WO2014096186A1

WO2014096186A1 - Method of fracturing subterranean formations

Info

Publication number: WO2014096186A1
Application number: PCT/EP2013/077367
Authority: WO
Inventors: Lorenzo GIARDINI; Laura Vigano'; Luigi Merli; Chiara Cipriani; Dario Chiavacci; Pierangelo Pirovano; Giovanni Floridi; Giuseppe Li Bassi
Original assignee: Lamberti Spa
Priority date: 2012-12-21
Filing date: 2013-12-19
Publication date: 2014-06-26
Also published as: ITVA20120052A1

Abstract

Method of fracturing a portion of a subterranean formation through the use of an aqueous fracturing fluid having pH between 10 and 14 and comprising dissolved therein a crosslinked viscolelastic carboxymethyl cellulose.

Description

METHOD OF FRACTURING SUBTERRANEAN FORMATIONS

TECHN ICAL FIELD

The present disclosure relates to a method of fracturing a portion of a subterranean formation through the use of an aqueous fracturing fluid comprising dissolved therein a crosslinked carboxymethyi cellulose.

BACKGROUND OF THE ART

Hydraulic fracturing is widely used for stimulating petroleu m and gas production and recovery from subterranean formations.

It involves the injection of a suitable fluid down a wellbore to reach a formation ; the fluid shall be injected under sufficient pressure to extensively crack the formation and to provide passageways for the oil and gas that are contained in the pores spaces of the formation and help them flowing to the wel lbore. Suitable particu late materials (proppants) are often injected into the formation to prevent the closure of the fractures.

Usually, fracturing fluids are viscosified with polymeric gelling agents, especially with natura l polymers or chemically modified natura l polymers, such as etherified natura l polymers, to most effectively widen the fractures and inhibit fluid loss. Examples of these polymeric gelling agents are guar gum, gua r gu m derivatives, locust bean gu m, Karaya gu m, carboxymethyi cel lulose (CMC), carboxymethyi hydroxyethyl cellulose, and hydroxyethyl cell ulose . In high temperature applications the employment of these polymers as the sole viscosifier is quite limited owing to the large temperature coefficient of viscosity of aqueous thickened solutions. As such solutions are injected into a subterranean formation, the increased temperature of the formation can cause the fluid to lose viscosity and drop out the proppant before it can be deployed in propping open the formation fractures. To overcome this negative effect, it is necessary to introduce a crosslinker, such as polyvalent metal ions, which create bridges between the polymer chains increasing the viscosity of the system and its stability at high temperature.

Guar, guar derivatives and the other gums are conventional polymeric gelling agents, but unfortunately they contain substantial amounts, that is from about 1.5 to in excess of 10 percent by weight, of insoluble matter, The presence of such insoluble matter in a fracturing fluid is highly undesirable since it may clog the pores of the formation or the fracture. Cellulose ethers, such as carboxymethylated cellulose, have been proposed as an alternative to guar gum, guar derivatives or other gums for use in fracturing fluids because of their substantially lower insoluble matter content.

For example US 3,845,822 describes methods of treating fractures using primarily carboxymethyl cellulose, a CrF₆ crosslinker and a reducing compound suitable for reducing the chromium compound to a lower valence state such as Cr⁺³, WO 2011/107759, WO 2011/107758 and WO 2011/107757 describe fracturing fluids comprising an aqueous base fluid, a compliant ceilulosic viscosifying agent, which can be a carboxymethyl cellulose and a compliant crosslinking agent, usually a metal cation such as aluminum ion, iron ion or zirconium ion. The pH of the aqueous fracturing fluid is preferably acidic, generally between 3.5 and 6.

EP 104009 relates to a carboxymethyl hydroxyethyl cellulose which when crosslinked with a suitable aluminum ion in an aqueous solution is devoid of thinning when used in environments at temperatures up to about 200°F (93 °C)

US 4,749,040 describes a crosslinker composition that can produce delayed crosslinking of an aqueous solution of a crosslinkable organic polymer. The composition comprises an organic titanium complex and an organic alpha-hydroxymonocarboxyfic acid, preferably hydroxyacetic acid, and is particularly useful in fracturing subterranean formations. Aqueous organic poiymer solutions, which can be crosslinked by the composition, include carboxymethylhydroxyethyl cellulose solutions.

