WO2014042552A1

WO2014042552A1 - Shapeable particles in oilfield fluids

Info

Publication number: WO2014042552A1
Application number: PCT/RU2012/000761
Authority: WO
Inventors: Patric SCHORN; Irina Aleksandrovna LOMOVSKAYA; Diankui Fu
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

Abstract

A method of treating a wellbore penetrating subterranean formations including injecting into the wellbore a treatment fluid including a drilling fluid and a mixture of elongated shapeable particles, 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 in the drilling fluid causes the reorganization and agglomeration of the solid particles. The treatment may be mitigation of fluid loss or hydraulic fracturing

Description

SHAPEABLE PARTICLES IN OILFIELD FLUIDS

Background

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

During the drilling of wells, for example injection wells or wells for the production of hydrocarbons, drilling fluid is injected into the well through a drill pipe and circulated to the surface in the annular area between the wellbore wall and the drill string. In some cases practically all of the wellbore fluid may be lost to the formation. For example, wellbore fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole. Also, the drill may penetrate large vugs. The loss of drilling fluid during drilling operations may lead to the phenomenon termed lost circulation, which is detrimental to drilling operations. In the worst cases, lost circulation of drilling fluid can result in well control issues; sometimes little or no fluid returns to the surface and sometimes it may be difficult to maintain the pressure in the hole without the expected weight of the fluid column. Therefore, providing effective fluid loss control is highly desirable to ensure drilling integrity and safety. Currently there are many lost circulation control products available on the market, but most of them work effectively only in cases of less severe lost circulation. When losses occur during drilling, the time needed to cure the losses is critical to well control and to minimizing the financial impact on production companies. Traditional lost circulation treatments generally require the preparation of fluids separately, which requires extra time and equipment. It is highly desirable to have a reliable system to address severe to total losses into fractures or vugs. The ability to turn the drilling fluid itself into a lost circulation control solution will be of great benefit.

After a well is drilled, hydraulic fracturing is one of the most effective methods for increasing the flow of fluids to and from the wellbore, for example for increasing the production of oil and gas from subterranean formations. A hydraulic fracture is created by injection of fluid, for example a water-based gelled slurry having a certain necessary viscosity and containing propping agents such as sands or ceramic materials, down the borehole and into the targeted reservoir interval at an injection rate and pressure sufficient to cause the reservoir rock to fracture. The proppant is typically included in the fracturing fluid to prevent fracture closure after pumping is stopped and the pressure is relieved, and to optimize fracture conductivity. The productivity of the well is determined by the geometry and the permeability (conductivity) of the propped fracture as well as formation properties; proppant packs themselves may impede fluid flow. 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. Furthermore, there has been a dramatic increase in the consumption of water and propping agents in recent years, for example because of the widespread application of hydraulic fracturing to shale formations. A method for the use of drilling fluids and drill cuttings in hydraulic fracturing treatments reduces the environmental footprint of drilling operations and the costs of fracturing materials, including water.

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.

One embodiment presented here is a method of treating a wellbore penetrating subterranean formations. The method includes injecting into the wellbore a treatment fluid, that contains a drilling fluid and a mixture of elongated shapeable particles, 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, having aspect ratios greater than or equal to about 20. The elongated shapeable particles may be shrinkable fibers that may be degradable. Such degradable fibers may be polyesters, for example polylactic acid, polyglycolic acid, polyethylene terephthalate, poly(hydroxyalkanoate) and copolymers of these. Alternatively the degradable elongated shapeable particles, for example fibers, may be polyamides, for example nylon 6; nylon 6,6; or nylon 6,12. The degradable fibers may also be polyolefins, for example polyethylene, polypropylene, polystyrene, poly(ethylene vinyl acetate), polyvinyl alcohol and copolymers of these. The shrinkable fibers may have a sheath and core structure, for example in which the sheath and core have different crystallinities. In other embodiments, the elongated shapeable particles may be shrinkable films, for example polyurethanes. The shrinkable films may have two or more layers. In yet other embodiments, the elongated shapeable particles may be non-degradable fibers, or may be made of two or more substances having differing coefficients of thermal expansion. In further embodiments, 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.

In one embodiment, the treatment fluid is injected into a wellbore during a drilling operation to mitigate lost circulation. In another embodiment, the treatment fluid is injected into a wellbore and into at least one formation above its fracture pressure to elongate and prop a fracture.

Another embodiment is a composition including a drilling fluid containing elongated shapeable particles.

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 agglomerate structure.

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

Figure 4 is a schematic of the formation of a channel-like structure in hydraulic fracturing.

