an aqueous liquid, a gelling agent, a cross-linking agent comprising a zirconium chelate or an aluminum chelate and a sufficient amount of carbon dioxide to reduce the pH of the fracturing mud to a level below about 5.5 . The gelling agent includes, among others, carboxyalkyl cellulose or carboxyalkyl hydroxyalkyl cellulose.

Unfortunately, carboxymethyl celluloses produce strong gels only under acidic conditions, at low pH, and are unable to provide gels with stability at high temperatures such as those obtained with guar and guar derivatives. Furthermore, the use of highly acidic fluids, such as those described in the cited literature, can cause corrosion of tanks or metal pipes commonly used in well service systems.

Therefore, other solutions were sought. For example, US 4,553,601 describes a gelling agent prepared by introducing dihydroxy-vicinal hanging structures into hydroxyethyl cellulose or other selected cellulose ethers, including, for example, carboxymethyl cellulose. This gelling agent can be cross-linked with a variety of metal ions to produce gelled fracturing fluids that exhibit high thermal stability up to pH 14.

US 2002125012 discloses a fracturing fluid comprising a solvent, a polymer that is soluble or hydratable in said solvent, a crosslinking agent, an inorganic destructuring agent, and an ester. Carboxymethyl cellulose is mentioned among the useful soluble polymers, but no further characterizing description is provided and no fracturing fluid comprising a CMC is disclosed.

However, it is still highly desirable to provide a cross-linked carboxymethyl cellulose-based gelled fracturing fluid that can be used at pH above 10 and is stable at temperatures well above 200 ° F (93 ° C).

It has now been found that these requirements are fully met when the aqueous fracturing fluid is formulated using a viscoelastic carboxymethyl cellulose (CMC) as a gelling agent. Surprisingly, viscoelastic carboxymethyl cellulose can be crosslinked at basic pH with compounds containing titanium (IV), zirconium (IV), antimony (III), antimony (V) to give stable gels both at high shear stresses and at high temperatures. These gels are stable up to 300 ° F (149 ° C).

As far as the applicant is aware, no one has ever described this technical solution before.

In this text, the expression "viscoelastic carboxymethyl cellulose" means a CMC which, in aqueous solution at a concentration equal to 1.15% by weight of active substance, has an elastic modulus G 'higher than the viscous modulus G' 'at least up to 5% angular amplitude (deformation), measuring Gâ € ™ and Gâ € ™ â € ™ with a flat-cone rotational rheometer varying the% of deformation between 0.01 and 1000, at a frequency of 0.5 Hz, at a temperature of 25 ° C and with a steel cone of 60 mm and an angle between plate and cone equal to 1 °.

It should be emphasized that, although some syntheses of viscoelastic CMCs are reported in the literature, the CMCs present on the market do not normally have viscoelastic behavior; furthermore, publications describing the preparation of viscoelastic CMCs do not suggest their use in aqueous fracturing fluids. EP 1682630 describes, for example, a water-based drilling fluid, usable for drilling operations, comprising a particular carboxymethyl cellulose. Said CMC is characterized by viscoelastic characteristics since it forms a gel in an aqueous solution of sodium chloride at 0.3% by weight, the gel being a fluid having an elastic modulus (G ') that exceeds the viscous modulus (G ") in the entire range of frequencies between 0.01 and 10 Hz when measured with an oscillatory rheometer operating at a deformation of 0.2. In this patent no mention is made of the use of fluid for the fracturing of underground formations and the possibility of producing very strong gels, even at high temperatures, by crosslinking the CMC at basic pH.

With the expression "degree of substitution" (DS), we mean the average number of hydroxyl groups substituted for each anhydroglucoside unit of carboxymethyl cellulose, which can be measured, for example, according to the ASTM D1439-03 standard.

The active substance content in carboxymethyl cellulose samples can be determined, on an anhydrous basis, according to the ASTM D1439-03 (Purity) standard.

DESCRIPTION OF THE INVENTION

The object of the present invention is therefore a method for fracturing a portion of an underground formation comprising:

I. provide an aqueous fluid for fracturing with a pH between 10 and 14 comprising dissolved therein:

a) from 0.1 to 5.0% by weight, preferably from 0.2% to 1.8% by weight, of a carboxymethyl cellulose (CMC) which in aqueous solution at a concentration equal to 1.15% by weight of active substance, has an elastic modulus G 'higher than the viscous modulus G' 'at least up to 5% of angular amplitude (deformation), measuring Gâ € ™ and Gâ € ™ â € ™ with a flat-cone rotational rheometer varying the% deformation between 0.01 and 1000, at a frequency of 0.5 Hz, at a temperature of 25 ° C and with a steel cone of 60 mm and an angle between plate and cone equal to 1 °;

b) from 0.001 to 0.5% by weight, preferably from 0.005 to 0.2% by weight, of a crosslinker selected from the group consisting of titanium (IV), zirconium (IV), antimony (III), antimony (V) and their mixtures;