EP 112102 provides a fracturing fluid composition and method for fracturing subterranean formations penetrated by a well bore. The fracturing fluid comprises an aqueous fluid, a gelling agent, a crosslinking agent comprising a zirconium chelate or an aluminum chelate and a sufficient quantity of carbon dioxide to reduce the pH of the fracturing fluid to a level below about 5.5. The gelling agent includes, among others, carboxyalkyf cellulose or carboxyalkylhydroxyalkyl cellulose.

Unfortunately, carboxymethylated celluloses produce strong gel only in acidic conditions and at low pH they are not able to provide gels having the stability at elevated temperatures which is achieved through the use of guar gum and guar derivatives. Moreover, the use of a strongly acid fluid, such as those described in the cited literature, may cause corrosion of the metallic tanks or pipeline commonly used on well locations.

Other solutions have been looked for. For example, US 4,553,601 describes a gelling agent prepared by introducing a pendent vicinal dihydroxy structure in hydroxyethyl cellulose or other selected cellulose ether, among which carboxymethyl cellulose. This gelling agent can be crosslinked by a variety of metal ions to provide gelled fracturing fluids which exhibit high thermal stability at pH up to 14.

US 2002125012 provides a fracturing fluid comprising a solvent, a polymer which is soluble or hydratable in the solvent, a crossiinking agent, an inorganic breaking agent, and an ester compound. Carboxymethyl cellulose is mentioned among the useful soluble polymers, but not further characterizing description is provided and no example of fracturing fluid comprising CMC are reported.

However, it would be still highly desirable to provide gelled fracturing fluid based on crosslinked carboxymethyl cellulose which can be used at pH above 10 and are stable at temperature of the well above 200 °F (93 °C). It has now been found that these requirements are fully satisfied when a viscoelastic carboxymethyl cellulose (CMC) is used as gelling agent for the formulation of aqueous fracturing fluids. Surprisingly the viscoelastic carboxymethyl cellulose can be crosslinked at basic pH with titanium (IV), zirconium (IV), antimony (III), antimony (V) containing compounds to give both shear and thermal stable gel. These gels are stable even at temperature up to 300 °F (149 °C).

The rheological behaviour of viscoelastic CMC in aqueous solutions is rather complex and depends on a number of parameters including the degree of substitution of the carboxymethyl groups, the degree of polymerization of the cellulose and, in particular, its depends on the uniformity or non-uniformity of substitution, i.e. the distribution of carboxymethyl groups along the polysaccharide chains. As far as the Applicant knows, no one has found any clear relationship between all these paramenters and the viscoelastic behaviour. For this reason the characterization of these CMC depends mainly on rheology measurements, in particular viscosity measurements, and usually they are not defined with the usual parameters (DS, molecular weight etc.), but with viscosity parameters.

As an example, US6593468 describes, among other cellulose ethers, CMC with elastic properties, to be used as a superabsorbent material. Aqueous solutions of these CMC at concentrations up to a maximum of 0.5% by weight, show a value of the loss factor tan6 at an angular frequency of 1 Hz below 1.0, and in particular below 0.8,

EP 1682630 describes a water-based drilling fluid composition, useful for well-drilling operations, comprising a particular carboxymethyl cellulose (CMC). Said CMC is characterized by viscoelastic characteristics since it forms a gel in a 0.3 wt% aqueous sodium chloride solution, the gel being a fluid having a storage shear modulus (G') which exceeds the loss shear modulus (G") over the entire frequency region of 0.01-10 Hz when measured on an oscillatory rheometer operating at a strain of 0.2.

In the present text, with the expression "viscoelastic carboxymethyl cellulose" we mean a CMC that exhibits an storage shear modulus G' higher than the loss shear modulus G" at least till 5% of angular amplitude (% strain) in strain amplitude sweep test, when said test is carried out at a concentration of 1.15 % by weight (%wt) of active matter in water with a rotational cone-plate rheometer, at from 0.01 to 1000 % strain, 0.5 Hz, 25°C and with a geometry of 60 mm 1° steel cone.

It should be underlined that, although some syntheses of viscoelastic CMC are reported in the literature, the CMC on the market normally do not have viscoelastic behavior; moreover, the few publications describing the preparation of viscoelastic CMC do not suggest its use in aqueous fracturing fluids at high pH.

As far as the Applicant knows, no one has described the technical solution of the present disclosure before. US 5,067,566 describes a method of fracturing a subterranean formation comprising the steps of: formulating a gellab!e fracturing fluid by blending together an aqueous fluid, a hydratable polymer, a suitable crosslinking agent for crosslinking the hydratable polymer to form a polymer gel and

5 an enzyme breaker; raising the pH of the fracturing fluid above about 9.0 to 10.5 and fracturing such a subterranean formation . Carboxymethyl cel lulose is explicitly mentioned among the hydratable polymers.