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. Many materials are available for treating lost circulation while drilling; the main approach is to create a pill (a slug of fluid containing the treatment material) when losses are detected. The use of fibers in treatments of lost circulation while drilling has been described in U. S. Patent Application Publication Nos. 2010/0152070 and 201 1/0183874; the methods use a pill composition that contains two types of fibers of different lengths and rigidities and at least two sizes of particles in a water-based fluid; if the mud is oil-based, spacers are injected before and after the pill. Degradable materials in kill pills, for example polylactic acid, which hydrolyzes and releases an organic acid, were described in U. S. Patent No. 7,281,583 and U. S. Patent Application Publication No. 2010/0319915. Addition of a fluid containing water-dispersible glass fibers with fine solid particles, for example cement particles, was disclosed in U. S. Patent No. 7,331,391 ; the fluid may be the drilling fluid or a small-volume pill specially mixed for curing the lost circulation. Shrinkable fibers have not previously been disclosed for lost circulation compositions or treatments. In hydraulic fracturing, heterogeneous proppant placement is recognized as an excellent method of increasing proppant pack fluid conductivity. Surface modification of propping agents to promote agglomeration has been employed. In one approach to enhancing fracture conductivity, free-flow channels are created in the proppant pack by heterogeneous placement of proppants. One particular option is described in U. S. Patent Nos. 6,776,235, 7,581,590, and 8,061,424; U. S. Patent Application Publication Nos. 2009/0286700, 201 1/0083849 and 201 1/01 14313 and PCT Application Publication No WO2009/096805, 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, or fibers, which maintain the integrity of the proppant batches. One such scheme involves the inclusion of fibers in the fluid, with pulsing of batches of proppant/fibers and fibers alternately. The use of drilling fluid for hydraulic fracturing with shapeable materials has not been disclosed.

We have found that shrinkable fibers that change their shape when heated may be included in fluids used in certain oilfield treatments. We describe here the use of such elongated shapeable particles in drilling and in hydraulic fracturing. Note that whenever we refer to drilling and drilling fluids, we include completion and completion fluids in the term.

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; we define these as shapeable. 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], after an increase in temperature, 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. The change of shape or reshaping of embodiments described here is not the same as other shape changes of polymers such as fragmentation, assembling, flocculation, dispersion, or swelling, for example due to absorption of fluids, for example water. The change of shape or reshaping of embodiments described here is also not the same as coiling or uncoiling or other changes due to crosslinking, uncrosslinking, chemical reaction (such as addition or removal of chemical groups), hydrolysis, or change of ionic charge, for example by providing or by removing conjugate ions.

Embodiments described here relate to the use of such materials to create agglomerates larger than the particles in the drilling muds or to provide heterogeneous proppant placement (where the "proppant" is agglomerated drilling mud) during hydraulic fracturing treatments to create channel like structures. The shapeable materials, such as fibers, are mixed with the mud on the surface and pumped into a wellbore to stop fluid loss or into a fracture as it is forming. Under reservoir conditions such materials undergo physical changes, for example to create a more compact structure (agglomerates) [10] of fibers [4] with other solids [8] (as shown in Figure 2). If the treatment is fracturing, the agglomerates may form pillar like structures between the fracture faces [12] inside the hydraulic fracture (as shown in Figure 3). In other 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. On the other hand, Figure 4 shows the formation of a channel-like structure of agglomerates [10] between hydraulic fracture faces [12], from shrinkable fibers [4], and solid particles [8] from a drilling mud, for example calcium carbonate or barium sulfate, or cuttings.

We define the three-dimensional structure of a shapeable material when the shapeable material is initially mixed with drilling fluid or drilling fluid carrying cuttings (and optionally other solids such as non-shapeable fibers and fluid loss additives), 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 drilling fluid solids or drilling fluid solids plus cuttings (and optionally other solid materials, for example fluid loss agents and non-shapeable fibers used to improve solids 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 drilling fluid solids or drilling fluid solids plus cuttings (and optionally other solid materials, for example fluid loss agents and non-shapeable fibers used to improve solids transport) mixed with the shapeable materials, after reshaping of the shapeable materials as the "subsequent structure".

