WO2014042548A1

WO2014042548A1 - Hydraulic fracturing with shapeable particles

Info

Publication number: WO2014042548A1
Application number: PCT/RU2012/000756
Authority: WO
Inventors: Diankui Fu; Irina Aleksandrovna LOMOVSKAYA; Vladimir Alexandrovich PLYASHKEVICH; Vadim Kamil'evich Khlestkin
Original assignee: Schlumberger, Canada Limited; Services, Petroliers Schlumberger; Schlumberger, Holdings Limited; Schlumberger, Technology B.V.; Prad, Research And Development Limited
Priority date: 2012-09-13
Filing date: 2012-09-13
Publication date: 2014-03-20
Also published as: AR092543A1

Abstract

A method of hydraulic fracturing involving the steps of injecting into a wellbore penetrating a subterranean formation above fracture pressure a fluid made with a carrier fluid and a mixture of elongated shapeable particles and proppant, and allowing the elongated shapeable particles to undergo a change in shape. The elongated shapeable particles are selected from fibers, ribbons, flakes, films, sheets, platelets, and flakes, and have aspect ratios greater than or equal to 20. The change in shape is most commonly due to an increase in temperature of the elongated shapeable particles. The elongated shapeable particles may be made of multiple polymer components. Preferred elongated shapeable particles are shrinkable fibers. The change of shape of the elongated shapeable particles distributed within the proppant pack causes the reorganization of the proppant particles inside the hydraulic fracture to create channel-like structures.

Description

HYDRAULIC FRACTURING WITH SHAPEABLE PARTICLES

Background

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Embodiments relate to hydraulic fracturing treatments, which have been one of the most effective methods to improve hydrocarbon production, especially for reservoirs with low permeabilities. Oil or gas production from shale, for example, would not often be economically feasible without hydraulic fracturing treatments being performed. During hydraulic fracturing treatments, propping agents such as sands or ceramic materials are injected into the formation, together with fluids that have a certain required viscosity, at pressures sufficient to crack the formation rock. The productivity of the well is determined by the geometry and the permeability (conductivity) of the propped fracture as well as formation properties. The conductivity of hydraulic fractures is determined by a number of factors including, among others, the size and strength of the proppants, the nature of the fracturing fluids, the distribution of proppant in the fracture, the breaker efficiency, and the formation closure stress.

Summary

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

An embodiment is a method of fracturing involving injecting, into a wellbore penetrating a subterranean formation, above fracture pressure, a fluid including a carrier fluid and a mixture of elongated shapeable particles and proppant, and allowing the elongated shapeable particles to undergo a change in shape. Typically, the elongated shapeable particles are selected from fibers, ribbons, flakes, films, sheets, platelets, and flakes, and have aspect ratios greater than or equal to about 20. In one embodiment, the elongated shapeable particles are shrinkable fibers; in another embodiment, the shrinkable fibers are degradable. Such degradable fibers may be, for example, polyesters, for example polylactic acid, polyglycolic acid, polyethylene terephthalate, poly(hydroxyalkanoate) and copolymers of these. The degradable fibers may include polyamides, for example nylon 6; nylon 6,6; and nylon 6,12. The degradable fibers may also include polyolefms, for example polyethylene, polypropylene, polystyrene, poly(ethylene vinyl acetate), polyvinyl alcohol and copolymers of these olefins.

In another embodiment, the shrinkable fibers may have a sheath and core structure; the sheath and core may have different crystallinities. In yet another embodiment, the elongated shapeable particles are shrinkable, for example including polyurethanes. The films may have two or more layers. In yet another embodiment, the elongated shapeable particles are non-degradable fibers. In a further embodiment, the shapeable elongated particles may include two or more substances having differing coefficients of thermal expansion. In another embodiment, the elongated shapeable particles are fibers selected from eccentric or concentric side-by- side multicomponent fibers, islands-in-the-sea multi-component fibers, segmented-pie cross-section type multi-component fibers, radial type multi-component fibers, and core-sheath multicomponent fibers.

Brief Description of the Drawings

Embodiments are described with reference to the following figures. The same numbers are used throughout the figures to reference like features and components.

Figure 1 shows schematically how elongated reshapeable fibers are reshaped into a tighter ball-like structure.

Figure 2 shows schematically how a mixture of elongated reshapeable fibers and proppant is reshaped into a tighter ball-like structure including proppant. Figure 3 shows schematically how elongated reshapeable fibers mixed with proppant are reshaped into a tighter ball-like structure, including proppant that forms pillar-like structures inside hydraulic fractures.

Figure 4 shows (in 4-1) a dense fiber-proppant structure with no voids, and (in 4-2) a fiber-proppant structure, with voids, having a fairly rigid network, formed by shrunken fibers, containing proppant particles.

Detailed Description

It should be noted that in the development of any actual embodiments, numerous implementation-specific decisions may be made to achieve the developer's specific goals, for example compliance with system- and business-related constraints, which can vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The description and examples are presented solely for the purpose of illustrating embodiments and should not be construed as a limitation to the scope and applicability. Embodiments may be described in terms of treatment of vertical wells, but are equally applicable to wells of any orientation. Embodiments may be described for hydrocarbon production wells, but it is to be understood that embodiments may be used for wells for production of other fluids, such as water or carbon dioxide, or, for example, for injection or storage wells. It should also be understood that throughout this specification, when a concentration or amount range is described as being useful, or suitable, or the like, it is intended that any and every concentration or amount within the range, including the end points, is to be considered as having been stated. Furthermore, each numerical value should be read once as modified by the term "about" (unless already expressly so modified) and then read again as not to be so modified unless otherwise stated in context. For example, "a range of from 1 to 10" is to be read as indicating each and every possible number along the continuum between about 1 and about 10. In other words, when a certain range is expressed, even if only a few specific data points are explicitly identified or referred to within the range, or even when no data points are referred to within the range, it is to be understood that the inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that the inventors have possession of the entire range and all points within the range.