However in this patent, no descriptions and/or examples about the kind of CMC to be used are provided and no mention is made about the use of i o viscoelastic CMCand about the possibility of producing very strong gels, even at high temperature , compared to non-viscoelastic CMC.

With the expression "degree of substitution" (DS), we mean the average nu mber of substituted hydroxyl g roups on each anhydroglucosidic unit of the carboxymethyl cellu lose, which can be measured, for example, by

15 ASTM Standard D1439-03.

The content of active ingredient in carboxymethyl cellu lose samples can be determined, on the dry basis, by ASTM Standard D1439-03 (Purity). DESCRIPTION OF TH E INVENTION

It is therefore object of the present invention a method of fracturing a portion of a subterranean formation comprising :

0 I. providing an aqueous fracturing fluid 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 showing a storage shear modulus G' higher than the loss shear modulus G" at least till 5 % of angular a mplitude (% strain) in a strain amplitude sweep test, said test being carried out at a concentration of 1.15% by weight of active matter in water with a rheometer, at from 0.01 to 1000 % strain, 0.5 Hz, 25 °C and with a geometry of 60 mm 1° steei cone; b) from 0.001 to 0.5 % by weight, preferably from 0.005 to 0.2 % by weight, of a cross!in ker chosen a mong titanium (IV), zirconiu m (IV), antimony (III), antimony (V) and mixtures thereof;

I. placing said aqueous fracturing fluid into a portion of a subterranean formation .

DETAILED DESCRIPTION OF THE INVENTION

The viscoelastic carboxymethyl cellulose a), which characterizes the aq ueous fracturing fl uid of the present invention, has a linear viscosity at a concentration of 0.48 % by weight of active matter in water, 25°C and 300 rpm comprised between 10 and 150 mPa*s, preferably from 25 to 100 mPa*s, most preferably from 45 to 90 m Pa*s.

Preferably, the viscoelastic CMC of the invention, in the strain amplitude sweep test described above, shows a tan5, the ratio between the loss shear modulus G" and the storage shear modulus G', lower than 0.65 at 1% of strain .

Viscoelastic CMC can be prepared, by way of example, following the procedures described in DD 233 377; EP 1025130; Macromolecuies, 22, 364-366, 1989 or Polymer, 39, 3155-3165, 1998. The degree of substitution of the useful viscoelastic CMC can vary between 0.4 and 1.7, preferably between 0.5 and 1.2, more preferably from 0.55 to 1.0.

The viscoelastic CMC useful for the realization of the present invention can be a technical grade or a purified grade carboxymethyl cellulose, having a percentage of active matter (on the dry basis) comprised between 50 and 100 % by weight, preferably from 70 to 98.5, and a content of water of from about 2 to about 12 % by weight.

Suitable compounds which can be used to release the crosslinkers b) in the aqueous fracturing fluids of the present invention are those commonly used in the field.

Example of compounds which can be used to release the titanium (IV) ions are organotitanate compounds such as titanium acetylacetonate chelate, titanium triethanolamine and titanium ammonium lactate.

Example of compounds which supply zirconium (IV) ions are zirconium acetylacetonate, zirconium lactate, zirconium carbonate and zirconium dfisopropylamine lactate.

Examples of compounds providing antimony (III) ions are antimony tartrate and antimony oxalate. Antimony (V) ions may be derived, for example, from potassium pyroantimonate.

The preferred crosslinkers are titanium (IV), zirconium (IV) and mixtures thereof. The pH of the aqueous fracturing fluid of the invention can be adjusted at values comprised between 10 and 14 with bases or buffers, i.e. mixtures of acids and bases, commonly used in the field. For example, sodium bicarbonate, disodium hydrogen phosphate, potassium carbonate, sodium hydroxide, potassium hydroxide and sodium carbonate are typical pH adjusting agents. Also a slow dissolving base, such as magnesium hydroxide, can be profitably used. Preferably, the pH adjusting material is added to the aqueous fluid after the addition of the polymer to the aqueous fluid. Preferably, the pH of the fracturing fluid is kept from 11 to 13.