We disclose here embodiments of a method that utilizes this unique property of certain elongated shapeable particles which enables them to re-organize the solids in drilling muds (including cuttings, if present) to treat lost circulation and so that such drilling muds (including cuttings, if present) can be used in hydraulic fracturing to create channel-like structures or pillars between the fracture faces. It will be understood that throughout this disclosure, drilling muds or drilling muds solids are to be understood to include rock cuttings if they have been introduced into the fluid during drilling. Previously-removed cuttings, for example removed by a shale shaker, being held at the surface may be added back into the fluid; these cuttings, when the elongated shapeable particles are reshaped by heating downhole, will participate in fluid loss control or formation of pillars and/or solids-free channels in fractures. Typically, drill cuttings range in size from a lower value of tens of microns (for example a smallest size of from about 10 to 40 microns) to a largest size of hundreds of microns (for example a largest size of from about 200 to 300 microns). All of these sizes, and any sizes typically used for proppants in conventional hydraulic fracturing, are suitable. Although an advantage of the present method is that proppant is not needed on location, if proppant is available, it may be added. If cuttings, or proppant, are added, they are preferably added in the same concentration range as proppant in conventional hydraulic fracturing, for example from about 60 g/1 (0.5 lb/gal) to about 600 g/1 (10 lb/gal), although lower concentrations may be added if that is all that is available. This approach does not require any significant change in current pumping services. Pumping of certain shapeable materials with drilling muds into hot subterranean locations creates agglomerates that may be used to treat lost circulation, and may be used to create pillars and channel-like structures during hydraulic fracturing treatments when such muds containing suitable elongated shapeable particles are used as the fracturing fluid. The shapeable materials of particular interest are fibers that can shrink at the higher temperatures downhole; this action allows the mixtures of muds and fibers to self-organize to create agglomerates. The shapeable materials may also serve as consolidating materials for the agglomerates.

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 transport of the agglomerates formed. The concentration of elongated shapeable particles must be sufficient to consolidate the solids.

One embodiment is the addition of shapeable materials to drilling fluid to create fluid loss control pills during drilling by consolidating at least some of the solid materials in the mud to produce large mesh agglomerates for control of fluid losses into fractures or vugs while drilling. Suitable agglomerates are larger than about 100 microns, for example about 500 microns in cross section. The shapeable materials of most interest are fibers that can shrink under downhole conditions. Suitable concentrations of fibers are from about 2.4 to about 18 g/L, preferably from about 6 to about 12 g/L; a suitable total pill size is typically from about 6 m³ (50 bbl) to about 12 m³ (100 bbl). On the surface, the shapeable materials such as the fibers described below, and optionally other lost circulation materials, are mixed with water or oil- based drilling mud and injected into the well through a drill pipe and re-circulated to the surface in the annular area between a wellbore wall and a drill string. Under reservoir conditions such materials undergo the physical change described above to create a more compact structure (as shown in Figure 2) with the small particles which are usually used in drilling muds, resulting in bridging and plugging, even of large fractures or vugs. The elongated shapeable particles may be made from degradable materials in case clean up of the plug is needed for production; in this case the plug may be loosened within days to weeks as a result of degradation at depending upon the downhole temperature. When lost circulation is observed, drilling is stopped and a pill of elongated shapeable particles and drilling mud is introduced into the well; if lost circulation is not mitigated, another pill, optionally larger, is introduced; this procedure is followed until the lost circulation is cured. Drilling is then resumed. Although reverse circulation (injecting through the annulus and returning through the drill pipe) is sometimes used in lost circulation treatments, and may be used in present embodiments, it is not preferred in present embodiments because of the risk of plugging the drill pipe and bottomhole assemblies. Embodiments for curing lost circulation may be used with all types of water-based muds, oil-based-muds, and invert-emulsion type muds. Conventional drilling equipment, for example mixers and pumps, may be used.

Another embodiment involves pumping of a drilling fluid, optionally containing cuttings, containing elongated shapeable materials with drilling fluids. When the elongated shapeable particles change shape, they cause agglomeration of the entrained particles to create channel like structures in hydraulic fracturing applications. The shapeable materials of particular interest are fibers that shrink at the higher temperatures downhole and cause the small solid particles in the drilling fluid, and drill cuttings if present, to self-organize to create pillar type structures or channels in the fracture.

Embodiments are useful regardless of the drilling mud type (water-based, oil- based or emulsion-type) because effectiveness depends upon the physical occurrence of heating the elongated shapeable particles, for example, fibers, that are intermixed with the mud solids, to cause agglomeration. If several different drilling muds are available on site they may be mixed for use. Conventional hydraulic fracturing pressure pumping equipment (optionally modified to mix in fibers) is used for the final fluid preparation and injection; drilling fluids are preferably stored in a frac tank and mixed, for example by continuous circulation, to keep the solids suspended; then typically fibers are added to the blender, and stored cuttings may be stored in, and added using, a sand chief. If there is insufficient drilling fluid available on site to complete a fracturing job, the job can be finished with conventional fluids and proppants, or the operator cam pump some slugs of drilling fluid carrying elongated shapeable particles and some slugs of fracturing fluid carrying proppant.