The discussion in this paragraph of possible alternatives to the presently disclosed embodiments merely provides context information related to the present disclosure and may not constitute prior art. Much effort has been made in the past to minimize the impact of fracturing fluids on conductivity. Efforts included reducing polymer loading, incorporation of encapsulated breakers, and, more recently, the use of solids-free viscoelastic surfactants (VES). Although the development of VES fluid systems provides improvements from a fluid standpoint, fracture conductivity is still sometimes largely governed by the permeability of the proppant pack. Heterogeneous proppant placement had been an intense topic of research for a number of years. One option is described in U. S. Patent No. 8,061,424, U. S. Patent Application 20090286700 and U. S. Patent Application No. 20110083849, in which free-flow channels are created with heterogeneous placement of proppants. Such channels are created by placing proppants in the fracture in batches rather than continuously either by programmed pumping of different fluids with or without proppant, or by pumping of fluids containing proppant and fluids containing a dissolvable material. An important component of the techniques is elongated particles, which maintain the integrity of the proppant batches.

We disclose here embodiments of a method that utilizes a unique property of certain elongated shapeable particles which enables them to re-organize proppants inside a hydraulic fracture to create channel-like structures. This approach does not require any significant change in current pumping services. Pumping of certain shapeable materials with proppants creates channel-like structures during hydraulic fracturing treatments. The shapeable materials of particular interest are fibers that can shrink under downhole conditions; this action allows the mixtures of proppants and fibers to self-organize to create pillar-type structures. The shapeable materials may also serve as consolidating materials for the pillars, allowing the use of crushable materials as propping agents, which makes hydraulic fracturing treatments more economical and logistically feasible in certain instances.

Note that mixtures of elongated shapeable particles (for example, fibers) and fibers known to be used in oilfield treatments (for example to transport proppant) may be used. This may be done, for example, if the elongated shapeable particles are fibers, but the concentration is insufficient for optimal proppant transport. The concentration of elongated shapeable. particles must be sufficient to consolidate the solids.

We define elongated particles as particles having an aspect ratio of at least about 20, for example fibers having a length greater than about 20 times their diameter. Some elongated particles, for example certain fibers, undergo physical changes, for example from long extended shapes to contracted structures, at certain temperatures. Figure 1 shows schematically a change in the initial structure of a loose aggregation or collection (2] of individual fibers [4] in their initial shapes into a tighter ball-like subsequent structure [6] made up of fibers in their subsequent shapes. It is known that many plastic or thermoplastic materials undergo such transformations when heated. For example, in addition to fibers, elongated shapeable particles include films (sheets), platelets (flakes), ribbons and other shapes formed from suitable materials may crumple up into contracted shapes.

We define the three-dimensional structure of a shapeable material when the shapeable material is initially mixed with a fluid and proppant (and optionally other solids such as non-shapeable fibers used for proppant transports), pumped downhole, and deposited in a subterranean location in a wellbore or in a formation, as the "initial shape" and the three-dimensional structure of the shapeable material after reshaping of the shapeable material as the "subsequent shape". We define the three-dimensional structure of an accumulation of shapeable materials, and proppant and optionally other solid materials (for example fluid loss agents and non-shapeable fibers used for proppant transport) mixed with the shapeable materials, when this accumulation is initially deposited in a subterranean location in a wellbore or in a formation, as the "initial structure" and the three-dimensional structure of the shapeable materials, and proppant and optionally other solid materials (for example fluid loss agents and non- shapeable fibers) mixed with the shapeable materials, after reshaping of the shapeable materials as the "subsequent structure".

Embodiments described here relate to the use of such materials to create heterogeneous proppant placement during hydraulic fracturing treatments to create channel like structures. The shapeable materials, such as fibers, are mixed with proppants on the surface and pumped into a fracture as it is forming. Under reservoir conditions such materials undergo physical changes, for example to create a more compact structure [10] of fibers [4] with proppants [8] (as shown in Figure 2) resulting in pillar like structures between the fracture faces [12] inside the hydraulic fracture (as shown in Figure 3). In another words, the more compact structures, or bundles, form "islands" that keep the fracture open along its length but provide a lot of channels for the formation fluids to circulate.

In some embodiments, using shapeable materials is the only method of creating proppant heterogeneity in fractures. In such cases, the ratio of shapeable elongated materials to proppant may be constant or may vary as the proppant concentration varies, if it does, from stage to stage. Shapeable elongated particles may be included in all stages or in some stages. Typically, proppant concentrations range from 0 to about 960 g/L (about 8 ppa (pounds proppant added)) and the concentration of shapeable elongated particles may be up to about 24 g/L (about 200 lb/1000 gal) (which is typically the pump and pumping rate limit of available oilfield equipment). The ratio of proppant to shapeable elongated particles may vary, in part depending upon whether dense or fluid-permeable pillars are desired. We have found a particularly desirable concentration of shapeable elongated particles to be in the range of from about 2.4 g/L to about 24 g/L, especially about 12 g/L.

We have found that the best ratio of proppant to fiber, to form fluid-permeable pillars, is about 10/1 (for example 120 g/L of proppant and 12 g/L of shrinkable fibers, as shown in examples 2 and 3 below). The greater the ratio of proppant to fiber, the denser the pillar and the less fluid conductivity it has. Therefore, increasing the fiber concentration, and/or decreasing the proppant concentration, results in a greater probability of creating a material of "shape B" (see below), while decreasing the fiber concentration, and/or or increasing the proppant concentration, results in a greater probability of creating a material of "shape A" (see below).

