Abstract:
A fluid loss control system for use in a well penetrating a subterranean formation includes a microgel for forming a monolayer on rock surfaces in fractures in the subterranean formation. A method for fluid loss control in a subterranean formation includes placing a fluid loss control solution in a wellbore penetrating the subterranean formation, wherein the fluid loss control solution comprises a microgel for forming a monolayer on rock surfaces in fractures in the subterranean formation.

Description:
BACKGROUND OF INVENTION 
       [0001]    The invention relates generally to well fluid loss control systems and methods. Particularly, the invention relates to fluid loss control systems using microgels. 
         [0002]    Production of hydrocarbon fluid from a subterranean formation frequently involves the use of treatment fluids intended for specific zones. Unfortunately, hydrocarbon fluid bearing subterranean formations or strata are usually structurally heterogeneous, often containing zones of varying permeability and/or containing fractures. Thus, channeling of the treatment fluids through the zones of greater permeability or fractures can occur and the benefit of the treatment fluids is substantially reduced. 
         [0003]    For example, when production from a well is slow, one can perform “stimulation” to improve the production. Well stimulation may be performed by (among other methods): (1) injecting chemicals into the well bore to react with and/or dissolve damage; (2) injecting chemicals through the well bore and into the formation to react with and/or dissolve small portions of the formation to create alternative flowpaths for the hydrocarbon (thus rather than removing the damage, redirecting the migrating oil around the damage); or (3) injecting chemicals through the well bore and into the formation at pressures sufficient to fracture the formation, thereby creating a channel through which hydrocarbon can more readily flow from the formation and into the well bore. The last approach is referred to as hydraulic fracturing, or acid fracturing if the chemicals injected (such as acids and/or chelating agents) also can dissolve portions of the formation. 
         [0004]    Hydraulic fracturing involves literally breaking or fracturing a portion of the surrounding strata, by injecting a specialized fluid into the well bore directed at the face of the geologic formation at pressures sufficient to initiate and extend a fracture in the formation. Actually, what is created by this process is not always a single fracture, but a fracture zone, that is, a zone in the formation having multiple fractures, through which hydrocarbon can more easily flow to the well bore. 
         [0005]    Commonly used hydraulic fracturing fluids comprise at least three principal components: a carrier fluid (usually water or brine), a polymer, and a proppant. (In acid fracturing, proppant is usually not used, and a chemical that can dissolve damage and/or a portion of the formation is used.) In addition, some polymers used in fracturing fluids require the use of crosslinkers. Other compositions used as fracturing fluids include water with additives (for example in slickwater treatments), viscoelastic surfactant gels, and gelled oils. The purpose of these fracturing fluids is first to create and extend a fracture, and then once it is opened sufficiently, to deliver proppant into the fracture, which keeps the fracture from closing once the pumping operation is completed. The carrier fluid is the means by which proppant and breaker are carried into the formation. 
         [0006]    A typical fracturing fluid can be prepared by blending a polymer with an aqueous solution (sometimes an oil-based or a multi-phase fluid is desirable). One type of polymer used is a solvatable polysaccharide. The purpose of the polymer is to increase the viscosity of the fracturing fluid which aids in the creation of a fracture and which thickens the aqueous solution so that solid particles of proppant can be suspended in the solution for delivery into the fracture. In many fracturing treatments, a crosslinking agent is added which further increases the viscosity of the fluid composition by crosslinking the polymers. 
         [0007]    Some fracturing fluids use viscoelastic surfactants (VES) instead of polymers and crosslinkers. Viscoelastic surfactant fluids are normally made by mixing in appropriate amounts suitable surfactants such as anionic, cationic, nonionic and zwitterionic surfactants. A surfactant typically comprises a long-chain hydrophobic group and a hydrophilic moiety. Cationic surfactants are often quaternary ammonium salts, such as cetyltrimethylammonium bromide (CTAB). The viscosity of viscoelastic surfactant fluids is attributed to the three dimensional structure formed by the components in the fluids. When the concentration of surfactants in a viscoelastic fluid significantly exceeds a critical concentration, and in most cases in the presence of an electrolyte, surfactant molecules aggregate into species such as micelles, which can interact to form a network exhibiting elastic behavior. There has been considerable interest in using viscoelastic surfactants in wellbore service fluids. See for example, U.S. Pat. Nos. 4,695,389, 4,725,372, 5,551,516, 5,964,295, and 5,979,557, all of which are hereby incorporated by reference in their entirety. 
         [0008]    One complication often encountered in well stimulation (fracturing) is the loss of fracturing fluids through the fracture faces and into the reservoir. Excessive fluid loss is a cause of premature screenout where the proppant can no longer be carried further and it bridges, essentially stopping the fracture growth. At permeabilities higher than, for example, about 5 md, fluid loss is excessive because it can no longer be controlled adequately by viscous forces. Inclusion of a fluid loss agent will minimize the amount of, for example, expensive VES fluid lost to the formation, reduce formation damage, and increase the fracture efficiency. In addition to fracturing operations, fluid loss control agents may also be used to control fluid loss during drilling, completion, workover and other stimulation operations. When using matrix stimulation fluids, drilling muds, workover fluids and completion fluids, and in cementing operations, fluid loss agents may also be required. 