A suitable concentration of elongated shapeable particles is from about 2.4 to about 18 g/L, preferably from about 6 to about 12 g/L. A conventional fracturing fluid should be used as a pad to create fractures and as post flush fluids to ensure that all solids are placed inside the hydraulic fracture. Typically the solids content is increased in successive stages, for example by adding drill cuttings or proppant; the fiber concentration may remain constant. The migration of fines is minimal because pillar formation using shrinkable fibers consolidates all the solids and keeps them in place. However, any additional methods may be used to mitigate fines migration. Some filter mud solids may act as fluid loss agents. The agglomerates formed may have some permeability, but shapeable fibers used with particles that are primarily smaller than proppant tend to create channels rather than solid pillars, in which case the permeability of pillars may not be a concern. On the other hand, degradable fibers may be used because if no pillars or channels, or poor pillars or channels, are formed, degradation of the fibers increases the permeability of the solid pack. In addition, in case there is a screenout, degradable fiber allows easy clean out operations. Embodiments may be used in slickwater, in hybrid waterfracs, with channelant, and in other methods of creating heterogeneous proppant placement, such as those described in U. S. Patent Nos. 6,776,235, 7,581,590, and 8,061,424; U. S. Patent Application Publication Nos. 2009/0286700, 2011/0083849 and 201 1/0114313 and PCT Application Publication No WO2009/096805, 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. Embodiments may be used in foamed or energized treatments.

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 PLA 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 11 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 1 1 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-2,2¹- 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 8113825. 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 pyrrol idone, 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 biaxially-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 other solids; 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:

To demonstrate the use of embodiments for curing fluid loss, we conducted an experiment using a polylactic acid shrinkable sheath/core fiber ("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). A well-mixed aqueous slurry containing 30 g/L of D020, 150 g/L of SafeCarb 500, 12 g/L of polylactic acid shrinkable sheath/core fibers ("Trevira" T266 series, 6 mm in length and 22 microns (4.4 dtex) in diameter) was placed in a glass bottle (total volume 100 mL). The bottle with testing mixture was placed in oven at 80 °C for 1 hour. After the mixture was heated, the fibers shrank and consolidated the drilling mud into clumps with approximately 1 cm diameter that settled, leaving solids-free fluid above.

Example 2:

To demonstrate the use of embodiments in fracturing, we conducted experiments using a polylactic acid shrinkable sheath/core fiber ("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). The drilling mud, contained 30 g/L of D020 and 150 g/L of SafeCarb 250, was mixed with fibers (12 g/L) in a blender at 3000 rpm for 5 min to form a uniform slurry. The mixture was placed in a vertical 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 mixture in the slot was heated at 100 °C for about 30 min, and the fibers shrank and consolidated the drilling mud into a channel-like structure. The channels had 1-5 mm width and contained transparent colorless fluid free of any solid particles. However, shapeable fibers with particles smaller than proppant tend to create channels rather than solid pillars.

Example 3:

A job design for hydraulic fracturing using shrinkable fibers with a drilling mud and cuttings is presented in Table 1. The base gel-like fluid used is a water solution of bentonite viscosifier at a concentration of 30 g/L. The solid particles in the drilling mud are calcium carbonate having a particle size distribution between about 100 and about 500 microns at a concentration of 120 g/L. The cuttings are solid particles having a particle size distribution from about 100 microns to about 0.85 mm. The fibers are polylactic acid shrinkable sheath/core fibers ("Trevira" T266 series, 6 mm in length and 12 microns in diameter) as used in the previous examples.

Table 1

Example 4:

A job design used in the event of severe losses into the formation during drilling is as follows: 5.8 m³ of a water based drilling mud, which consists of 4332 kg of water, 348 kg of bentonite and 1 160 kg of barite, is added to a 12 m³ batch mixer and allowed to mix for 15 minutes. 46 kg of polylactic acid shrinkable sheath/core fibers ("Trevira" T266 series, 6 mm in length and 22 microns (4.4 dtex) in diameter) as used in the previous examples, in diameter) is gradually added to the above drilling mud and allowed to mix for another 30 minutes. The fiber and drilling mud mixture is then pumped down hole at 0.5 m /min.

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 treating a wellbore penetrating subterranean formations comprising injecting into the wellbore a treatment fluid comprising a drilling fluid and a mixture of elongated shapeable particles 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 or claim 2 wherein the elongated shapeable particles are degradable shrinkable fibers selected from polyesters, polyamides, and polyolefins.

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

5. The method of claim 1 or claim 2 wherein the elongated shapeable particles are shrinkable films.

6. The method of claim 5 wherein the shrinkable films comprise polyurethanes.

7. The method of claim 5 or claim 6 wherein the shrinkable films comprise two or more layers.

8. The method of any of the preceding claims wherein the shapeable elongated particles comprise two or more substances having differing coefficients of thermal expansion.

9. The method of any of claims 1 through 4, or 8 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.

10. The method of any of the preceding claims wherein the treatment fluid is injected into a wellbore during a drilling operation to mitigate lost circulation.

1 1. The method of any of claims 1 through 9 wherein the treatment fluid is injected into a wellbore and into at least one formation above its fracture pressure to elongate and prop open a fracture.

12. A composition comprising a drilling fluid containing elongated shapeable particles.