II. placing said aqueous fracturing fluid in a portion of an underground formation.

DETAILED DESCRIPTION OF THE INVENTION

The viscoelastic carboxymethyl cellulose a), which characterizes the aqueous fluid for fracturing of the present invention, has a linear viscosity at a concentration equal to 0.48% by weight of active substance in water, at 25 ° C and 300 rpm between 10 and 150 mPa * s, preferably between 25 and 100 mPa * s, more preferably between 45 and 90 mPa * s.

Preferably, the viscoelastic CMC of the invention, in aqueous solution at a concentration equal to 1.15% by weight of active substance, has tan ï ¤, i.e. the ratio between the viscous modulus G "and the elastic modulus G ', lower than 0, 65 at 1% deformation, Gâ € ™ and Gâ € ™ â € ™ being measured with a flat-cone rotational rheometer by varying the% of deformation, at a frequency of 0.5 Hz, at a temperature of 25 ° C and with 60 mm steel cone and 1 ° angle between plate and cone.

Viscoelastic CMCs can be prepared, for example, following the procedures described in DD 233 377, EP 1025130, Macromolecules, 22, 364-366, 1989 or Polymer, 39, 3155-3165, 1998. The degree of substitution of the usable viscoelastic CMC in the invention it can vary between 0.4 and 1.7, preferably between 0.5 and 1.2, more preferably between 0.55 and 1.0.

The viscoelastic CMC useful for carrying out the present invention can be a technical grade or purified carboxymethyl cellulose, with an active substance content (on an anhydrous basis) ranging from 50 to 100% by weight, preferably from 70 to 98.5% by weight. weight, and a water content of from about 2 to about 12% by weight.

The compounds that can be used to release the crosslinkers b) in the aqueous fluids for fracturing of the present invention are those commonly used in the sector.

Examples of compounds that can be used to release titanium (IV) ions are organotitanates such as titanium acetylacetonate, titanium triethanolamine and ammonium and titanium lactate.

Examples of compounds that provide zirconium (IV) are zirconium acetylacetonate, zirconium lactate, zirconium carbonate and diisopropylamine lactate and zirconium.

Examples of compounds that provide antimony (III) are antimony tartrate and antimony oxalate. Antimony (V) can derive, for example, from potassium pyroantimonate.

The preferred crosslinkers are titanium (IV), zirconium (IV) and their mixtures.

The pH of the aqueous fluid for fracturing of the invention can be adjusted to values between 10 and 14 with bases or buffers, ie mixtures of acids and bases, commonly used in the sector. For example, sodium bicarbonate, dibasic sodium phosphate, potassium carbonate, sodium hydroxide, potassium hydroxide, and sodium carbonate are typical pH adjusting agents. Slowly dissolving bases, such as magnesium hydroxide, can also be used profitably. Preferably, the pH adjusting compound is added to the aqueous liquid after the addition of the polymer. Preferably, the pH of the fracturing fluid is maintained between 11 and 13.

The aqueous fracturing fluids according to the invention can comprise a retarder to slow down the crosslinking speed for a time sufficient to allow the thickened aqueous liquid to be easily pumped into the underground formation. The retarder may be a complexing agent which effectively complexes the cross-linking cation. Any of the complexing agents known in the art can be used. Examples of suitable complexing agents include, but are not necessarily limited to, polyols, gluconates, sorbitols, mannitols, carbonates, or any mixture thereof. The retarder can be present in the fracturing fluid up to 0.4% by weight, preferably in the range of 0.02 to 0.3% by weight.

Any type of aqueous fracturing fluid can be used in the method of the invention. The fluid can be, for example, a gelled liquid or a foam-gel, in which the foam bubbles help to transport and insert the proppants into the fractures.

The aqueous component of the fracturing fluid can be selected from fresh water, salt water, sea water, brine (brine), natural or synthetic, mixtures of water and water-soluble organic compounds, any other aqueous liquid that does not interact with the other components of the fracturing fluid negatively affecting its performance, and their mixtures.

Brines are solutions of water and inorganic and / or inorganic salts. Preferred inorganic salts include alkali metal halides, in particular potassium chloride. The brines can also comprise an organic salt, preferably sodium or potassium formate. Preferred divalent inorganic salts include calcium halides, more preferably calcium chloride or calcium bromide. Sodium bromide, potassium bromide or cesium bromide can also be used.