Aqueous fracturing fluids according to the invention can comprise a retarder which delay the rate of crosslinking reaction for a sufficient time to allow the aqueous thickened fluid to be easily pumped into the subterranean zone. The retarder may be a complexing agent that effectively complexes the crosslinker cation. Any suitable complexing agent known to those in the art may be used. Examples of suitable complexing agents include, but are not necessarily limited to, polyols, gluconates, sorbitols, mannitols, carbonates, or any mixtures thereof. The retarder can be present in the fracturing fluid up to 0,4 % by weight, preferably in the range of from 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 fluid or a foamed ge!, wherein foa m bubbles help to transport and to place proppants into fractures.

The aqueous component of the fracturing fluid may be selected from fresh water, salt water, seawater, natural or synthetic brine, mixtures of water and water sol uble organic compounds, any other aqueous liquid that does not interact with the other components of the fracturing fluid to adversely affect its performance, and mixtures thereof.

The brine is water comprising an inorganic salt and/or organic sa lt. Preferred inorganic salts include al kali metal halides, more preferably potassiu m ch loride. The brine phase may a!so comprise an organic salt, more preferably sodium or potassiu m formate. Preferred inorganic diva lent salts include calcium halides, more preferably calcium chloride or calciu m bromide. Sodium bromide, potassium bromide, or cesiu m bromide may a lso be used .

The aqueous fracturing fluid, beside the viscosifying agent, the crosslinker system and the aqueous component, normally contains additives that are wel l known by those skilled in the art, such as proppants, gel stabilizers, gel breakers, surfactants, clay stabilizers, alcohols, scale inhibitors, corrosion inhibitors, fluid-loss additives, bactericides, and the like.

Preferably, the fluid also includes one or more proppants suspended in the fluid . Useful proppants include, but are not limited to, gravel, sand, resin coated sand, ceramic beads, bauxite, glass, glass beads and mixtures thereof. Gel stabilizer are oxidation inhibitors or free radical scavengers which remove dissolved oxygen from water. Dissolved oxygen is the major cause of oxidative free radical polymer breakdown of water soluble natural polymers or their derivatives. Example of suitable gel stabilizer are sodium thiosulfate, substituted benzofuranones, hydroxylamine in the form of its salts and alkyl derivatives, trivalent phosphorus compounds, hydroquinone and hydroquinone formulated with amines, natural antioxidants, such as ascorbic acid and vitamin C, and methylethyl ketoxime.

Useful gel breakers include, but are not limited to, ammonium persulfate, sodium persulfate, sodium bromate and sodium chlorite, enzymes. Preferably, the gel breaker is a delayed gel breaker, such as encapsulated ammonium persulfate. A delayed gel breaker slowly releases the oxidizer to enable a strong initial gel to carry and to deposit the proppants in the formation.

The aqueous fracturing fluids of the present invention may be prepared by any method suitable for a given application. For example, certain components of the treatment fluid of the present invention may be provided in a pre-blended powder or a concentrated dispersion of powders in a nonaqueous liquid, which may be combined with the aqueous base fluid at a subsequent time. Simply the aqueous fracturing fluid of the invention can be prepared by blending together the aqueous fluid, the viscoelastic carboxymethyl cellulose, the crosslin king agent and the other additives in a blender. The blending typically occurs in the field .

Generaiiy the gelled aqueous fracturing fluids of the invention have a viscosity, measured with Grace Instrument M5500® rheometer at 180 °F (82.2 °C) of above 500 mPa*s at 100 sec^"1, and, more preferably, above 600 mPa*s at 100 sec^"1. These gelled fluids are therma lly stable under shear at temperatures of about 300 °F ( 149 °C) .

In the method of the disclosure, the aqueous fracturing fluid is fina lly pumped or injected into the subterranean formation (e.g . , from the surface through the wel ibore) . Preferably, the fluid is pumped or injected at a pressure sufficient to fracture the formation (e.g . , generate a plura lity of fractures) and thus to enable the optional particulate solid (proppants) suspended in the wel! treatment fluid to be carried into the fractures and to be there deposited .

The fol lowing exa mples are included to demonstrate preferred embodiments of the invention .

EXAMPLES

The main chemical and rheological parameters of the carboxymethyl cellulose used in the applicative tests (Examples 1- 12) are reported in Table 1.

For the linear viscosity determination, 500 m l of deionized water were transferred in a Waring® blender cup and conditioned at 25 °C. Subsequently the a mount of sample required to reach 2.40 g of active matter on dry basis were added in 60 seconds under stirring at 2000 rpm. The stirring was maintained for 30 minutes. After cooling to 25 °C the viscosity was determined at 300 rpm with a OFITE 800® viscometer.