In other embodiments the use of shapeable elongated particles may be used in conjunction with other methods of creating proppant heterogeneity in fractures. Such other methods include (a) sequentially injecting into the wellbore alternating stages of fracturing fluids having a contrast in their ability to transport propping agents to improve proppant placement, or having a contrast in the amount of transported propping agents; (b) pumping alternating fluid systems during the proppant stages applied to fracturing treatments using long pad stages and slurry stages at very low proppant concentrations; this is a form of what is commonly known as "waterfracs", also known in the industry as "slickwater" treatments or "hybrid waterfrac treatments"; (c) pumping a first stage that involves injection into a borehole of fracturing fluid containing thickeners to create a fracture in the formation; and a second stage that involves periodic introduction of proppant into the injected fracturing fluid to supply the proppant into a created fracture, to form proppant clusters within the fracture to prevent fracture closure and to form channels for flowing formation fluids between the clusters, in which the second stage or its sub- stages may involve additional introduction of either a reinforcing or consolidation material or both, thus increasing the strength of the proppant clusters formed in the fracture fluid; and (d) injecting a well treatment fluid containing proppant and proppant-spacing filler material (called a channelant) through a wellbore into the fracture, heterogeneously placing the proppant in the fracture in a plurality of proppant clusters or islands spaced apart by the channelant, and removing the channelant filler material to form open channels around the pillars for fluid flow from the formation through the fracture toward the wellbore. Elongated shapeable particles may be included in the proppant stages of any of these techniques to consolidate the solids and thus to increase the heterogeneity of the final proppant placement; this may provide additional channel creation and increase the porosity, and thus the conductivity, of the proppant pack. In method (d) above, the channelant itself may be an elongated particle such as a polyvinyl alcohol fiber or a polylactic acid fiber that can aid in proppant transport and placement, and is or is not reshapeable depending upon its structure and composition, and is soluble in the formation fluid and/or in the fracturing fluid at formation temperature.

Some single-component materials, for example PLA fibers, are shrinkable; in general, fibers made from many amorphous polymers may be shrinkable. Most suitable shapeable materials are typically multicomponent materials, for example multicomponent fibers, for example two-component fibers. The initial shapes of suitable shapeable materials include fibers, films, ribbons, platelets, flakes and other shapes having an aspect ratio of greater than about 20 (the aspect ratio of a flake, ribbon or film is the ratio of the average surface area to the average thickness). Common structures of multicomponent fibers, for example side-by-side, sheath-core, segmented pie, islands-in-the-sea, and combination of such configurations, and methods of forming such multicomponent fibers, are well known to those of ordinary skill in the art of making fibers. For example, such fibers and methods of making them are described in U. S. Patent No. 7,851,391. The differences in the compositions of the different components, and their consequent differences in behavior when subjected to changes in conditions downhole (such as differences in shrinkage or elongation with differences in temperature or with sorption of fluids such as oil and water or, with differences of sorption of fluids such as oil and water, or with changes in pH or salinity) are responsible for the changes in shape.

Any elements of the disclosed embodiments may be replaced by any one of numerous equivalent alternatives, only some of which are disclosed in the specification.

Generally, a property of particular importance in making polymers shapeable (and therefore for making elongated shapeable particles) is shrinkage, which will be discussed here, as one example, for polylactic acid. It is known that simple PLA fibers and films are subject to substantial heat shrinkage, and that the motive for much research on PLA fibers has been to increase the melting point and increase resistance to heat shrinkage. Many patents describe improved PLA fibers that have up to "only" 15% or "only" 20 % shrinkage. On the other hand, PLA is commonly used as the shapeable component of "shrink-wrap" or "heat-shrink" films and research has been directed towards controlling and even increasing shrinkage. Following are discussions of shrinkage of PLA fibers and of PL A films; shrinkage of PLA fibers and films are, for the most part, governed by the same principles, as are the shrinkage of other polymeric fibers and films.

Shrinkage is generally measured by immersing a polymer in boiling water or treating it with hot air. The desired degree of fiber shrinkage relates to the intended application of the fiber. There are typically two types of fibers or components of fibers. The first type includes stable, or low shrinkage, fibers. These fibers usually have a shrinkage of less than about 20% and generally have shrinkage of less than about 15%. Researchers have studied the relationship between the shrinkage properties of polylactic acid fiber and other parameters including polymer composition, molecular weight, degree of branching or crosslinking, presence of additives including nucleating agents, and stress inducing techniques. The second type of fiber includes high shrinkage fibers. High shrinkage fibers will generally exhibit a degree of shrinkage of greater than 10%, and preferably greater than 15%. Useful values can be provided as high as 20-80%. High shrinkage values can be provided by drawing fiber filaments from melts at filament velocities which are below the critical velocity for establishing crystallization. This results in elongation and orientation of the fiber, but does not provide the physical crosslinks of crystallization to heat stabilize the fiber. Thus, high shrinkage fiber can be obtained. (During production, different conditions may be used, for example different stretching velocities or different temperatures during stretching; as a result, fibers accumulate internal tension in their structures and try to return to their initial undrawn structures upon being heated).

The extent to which fibers of PLA or PLA-based polymers shrink, when exposed to heat, relates to the extent to which the method by which the fibers were formed generated a completely relaxed fiber. A preferred process for generating a low shrink fiber is to make highly crystallized and stress-relaxed fiber. In addition, shrinkage is affected by the composition of the fiber. Other factors which affect the presence of stress are molecular weight, molecular weight distribution, degree of branching, melt temperature of the polymer, draw rate, mass throughput, quench rate, orientation and crystallinity. Generally, polylactide polymers provided with low D-lactide levels (and correspondingly high L-lactide levels) crystallize at lower levels of spin-line stress. Lower levels of D-lactide correspond to levels of less than about 5% R-lactic acid residues provided either through D-lactide or meso-lactide. The reduction in R-lactic acid residue corresponds with a decrease in spin-line stress required to induce a similar degree of crystallization. In addition, low R-lactic acid polymers can obtain a higher level of crystallinity. The essence of this is to provide polylactide polymer having high enantiomeric purity. This can be provided by either providing low R- lactic acid residues or low S-lactic acid residues. Higher crystallinity fibers will provide lower fiber shrinkage.