The aqueous fracturing fluid, in addition to the thickening agent, the crosslinker and the aqueous component, normally contains additives which are well known to those skilled in the art, such as proppants, gel stabilizers, gel destructuring agents, surfactants, clay stabilizers, absorbers of oxygen, alcohols, scale inhibitors, corrosion inhibitors, filtrate loss reducers, bactericides and the like.

Preferably, the fluid further comprises one or more proppants suspended in the fluid. Usable proppants include, but are not limited to, sand, resin coated sand, ceramic beads, bauxite, glass, glass beads, and mixtures thereof.

Gel stabilizers are oxidation inhibitors or free radical absorbers that remove dissolved oxygen in water. Dissolved oxygen is the main cause of radical / oxidative breakdown of natural polymers or their water-soluble derivatives. Example of suitable gel stabilizers are sodium thiosulfate, substituted benzofuranones, hydroxylamine both in the form of salt and its alkyl derivatives, compounds of trivalent phosphorus, hydroquinone and hydroquinone formulated with amines, natural antioxidants, such as ascorbic acid and vitamin C, and methylethyl kethoxime .

Usable gel destructuring agents include, but are not limited to, ammonium persulfate, sodium persulfate, sodium bromate and sodium chlorite, enzymes. Preferably, the destructuring agent is a retarded agent, for example encapsulated ammonium persulfate. A delayed destructuring agent slowly releases the oxidant to allow a very strong gel to form at the beginning that can transport and deposit the proppants in the formation.

The aqueous fracturing fluids of the present invention can be prepared by any method suitable for a given application. For example, some components of the treatment fluid of the present invention can be supplied premixed in the form of a powder or a concentrated dispersion of powders in a non-aqueous liquid, which can be combined with the aqueous fluid at a later time. Simply the aqueous fluid for fracturing of the invention can be prepared by mixing the viscoelastic carboxymethylcellulose, the crosslinking agent and the other additives in a mixer together with the aqueous liquid. Mixing usually takes place in the field.

Generally the aqueous fluids gelled by fracturing of the invention have a viscosity, measured with Grace Instrument M5500® rheometer at 180 ° F (82.2 ° C) above 500 mPa * s at 100 sec <-1>, and, more preferably, above 600 mPa * s at 100 sec <-1>. These gelled fluids are thermally stable under high deformations at temperatures of approximately 300 ° F (149 ° C).

In the method of the disclosure, the aqueous fracturing fluid is then pumped or injected into the underground formation (e.g., from the surface through a well). Preferably, the fluid is pumped or injected at a pressure sufficient to fracture the formation (for example, generate a plurality of fractures) and thus allow the transfer into the fractures of the optional solid particles (proppants), suspended in the treatment fluid, and to be deposited there.

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

EXAMPLES

The main chemical and rheological parameters of the carboxymethyl cellulose used in the application tests (Examples 1-12) are shown in Table 1. For the determination of the linear viscosity, 500 ml of deionized water were transferred into a cup of a Waring® mixer and brought at 25 ° C. Subsequently, in 60 seconds under agitation at 2000 rpm, the quantity of sample necessary to obtain 2.40 g of active substance (on an anhydrous basis) was added. The agitation was maintained for 30 minutes. After cooling to 25 ° C the viscosity was determined at 300 rpm with an OFITE 800® viscometer.

The viscoelastic behavior of the different CMCs was determined on aqueous solutions with a concentration of 0.7 and 1.15% by weight of active substance using an AR 2000 rheometer (TA Instruments), varying the deformation between 0.01 and 1000 %, at 0.5 Hz, at 25 ° C and with a steel cone with a geometry of 60 mm and an angle between plate and cone equal to 1 °. Table 1 shows the% strain value (% strain) at which the elastic modulus G 'below becomes less than the viscous modulus G' '(there is a crossing of the relative curves) and the tan ï ¤ (ratio G "/ G') at 1% deformation (for the solution at 1.15% by weight). In the table, "No" means that the curves G 'and G "do not cross. The degree of substitution (DS) was determined using the ASTM D 1439-03 standard, (Degree of etherification, Method B). The same standard was used for the determination of the percentage content of active substance (Purity).