The viscoelastic behaviour of the different CMC was determined on water solution with a concentration of 0.7 and 1.15 % by weight (%wt) of active matter using a AR 2000 Rheometer (TA Instruments) in strain amplitude sweep procedure, varying the % strain from 0.01 to 1000 %, at 0.5 Hz, 25 °C and with a geometry of 60 mm 1° steel cone. Table 1 reports the value of % strain at which the storage shear modulus G' become lower than the loss shear modulus G" (curve cross-over) and the tan5 (ratio

G'VG') at 1% of strain. "None" means that the CMC solution did not show any G'/G" curve crossover.

The degree of substitution (DS) was determined using ASTM Standard D1439-03 (Degree of etherification, Method B). The same ASTM Standard was used for the determination of the active matter content (Purity),

The solutions prepared for the linear viscosity test were used for the preparation of crosslinked gel.

Diluted NaOH was added under mechanical stirring at 2000 rpm to 100 g of solution to bring the pH to about 11.

Then the CMC solutions were gelled by adding an aqueous solution containing 6 % by weight of zirconium (IV), as crosslinker. Table 1

* Comparative

The pH was subsequently raised to about 12 with diluted NaOH . 29 g of the gel so obtained were transferred in a Grace Intrument M5500® rotor cup for high-pressure high-temperature testing.

The test were performed at 180°F (82.2 °C) and 400 psi (2.76 MPa), in a preheated bath, at 100 sec^"1 constant shear rate for 1 hour.

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

The crosslinked gel viscosity after 1 hour measured for each gel is reported in Table 2.

The results show that the viscoeiastic carboxymethyl celluloses according to the disclosure have a superior ability to give a strong cross-linked gel that make them perfectly suitable as viscosifier for aqueous based fracturing fluids.

On the contrary, the carboxymethyi cellulose of the prior art do not show the same performances.

Table 2

* Comparative

High temperature test

A cross-linked gel was prepared as described above with the CMC of Example 2, but with the further addition, before the addition of the cross- linker, of 0.15 % by weight of tetramethylammonium chloride (TMAC), a typical clay stabilizer used in fracturing operations, and 0.24 % by weight of sodium thiosuifate, as gel stabilizer. 29 g of the gel so obtained were transferred in a Grace Instrument M5500® rotor cup for high-pressure high-temperature testing. The gel was treated for 1 hour at 300 °F (149 °C) and 400 psi (2.76 MPa), in a preheated bath, at 100 sec ¹ constant shear rate. After an initial period of stabilization (about 15 minutes), the gel reached a viscosity of 400 mPa_*s and this value was maintained til! the end of the test.

Claims

O 2014/096186

1 8

1) Method of fracturing a portion of a subterranean formation comprising :

L providing an aqueous fracturing fluid with a pH between 10 and 14 comprising dissolved therein ;

a) from 0.1 to 5.0 % by weight, of a carboxymethyl cellulose showing an storage shear modulus G' higher than the loss shear modulus G" at least till 5 % of angular amplitude (% strain) in a strain amplitude sweep test, said test being carried out at a concentration of 1.15% by weight of active matter in water with a rotational cone-plate rheometer, at from 0.01 to 1000 % strain, 0.5 Hz, 25°C and with a geometry of 60 mm 1° steel cone;

b) from 0.001 to 0.5 % by weight of a crosslinker chosen among titanium (IV), zirconium (IV), antimony (III), antimony (V) and mixtures thereof;

II. placing said aqueous fracturing fluid into a portion of a subterranean formation.

2) The method according to claim 1), wherein said aqueous fracturing fluid has a pH ranging from 11 to 13.

3) The method according to claim 1), wherein the aqueous fracturing fluid comprises: a) from 0.20% to about 1.8% by weight, of said carboxymethyl cellulose.

The method according to claim 1), wherein said carboxymethyl cellulose shows a tan5 lower than 0.65 at 1% of strain in a strain amplitude sweep test, said test being carried out at a concentration of 1.15% by weight of active matter in water with a rotational cone- plate rheometer, at from 0.01 to 1000% strain, 0.5 Hz, 25 °C and with a geometry of 60 mm 1° steel cone.

The method according to claim 1), wherein said the aqueous fracturing fluid comprises:

b) from 0.005 to 0.2 % by weight of said crosslinker.

The method according to claim 1), wherein said crosslinker is chosen among titanium (IV), zirconium (IV) and mixtures thereof.

The method according to claim 1), wherein said aqueous fracturing fluid further comprises;

c) up to 0.4% by weight of a retarder.