It has also been found that a linear polylactide results in lower shrinkage; branching and crosslinking tend to result in higher shrinkage. However, branching and crosslinking appear to increase spin-line stress at a given filament velocity and therefore have a lower velocity for the onset of shrinkage reduction.

As described in U. S. Patent No. 7,846,517, for film manufacture, PLA optionally contains small amounts of antiblock additives, skip additives, viscosity enhancers, and impact modifiers. After the polymer composition of the film is selected, the polymer is then processed to generate a film with desirable shrink properties. Generally, the polymers are procured in pellets or grains. In cases where multiple polymers are to be included, the polymers pellets are first dry blended. That is, the pellets are mixed together. In a preferred embodiment, the pellets are then processed into film by blown film technology, which typically includes five steps: extrusion, temperature conditioning, orienting, and collapsing. A preliminary step of drying the polymer pellets is preferable, but not required. As well, a terminal step of annealing may be preferable, but not required.

The polylactide used in film form in embodiments may be made from L-, D- or D,L-lactide, or blends thereof, by any polymerization process. A high molecular weight polymer can be produced by ring-opening polymerization from lactic acid dimer (lactide). Lactic acid is optically active, and thus its dimer appears in four different forms: L,L-lactide; D,D-lactide; L,D-lactide ("mesolactide"); and a racemic mixture of L,L- and D,D-lactides. By polymerizing these dimers either as pure compounds or at different blend proportions, polymers are obtained which have different stereochemical structures affecting their resilience and crystallinity and, consequently, also their mechanical and thermal properties. Common PLA polymers may comprise about 1 to about 2 percent by weight D-lactide; about 3 to about 5 percent by weight D-lactide; or about 1 1 to about 13 percent by weight D-lactide. In some embodiments, the shrink films may comprise about 50 weight percent to about 90 weight percent of PLA polymer having about 11 to about 13 percent by weight D- lactide; and about 10 weight percent to about 50 weight percent of a PLA polymer having about 1 to about 2 percent by weight D-lactide. Polymers and/or polymer blends with higher levels of D-lactide can yield films that begin to shrink at lower temperatures when exposed to heat; these films also tend to exhibit more "gentle" shrink curves, i.e., less shrinkage per rise in temperature. Conversely, films comprising polymers with relatively low D-lactide concentration generally require exposure to higher temperatures to shrink.

After melting the polymer, the next step in preparation of films is typically orienting, also known as stretching. This step imparts the shrink "memory" into the film where it is "stored" by the polymer blend. Orienting can be accomplished by many methods and associated equipment known to one of ordinary skill in the art, including, for example, machine/cross direction orientation and blown film orientation. All methods are preferably designed to first control the temperature of the polymer, followed by a controlled stretching operation. Without being limited to or bound by theory, it is believed that the orienting process conveys strength and flexibility to the film product. Annealing, also called crystallization or relaxation, is typically the final step in the preparation of films. An annealing step is optional. When desired, annealing is generally accomplished post-orienting, and performed at temperatures between about 120° F. to about 285° F. in some embodiments. This process is accomplished by rotating heated cylinders that contact the film just prior to the winding process where the finished roll of plastic film is generated. Film properties can be manipulated as desired with nominal trial and error by one of ordinary skill in the art. Films can generally have characteristics that fall into the following ranges after heating at 95° C for about 10 seconds: o Longitudinal direction: about 10% to about 90% shrinkage, with an average of about 50%.

o Cross direction: about 0% to about -30% shrinkage (growth), with an average of about -5%.

Following are non-limiting examples of shapeable materials that may be used in embodiments disclosed herein. For shrinkable materials, shrinkage of from about 20 to about 80 per cent is preferred; shrinkage of from about 40 to about 80 per cent is more preferred, although less or more shrinkage is suitable. Other suitable materials may readily be identified or conceived of by readers of this disclosure.

One example of suitable shapeable material is two-component fibers made of a core material and a sheath material that have different melting points. The core material (for example a thermoplastic resin, for example a polypropylene or a polyester) normally is used to ensure the integrity of the material during use; this core is not normally melted as the shapeable material is reshaped, and may, for example, form a three-dimensional network in the newly shaped subsequent structure, giving the subsequent structure strength. The sheath material (for example a thermoplastic resin, for example a polyethylene) has a lower melting and bonding temperature and thus may be used to hold the subsequent structure together and in the new shape. The melting point of the sheath material may be about 80 °C; the melting point of the core material may commonly be up to about 160 °C. Such materials may be manufactured with the sheath and core eccentric or concentric, and the fibers may be available in conventional form or available commercially already in a crimped (zigzag), wavy, or spiral form. Such fibers are available, for example, from ES FIBERVISIONS. Such shrinkable fibers are described in U. S. Patent Application Publication No. 2010/0227166.

Another example of suitable shapeable materials is highly shrinkable copolyamide fiber (having high wet heat shrinkage characteristics and low dry heat shrinkage characteristics) as disclosed by Toray Industries, Inc. An example of a suitable fiber is described in JP08209444. Another example is a staple fiber obtained by extruding a copolyester including (A) isophthalic acid and (B) 2,2-bis{4-(2- hydroxyethoxy)phenyl} propane as copolymerizing components, as described in JP 10204722. This latter fiber undergoes less than or equal to 20 percent shrinkage in boiling water, and 12 to 40 percent shrinkage in 160 °C dry air after treating in boiling water.