         [0009]    Conventional fluid loss control agents include, for example oil-soluble resins, calcium carbonate, mica, starch and graded salt fluid loss additives. These agents achieve their fluid loss control by having solids or particulates that form a filter-cake on the face of the formation to inhibit flow into and through the formation. In addition to the particulate fluid loss control agents, other fluid loss control agents include viscoelastic surfactants and linear or metal-crosslinked polymers. Boron or metal-crosslinked polymers, such as hydroxyethylcelluloses (HEC) and various guars or modified guars, are among the most commonly used fluid loss control agents. Normally, these polymers do not form rigid gels, but they can be made viscous by crosslinking with borate, zirconium, or other suitable metal ions. Examples of fluid control using such polymers can be found, for example, in M. E. Blauch et al., SPE 19752, “Fluid Loss Control Using Crosslinkable HEC in High-Permeability Offshore Flexure Trend Completions,” pages 465-476 (1989); U.S. Pat. No. 4,552,215, issued to Almond et al., SPE 29525, “A New Environmentally Safe Crosslinked Polymer for Fluid Loss Control,” pages 743-753 (1995), by R. C. Cole et al.; SPE 36676, “Development and Field Application of a New Fluid Loss Control Material,” pages 933-941 (1996), by P. D. Nguyen et al.; and U.S. Pat. No. 5,372,732, issued to Harris et al. 
         [0010]    While the conventional fluid loss control agents have been successfully used in fluid loss control, there still exists a need for better fluid loss control systems. 
       SUMMARY OF INVENTION 
       [0011]    In one aspect, the present invention relates to a fluid loss control system for use in a well penetrating a subterranean formation. A fluid loss control system in accordance with one embodiment of the invention includes a microgel for forming a monolayer on rock surfaces in fractures in the subterranean formation. 
         [0012]    In another aspect, the present invention relates to methods for fluid loss control in a subterranean formation. A method in accordance with one embodiment of the invention includes placing a fluid loss control solution in a wellbore penetrating the subterranean formation, wherein the fluid loss control solution comprises a microgel for forming a monolayer on rock surfaces, in fractures or in a wellbore, in the subterranean formation. 
         [0013]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     DETAILED DESCRIPTION 
       [0014]    The following description illustrates embodiments of the invention. The described examples are for illustrative purpose only. One of ordinary skill in the art would appreciate that these examples are not exhaustive and they are not intended to limit the scope of the invention. It should 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 inventor appreciates and understands that any and all data points within the range are to be considered to have been specified, and that the inventor has possession of the entire range and all points within the range. 
         [0015]    Embodiments of the invention relate to fluid loss control systems and methods, using microgels. These microgels are soft, crosslinked polymers, i.e., intramolecularly crosslinked macromolecules. They are typically formed by polymerization of polyfunctional precursors. Microgels suitable for use in fluid loss control in accordance with embodiments of the invention, for example, may be based on AMPS (2-acrylamido-2-methylpropane sulfonic acid) polymers. One example of microgels may be manufactured using a copolymer of acrylamide and AMPS monomers. The manufacturing process may involve crosslinking the acrylamide and AMPS monomers, with or without the help of another cross-linking agent, such as N,N′-methylenebisacrylamide (MbisAM) or multifunctional aldehydes such as glutaraldehyde. The polymerization reaction may be initiated with a radical, with radiation, or with other reagents, as known in the polymer art. Other examples include terpolymers of AMPS, acrylamide and vinyl pyrrolidone and various terpolymers formed from acrylamide, acrylic acid and either AMPS or vinyl pyrollidone. It should be appreciated that a portion of the acrylamide in a polymer can be converted to acrylic acid by treatment with either acid or base at elevated temperatures and these hydrolyzed acrylamides are included in the definition of acrylamide. Alternatively, polymerization in the presence of both acrylamide and acrylic acid can be performed in conjunction with another monomer. 
         [0016]    In accordance with embodiments of the invention, microgels for use in fluid loss control can be adsorbed onto rock surfaces (e.g., sandstone rocks). Preferably, these microgel molecules do not have significant attraction to each other, so that they prefer to form a monolayer on the rocks. Formation of a monolayer of such microgels on rocks has been found to result in significant reduction in water permeability, without reduction in hydrocarbon permeability. To reduce attraction between molecules, the crosslinker type and amount is selected to promote intramolecular crosslinking which yields distinct microgels. The charge of the polymer is typically slightly negative which also minimizes attraction between microgels. 
         [0017]    Reduction of water permeability by monolayers of microgels on rock surfaces has been observed with different types of polymers. For example, U.S. Pat. No. 4,617,132 discloses a method, in which a sandstone formation is first contacted with an aqueous solution containing a water soluble anionic polymer having a molecular weight greater than 100,000. Then, the anionic polymer is contacted with a fluid containing a water soluble cationic polymer having a molecular weight greater than 1,000. As a result of the contact of the anionic polymer with the cationic polymer, coacervation occurs between the two polymers. The presence of stabilized polymer in the sandstone leads to reduced water permeability. 