Table 1

% Viscosity% Strain% Strain TanÎ ́ CMC Substance DS Linear 0.7% in 1.15% in 1.15% in Active mPa * s weight weight weight Example 1 75.6 0.69 68 16 <102> 0.50 Example 2 73.1 0.69 60 16 <127> 0.50 <Example 3> 72.7 0.69 44 No 16 0.86 <Example 4> 78.6 0.71 61 No 10 0.95 <Example 5> 68.3 0.84 57 No 100 0.76 <Example 6> 99.4 0.84 47 No 40 0.70 <Example 7 *> 68.1 0.86 51 No No 1.12 < Example 8 *> 98.6 0.92 62 No No 1.34 <Example 9 *> 99.1 0.92 68 No No 1.07 <Example 10 *> 72.8 0.67 57 No 2 0.83 * Comparative

The solutions prepared for linear viscosity measurement were also used for the preparation of the cross-linked gel. For each sample, 100 g of solution were brought to pH about 11 by adding diluted NaOH under mechanical stirring at 2000 rpm. Then the CMC solutions were gelled by adding an aqueous solution containing 6% by weight of zirconium (IV), as crosslinker. The pH was then brought to about 12 with diluted NaOH. 29 g of the resulting gel was transferred to a high pressure, high temperature test rotor cup of a Grace Instrument M5500® rheometer. The tests were performed at 180 ° F (82.2 ° C) and 400 psi (2.76 MPa), in a preheated bath, at a constant shear rate of 100 sec <-1> for 1 hour.

The amounts of crosslinker, reported in Table 2, have been optimized in order to reach the maximum viscosity of the gel.

The viscosity of the cross-linked gel after 1 hour is shown in Table 2.

Table 2

Viscosity Viscosity

Linear Cross-linking Gel Cross-linked gel

mPa * s (mL / L) mPa * s

<Example 1> 68 3.5 1200

Example 260 3.5 950

Example 344 3.5 700

Example 461 3.5 900

Example 557 4 900

Example 647 4.5 700

Example 7 * 51 4.5 350

Example 8 * 62 4.5 240

Example 9 * 68 4.5 450

Example 10 * 57 3.5 540

* Comparative

The results show that the viscoelastic carboxymethyl celluloses according to the invention have the ability to form a very strong cross-linked gel and this makes them perfectly suitable for use as thickeners for aqueous fracturing fluids. On the contrary, the carboxymethyl celluloses of the known art do not show the same performances.

High temperature test.

With the CMC of Example 2 a cross-linked gel was prepared as described above, but with the further addition, before the addition of the cross-linker, of 0.15% by weight of tetramethyl ammonium chloride (TMAC), a stabilizer of clays normally used in fracturing operations, and 0.24% by weight of sodium thiosulfate, as a stabilizer of the gel. 29 g of the resulting gel was transferred to a high temperature high pressure test rotor cup of a Grace Instrument M5500® rheometer. The gel was treated for 1 hour at 300 ° F (149 ° C) and 400 psi (2.76 MPa), in a preheated bath, at a shear rate of 100 sec <-1>. After an initial stabilization period (about 15 minutes), the gel reached a viscosity of 400 mPa * s and this value was maintained until the end of the test.

Claims (7)

CLAIMS 1) Method for fracturing a portion of an underground formation comprising: I. provide an aqueous fluid for fracturing with a pH between 10 and 14 comprising dissolved therein: a) from 0.1 to 5.0% by weight of a carboxymethyl cellulose (CMC) which in aqueous solution at a concentration equal to 1.15% by weight of active substance, has an elastic modulus G 'higher than the viscous modulus G '' at least up to 5% angular amplitude (deformation), measuring Gâ € ™ and Gâ € ™ â € ™ with a flat-cone rotational rheometer varying the% of deformation between 0.01 and 1000, at a frequency of 0, 5 Hz, at a temperature of 25 ° C and with a steel cone with a geometry of 60 mm and an angle between plate and cone equal to 1 °; b) from 0.001 to 0.5% by weight of a crosslinker selected from the group composed of titanium (IV), zirconium (IV), antimony (III), antimony (V) and their mixtures; II. placing said aqueous fracturing fluid in a portion of an underground formation. 2) The method according to claim 1), wherein said aqueous fluid for fracturing has a pH between 11 and 13. 3) The method according to claim 1), wherein said aqueous fluid for fracturing comprises: a) from 0.2% to 1.8% by weight of said carboxymethyl cellulose. 4) The method according to claim 1), in which said carboxymethyl cellulose, in aqueous solution at a concentration equal to 1.15% by weight of active substance, has a tan ï ¤ lower than 0.65 to 1% of deformation, Gâ € ™ and Gâ € ™ â € ™ being measured with a flat-cone rotational rheometer varying the% of deformation, at a frequency of 0.5 Hz, at a temperature of 25 ° C and with a steel cone of 60 mm and angle between plate and cone equal 1 °. 5) The method according to claim 1), wherein said aqueous fluid for fracturing comprises: b) from 0.005 to 0.2% by weight of said crosslinker. 6) The method according to claim 1), in which said crosslinker is selected from titanium (IV), zirconium (IV) and their mixtures. 7) The method according to claim 1), wherein said aqueous billing fluid further comprises: c) up to 0.4% by weight of a retarder.