Yet another example of suitable shapeable materials is a polyester fiber having a diol component and a dicarboxylic acid component; for example the diol may be 1 ,1-cyclohexanedimethanol or its ester-forming derivative (or biphenyl-2,2'- dicarboxylic acid or its ester- forming derivative) in an amount of 2 to 20 mole percent based on the whole dicarboxylic acid component. Such fibers were disclosed by Kuraray in JP 9078345 and JP 81 13825. Other suitable materials from Kuraray include the polyester fibers described in U. S. Patent No. 5567796.

Nippon Ester Company Ltd. has described several fibers suitable for use as shapeable materials. A highly shrinkable conjugated fiber disclosed in Japanese Patent Application No. JP 2003-221737 is composed of a polyester, A, containing polyethylene terephthalate as a main component (prepared by copolymerizing an aromatic dicarboxylic acid having a metal sulfonate group in an amount of from 3 to 7 mole percent based on the whole acid component or an isophthalic acid in an amount of from 8 to 40 mole percent) and a polyester, B, that is ethylene terephthalate. The difference in melting point between polyester A and polyester B is at least 5 °C and the difference between the heat of melting of polyester A and polyester B is at least 20 J/g. The dry heat shrinkage at 170 °C is at least 15 percent. Another fiber described by Nippon Ester Company Ltd. in Japanese Patent No. JP 08035120 is a highly shrinkable polyester conjugated fiber obtained by conjugate spinning in a side-by-side fashion of polyethylene terephthalate and a polyethylene terephthalate copolymerized with 8 to 40 mole percent of isophthalic acid at a weight ratio of from 20:80 to 70:30. The product having a single fiber fineness of 1 to 20 denier has a hot water shrinkage at 90 °C of from 70 to 95 percent.

Kaneka Corporation has described several fibers suitable for use as shapeable materials in embodiments described herein in U. S. Patent Application Publication No. 2002/0122937 and U. S. Patent No. 7,612,000. They include a hollow shrinkable copolymer fiber made of acrylonitrile and a halogen-containing vinyl monomer manufactured by wet spinning followed by steam treatment, drying, and heating. Some examples contain one or more of acrylic acid, methacrylic acid, vinyl chloride, vinylidene chloride, vinyl esters (for example vinyl acetate, vinyl pyrrolidone, vinyl pyridine and their alkyl-substituted derivatives), amides, and methacrylic acid amides. In these references, one of the monomers may be halogen-containing to provide fire- resistance to the fiber; in the present application, this is not necessary. Other examples are modacrylic shrinkable fibers made from 50 to 99 parts by weight of a polymer (A) containing 40 to 80 weight percent acrylonitrile, 20 to 60 weight percent of a halogen-containing monomer, and 0 to 5 weight percent of a sulfonic acid- containing monomer, and 1 to 50 parts by weight of a polymer (B) containing 5 to 70 weight percent acrylonitrile, 20 to 94 weight percent of an acrylic ester, and 16 to 40 weight percent of a sulfonic acid-containing monomer containing a methallylsulfonic acid or methallylsulfonic acid metal salt, and no halogen-containing monomer. Some examples of the fibers contain from 10 to 50 percent voids, and shrink at least 15 percent (and often over 30 percent) at from 100 to 150 °C in 20 minutes. They may be crimped before use.

KB Seiren Ltd. has described in U. S. Patent Application Publication No. 2010/0137527 a fiber that is suitable for shapeable materials. It is a highly shrinkable (for example in boiling water) fiber that is composed of a mixture of a nylon-MXD6 polymer (a crystalline polyamide obtained from a polymerization reaction of metaxylenediamine and adipic acid) and a nylon-6 polymer in a weight ratio of from 35:65 to 70:30. The fiber is made by melt spinning and drawing or draw-twisting. The fiber shrinks 43 to 53 percent in hot water at from 90 to 100 °C. Inorganic particles, for example Ti0₂, may be added to improve the spinning process.

Shimadzu Corporation described in U. S. Patent No. 6,844,063 a core-sheath conjugated fiber (that is a fiber having two or more different polymers in a single filament), that is suitable as a shapeable material, made from a sheath of (A) a low heat-shrinkability component that is a highly crystalline aliphatic polyester (having a melting point above 140 °C) and a core of (B) a high heat-shrinkability polymer containing at least 10 percent by weight of a low crystallinity aliphatic polyester having a melting point lower than that of component (A) by at least 20 °C. The difference in shrinkability is at least 3 percent, preferably 5 to 70 percent, and more preferably about 10 to about 50 percent. In addition to the core-sheath structure, U. S. Patent No. 6,844,063 also describes other suitable conjugated structures such as concentric core-sheath, eccentric (non-concentric) core-sheath, parallel, keyhole, hollow, double core, non-circular (for example trilobe cross- section), hollow parallel, three-layered parallel, multi-layered parallel, one polymer disposed in radial alignment, sea-islands (or islands-in-the-sea), and others.

Kanebo Ltd. described, in Japanese Patent No. JP7305225, highly shrinkable polyester staple polymers obtained by melt-spinning a polymer made from a polyethylene terephthalate and subjecting it to specified melt-spinning drawing and post-treating processes under specified conditions. Examples are polyethylene terephthalate core-sheath structures with in which the core and sheath have different crystallinities.

U. S. Patent No. 6,844,062 describes spontaneously degradable fibers and goods made with fibers having a core-sheath structure including (A) a low heat- shrinkable fiber component comprising a high crystalline aliphatic polyester and (B) a high heat-shrinkable fiber component comprising an aliphatic polyester, for example a low crystalline or non-crystalline aliphatic polyester. Examples of polymer (A) include homopolymers such as polybutylene succinate (melting point about 1 16° C), poly-L-lactic acid (m.p. 175° C), poly-D-lactic acid (m.p. 175° C), polyhydroxybutyrate (m. p. 180° C.) and polyglycolic acid (m.p. 230° C), and copolymers or mixtures of these with small amounts of other components. Polymer (B) is a component having a low crystallinity and a high heat shrinkability. The component used for the copolymerization or mixing with the homopolymers with high melting point such as polybutylene succinate, polylactic acid, polyhydroxybutyrate and polyglycolic acid can be suitably selected from the raw materials for the preparation of the above-mentioned aliphatic polyesters.

Yet another suitable shapeable material was described in U. S. Patent No. 5,635,298. It is a monofilament having a core-sheath structure including a core of a thermoplastic polyester or copolyester and a sheath of a thermoplastic polyester, in which the polyester or copolyester of the core has a melting point of 200 to 300 °C, preferably of 220 to 285 °C, and includes at least 70 mole percent, based on the totality of all polyester structural units, of structural units derived from aromatic dicarboxylic acids and from aliphatic diols, and not more than 30 mole percent, based on the totality of all polyester structural units, of dicarboxylic acid units which differ from the aromatic dicarboxylic acid units which form the predominant portion of the dicarboxylic acid units, and diol units derived from aliphatic diols and which differ from the diol units which form the predominant portion of the diol units, and the sheath is made of a polyester mixture containing a thermoplastic polyester whose melting point is between 200 and 300 °C, preferably between 220 and 285 °C, and a thermoplastic, elastomeric copolyether-ester with or without customary nonpolymeric additives. The core-sheath monofilaments, if the core and sheath materials are separately melted and extruded, then cooled, then subjected to an afterdraw and subsequently heat-set, all under conditions as specified in the patent, preferably have a dry hear shrinkage at 180 °C of from 2 to 30 percent.

U. S. Patent No. 5,688,594 describes a hybrid yarn, the fibers of which are suitable shapeable materials for embodiments described herein. The hybrid yarn contains at least two varieties of filaments: (A) has a dry heat shrinkage of less than 7.5%, and (B) has a dry heat shrinkage of above 10%. Appropriate heating forces the lower-shrinking filaments to undergo crimping or curling. (A) is, for example, aramid, polyester, polyacrylonitrile, polypropylene, polyetherketone, polyetheretherketone, polyoxymethylene, metal, glass, ceramic or carbon, and (B) is, for example, drawn polyester, polyamide, polyethylene terephthalate, or polyetherimide.

U. S. Patent Application No. 20100227166 describes the preparation and use of a shrinkable fiber composed of a first thermoplastic resin and optionally a second thermoplastic resin having a higher melting point than the first thermoplastic resin. Examples of suitable thermoplastic resins include ethylene copolymers such as ethylene-vinyl acetate copolymer, ethylene-methacrylic acid copolymer and ethylene- acrylate copolymer, elastomer resins such as poly-alpha-olefin and styrene-ethylene- butylene-styrene copolymer, low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene, and propylene copolymers such as ethylene-propylene copolymer and ethylene-butene-propylene copolymer. Examples of suitable thermoplastic resin combinations given are low-density polyethylene/polypropylene, linear low-density polyethylene/polypropylene, ethylene-vinyl acetate copolymer/polypropylene, ethylene-methacrylic acid copolymer/polypropylene, propylene copolymer/polypropylene, low-density polyethylene/propylene copolymer, ethylene-vinyl acetate copolymer/propylene copolymer, and ethylene-methacrylic acid copolymer/propylene copolymer.

U. S. Patent No. 4,857,399 describes a four-layer shrink film, pieces of which are suitable shapeable materials for embodiments described herein. The film comprises an ethylene-propylene random copolymer first layer, a blend of anhydride- modified ethylene copolymer adhesive and ethylene vinyl acetate as an inner core second layer, a blend of partially hydrolyzed ethylene vinyl acetate copolymer and amide polymer as a third layer, and a blend of anhydride-modified ethylene copolymer adhesive and ethylene vinyl acetate as a fourth layer.

U. S. Patent Application No. 20070298273 discloses biaxial ly-oriented multilayer thermoplastic heat shrinkable films, small pieces of which are suitable shapeable materials for embodiments described herein. Such films are made in one embodiment from (a) two outer-film layers each comprising a polyolefin, and (b) a core layer comprising a blend of at least 50% by weight relative to the core layer of a first material comprising an ethylene unsaturated-ester copolymer and a second material selected from ionomers (ionic copolymers and terpolymers formed from an olefin and an ethylenically unsaturated monocarboxylic acid having the carboxylic acid moieties partially or completely neutralized by a metal ion), ethylene/acid copolymers and terpolymers and blends thereof. In a second embodiment, such films are made from (a) two outer-film layers each comprising a blend of a linear low- density polyethylene, a very low-density polyethylene or an ultra low-density polyethylene copolymer and a low-density polyethylene, and (b) a core layer comprising a blend of at least 50% by weight relative to the core layer of a first material selected from the group consisting of ethylene vinyl acetate copolymer, ethylene butyl acetate copolymer, ethylene methyl acetate copolymer, ethylene ethyl acetate copolymer, and blends thereof, and a second material selected from the group consisting of ionomers, ethylene/acid copolymers and terpolymers, and blends thereof, and at least 20% by weight relative to the core layer of a second material selected from the group consisting of ionomers, ethylene/acid copolymers and terpolymers, and blends thereof. In a third embodiment, such films are made from (a) a first and a second outer-film layer each comprising a blend of a linear low-density polyethylene, a very low-density polyethylene or an ultra low-density polyethylene copolymer and a low-density polyethylene; (b) a core layer disposed between the first and second outer-film layers and comprising a blend of at least 50% by weight relative to the core layer of a first material selected from the group consisting of ethylene vinyl acetate copolymer, ethylene butyl acetate copolymer, ethylene methyl acetate copolymer, ethylene ethyl acetate copolymer, and blends thereof, and a second material selected from the group consisting of ionomers, ethylene/acid copolymers and terpolymers, and blends thereof, at least 20% by weight relative to the core layer of a second material selected from the group consisting of ionomers, ethylene/acid copolymers and terpolymers, and blends thereof, and between 0.2 to 1.0% by weight of an amide slip agent; in which the core layer has a thickness of at least 50% of the total thickness of the film; and (c) a first intermediate layer positioned between the first outer-film layer and the core layer, and a second intermediate layer positioned between the second outer-film layer and the core layer; and where each of the intermediated layers comprises a polyolefin.

U. S. Patent No. 8,0217,60 describes the preparation of multilayer heat shrinkable films made with homopolymers and copolymers of a variety of resins such as the following polymers, their copolymers, or blends: polyolefin, polyethylene, ethylene/alpha olefin copolymer, ethylene/vinyl acetate copolymer; ionomer resin; ethylene/acrylic or methacrylic acid copolymer; ethylene/acrylate or methacrylate copolymer; low density polyethylene, polypropylene, polystyrene, polycarbonate, polyamide (nylon), acrylic polymer, polyurethane, polyvinyl chloride, polyvinylidene chloride, polyester, ethylene/styrene copolymer, norbornene/ethylene copolymer, and ethylene/vinyl alcohol copolymer.

Note that not all elongated particles, such as fibers and films, made with the compositions described above, are shapeable. The shapeability may depend upon such factors as crystallinity, branching, and molecular weight, and, in the case of copolymers, the relative ratios of the monomers. Furthermore, elongated shapeable particles, for example shrinkable fibers, may include shapeable portions, for example strands, and non-shapeable portions. The non-shapeable portions may be inert or may be removable, for example by melting, dissolving, or degrading. Suitable elongated shapeable particles may be obtained commercially or may be synthesized by those skilled in the art of making plastic materials.

In general, the lower limit for fiber diameter for typical shrinkable organic fibers is about 1.3 dtex (1 1 microns), which is based primarily on current manufacturing limitations. The upper limit is based on limitations of typical oilfield pumping equipment. On a weight basis, the larger the fiber diameter, the less the total fiber length that is pumped and the fewer fiber filaments are pumped. However, in embodiments described here, shapeable fibers are pumped with proppant; under such circumstances it is believed that 4.4 dtex fibers can be pumped with present-day equipment.

The elongated shapeable materials may reduce the bulk volume of the solids inside a hydraulic fracture, because, after they are placed but before they are reshaped, they are dispersed. Upon reshaping, the fibers tend to coil up and intertwine with one another to form a denser structure that occupies less space.

Embodiments can be further understood from the following examples. Example 1:

A homogeneous aqueous slurry containing 2.40 g/L of linear guar gel, 6.0 g/L mica (obtained from Mondo Minerals, Amsterdam, NL, having a specific gravity of 2.9 g/cc, and 0.1-0.3 mm average particle size (50/140 U. S. mesh) and 4.79 g/L of polylactic acid shrinkable sheath/core fibers ("Trevira" T266 series (containing equal parts of a more amorphous PLA sheath and a more crystalline PLA core), 6 mm in length and one of three different diameters (22 microns (4.4 dtex), 17 microns (2.2 dtex) or 14 microns (1.7 dtex)), obtained from Trevira, GmbH, DE) was placed in a flat slot (two panes of PLEXIGLASS™ organic glass 10 mm apart, stuck together with small panes of PLEXIGLASS™ organic glass 6 mm apart). The inner slot size was 220 x 220 x 6 mm, enclosing a volume of about 330 ml. The slot could be placed horizontally or vertically in an oven at 82 °C (180 °F). After one hour consolidated pillars approximately 3 to 6 cm in diameter containing a mixture of mica particles and fiber flocks were found in the slot, with channels free of any particles between the flocks; there was significant consolidation of the mica because of the fiber shrinkage. There was no significant difference observed in pillar shape or size whether the slot placement was horizontal or vertical.

Example 2.

A well-mixed aqueous slurry containing 5.4 g/L of linear guar gel, 12 g/L of polylactic acid shrinkable sheath/core fibers ("Trevira" T266 series (containing equal parts of a more amorphous PLA sheath and a more crystalline PLA core), 6 mm in length and 22 microns (4.4 dtex), 17 microns (2.2 dtex) or 14 microns (1.7 dtex) in diameter) and 120 g/L of proppant (HSP (High Strength Proppant) CarboProp with particle sizes of 0.42 - 0.85 mm (20/40 U. S. mesh), or 0.30 - 0.60 mm (30/50 U. S. mesh) or sand with 0.15 mm particle size (100 U. S. mesh size)) was placed in a flat slot as in Example 1. The slot could be placed horizontally or vertically in an oven at 82 °C (180 °F). After one hour, fiber-proppant pillars approximately 8 to 10 cm in diameter were found in the slot in all experiments. There was no significant difference observed in pillar shape or size whether the slot placement was horizontal or vertical. The difference in pillar size relative to Example 1 may be related to the different particles sizes. The fiber/proppant mass ratio used allowed formation of a consolidated pillar that included all of the proppant. Generally, there were two main fiber-proppant pillar structures, shown in Table 1 below^ obtained upon fiber shrinkage under these conditions. "Shape A" (Figure 4-1) was a dense fiber-proppant structure with no voids. It showed comparatively low conductivity values and can be used for solid pillar formation or for tail-in pumping stages (to create highly conductive channels (between the pillars) near the wellbore when other fracturing techniques, such as pulsing or conventional treatments, are used as the principal treatments). "Shape B" (Figure 4-2) was a fiber-proppant structure having voids and a relatively rigid network, containing proppant particles, formed by shrunken fibers. A number of channels free of fiber or proppant particles were observed in this structure, so it is expected to have high conductivity. Fiber

Proppant particle size, 1.7 dtex 2.2 dtex 4.4 dtex

mm

0.42 - 0.85 A A B

0.30 - 0.60 A B B

0.15 B B B

Table 1

Example 3.

A well-mixed aqueous slurry containing 2.4 g/L of linear guar gel solution with 1.2 ml/L of borate-based alkaline cross-linking agent (pFL=12), 12 g/L of polylactic acid shrinkable sheath core fibers ("Trevira" T266 series (containing equal parts of a more amorphous PLA sheath and a more crystalline PLA core), 6 mm in length and 22 microns (4.4 dtex), 17 microns (2.2 dtex) or 14 microns (1.7 dtex) in diameter) and 120 g/L of proppant (HSP CarboProp with particle sizes of 0.42 - 0.85 mm (20/40 U. S. mesh), or 0.30 - 0.60 mm (30/50 U. S. mesh) or sand with 0.15 mm particle size (100 U. S. mesh size)) was placed in a flat slot as in Example 1. The slot could be placed horizontally or vertically in an oven at 82 °C (180 °F). There was no significant difference observed in pillar shape or size whether the slot placement was horizontal or vertical. After one hour, fiber-proppant pillars approximately 8 to 10 cm in diameter were found in the slot in most experiments. The fiber/proppant mass ratios used allowed formation of consolidated pillars that included all the proppant. Generally, there were three main results obtained upon fiber shrinkage under these conditions, as shown in Table 2. "Shape A" (Figure 4-1) was a dense fiber-proppant structure having no voids. It is expected to have a comparatively low conductivity and can be used for solid pillar formation or for a tail-in pumping stage. "Shape B" (Figure 4-2) fiber-proppant structures had a relatively rigid network formed by shrunken fibers with proppant particles well-distributed within them. A number of channels free of fiber or proppant particles were observed in this structure, so it is expected to have high fluid conductivity. In the case of the fiber with the smallest diameter (1.7 dtex), no defined pillar structure was obtained, due to significant fiber decomposition in the alkali medium. The higher the pH, the faster the fiber degradation, and the larger the diameter that would be needed to survive at that pH.

Table 2

Example 4.

An example of a typical job design using elongated shapeable particles, in this case shrinkable fibers, during the tail-in stage is presented in Table 3 below, showing how an embodiment would be put into practice. The fracturing fluid is a well-mixed aqueous slurry containing 2.4 g/L of linear guar gel solution with 1.2 ml/L of a borate-based alkaline cross-linking agent at a pH of 12. The Slickwater fluid is a conventional water containing friction lowering agents. PTA (proppant transport additive) is non-shrinking polylactic acid fiber (monocomponent fibers 6 mm in length and 12 microns in diameter, available from Trevira). T266 is the polylactic acid shrinkable sheath/core fiber described above, (containing equal parts of a more amorphous PLA sheath and a more crystalline PLA core, 6 mm in length and 17 microns in diameter, also available from Trevira). The proppant used^' is 0.15 mm particles size sand, or HSP CarboProp 0.21 - 0.42 mm (40/70 U. S. Mesh) and 0.30 - 0.60 mm (30/50 U. S. mesh) . Table 3

Any element in the examples may be replaced by any one of numerous equivalent alternatives, only some of which are disclosed in the specification.

Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the concepts described herein. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus- function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.

Claims

We claim:

1. A method of fracturing comprising injecting into a wellbore penetrating a subterranean formation above fracture pressure a fluid comprising a carrier fluid and a mixture of elongated shapeable particles and proppant and allowing the elongated shapeable particles to undergo a change in shape.

2. The method of claim 1 wherein the elongated shapeable particles are selected from the group consisting of fibers, ribbons, flakes, films, sheets, platelets, and flakes, having aspect ratios greater than or equal to about 20.

3. The method of claim 1 wherein the elongated shapeable particles are shrinkable fibers.

4. The method of claim 3 wherein the shrinkable fibers are degradable.

5. The method of claim 4 wherein the degradable fibers comprise polyesters.

6. The method of claim 5 wherein the polyesters are selected from polylactic acid, polyglycolic acid, polyethylene terephthalate, poly(hydroxyalkanoate) and copolymers thereof.

7. The method of claim 4 wherein the degradable fibers comprise polyamides.

8. The method of claim 4 wherein the polyamides are selected from nylon 6; nylon 6,6; and nylon 6,12.

9. The method of claim 4 wherein the degradable fibers comprise polyolefins.

10. The method of claim 9 wherein the polyolefins are selected from polyethylene, polypropylene, polystyrene, poly(ethylene vinyl acetate), polyvinyl alcohol and copolymers thereof.

1 1. The method of claim 3 wherein the shrinkable fibers have a sheath and core structure.

12. The method of claim 12 wherein the sheath and core have different crystallinities.

13. The method of claim 1 wherein the elongated shapeable particles are shrinkable films.

14. The method of claim 1 wherein the shrinkable films comprise polyurethanes.

15. The method of claim 14 wherein the shrinkable films comprise two or more layers.

16. . The method of claim 1 wherein the elongated shapeable particles are non- degradable fibers.

17. The method of claim 1 wherein the shapeable elongated particles comprise two or more substances having differing coefficients of thermal expansion.

18. The method of claim 17 wherein the elongated shapeable particles are fibers selected from the group consisting of eccentric or concentric side-by-side multicomponent fibers, islands-in-the-sea multi-component fibers, segmented-pie cross-section type multi-component fibers, radial type multi -component fibers, and core-sheath multicomponent fibers.