         [0018]    Also, U.S. Pat. No. 5,146,986 discloses a method of selectively reducing water permeability in subterranean formations. The method uses a hydrocarbon carrier solution containing fatty acid imidazolyl compounds, which are surfactants. The surfactant is believed to adsorb on the walls of the interstitial passages in the formation, as a result of which, the flow of water through the passages is reduced. 
         [0019]    In fluid loss control, the particle size of the fluid loss control agent may require proper sizing based on the pore throat diameters of the formation rock. In accordance with embodiments of the invention, particles of the microgels are intended to be placed in the fractures. Therefore, it is preferred that the sizes of the microgels are not significantly larger than the pore sizes expected to be encountered in the formation. 
         [0020]    Although there is no universal agreement on the precise relationship between fluid loss control agent sizes and pore dimensions, the following guidelines may be used. These guidelines are disclosed in U.S. Patent Application Publication No. 2006/0157248 A1, which is assigned to the present assignee and incorporated by reference in its entirety. In general, particles having diameters greater than about one-third (although some researchers say up to one half) of a pore throat diameter are expected to bridge at or near the formation face, while particles smaller than this size but larger than about one-seventh of a pore throat diameter are expected to enter the formation and be trapped and form an internal filter cake. Particles smaller than about one-seventh of a pore throat diameter are expected to pass through the formation without substantially affecting the flow. It is to be understood that there are other important factors such as distributions of particle and pore sizes, flow rate, carrier fluid properties, particle concentration, and particle shape that can affect the tendency to form a filter cake. Specifically, microgels of the invention are flexible, and, therefore, they will be able to “squeeze” into smaller pores. 
         [0021]    The microgels are available in various sizes, which may be selected for different applications. For example, if the microgels are intended to be deployed into fractures, smaller particle sizes will be preferred. On the other hand, larger sizes microgels may be preferred for other applications. For example, larger sized microgels (e.g., Floperm® 2000 from SNF Floerger, Paris, France; or Temposcreen®, described in Soviet Union patents SU1663184 and SU1837105, are designed to reduce permeability in thief zones and high permeability channels. However, these larger sized microgels cannot penetrate a typical formation matrix. 
         [0022]    In accordance with embodiments of the invention, a fluid loss control agent comprises microgels that are sized to move into fractures in the formations. The sizes of these microgels, for example, may range from about 0.1 micron to about 150 microns, preferably from about 0.5 to about 10 microns, more preferably from about 1 to about 5 microns. Most preferably, a substantial portion of the microgels, for example more than about 99% are smaller than about 10 microns. In accordance with embodiments of the invention, the microgels preferably have a narrow distribution of sizes. For example, in a preferred embodiment, the microgels have a narrow size distribution around 2.5 microns. 
         [0023]    These microgels, being cross-linked polymers, are relatively robust. For example, they can be forced through pores of slightly smaller dimensions due to their property of being soft and deformable. In addition, their ability to adsorb onto rock surfaces can be manipulated by changing the salinity (or ionic strength) of the solution. In laboratory tests, it has been found that these microgels form monolayers on rocks. The unique property of these gels is the large reduction in water permeability (by a factor on the order of 30), while they do not significantly impact hydrocarbon (oil and gas) permeability. The gels should be hydrophilic in nature to promote the microgel&#39;s ability to impart the above-mentioned relative permeability effect. 
         [0024]    A fluid loss control system in accordance with embodiments of the invention may be used in fracturing operations to minimize the loss of fracturing fluids (e.g., VES fluids) into the formation. In such applications, the fluid loss control agents may be pumped together with the fracturing fluids, such as VES fluids. Alternatively, the microgels may be pre-mixed with the fracturing fluids before use. In preferred embodiments, microgels are added to the preflush and first stages of a VES fracturing fluid to minimize fluid loss. The microgels will form monolayers in tighter rocks along the fracture-rock interface. For higher permeability formations, the microgels may penetrate the rocks and drastically reduce water permeability of the rocks. As a result, loss of the fracturing fluids may be limited. 
         [0025]    Once well fracturing is complete, the microgels may be removed from the well to minimize potential damage to the formation permeability and well productivity. However, because these microgels do not significantly reduce the permeability to hydrocarbons, they will not have a significant impact on the production of hydrocarbons. Therefore, one may also choose to leave the microgels in the formation. 
         [0026]    If it is desired to remove the microgels after fracturing, then the microgel compositions may be selected for low adsorbtivity to the rocks. The adsorbtivity of the microgels may be controlled by adjusting the salinity (i.e., ionic strength) of the medium. Therefore, low salinity solution may be pumped to facilitate the removal of the microgels after the treatment. 
         [0027]    In addition to fluid loss control, the gels can also be used to inhibit water production with essentially no damage to oil or gas production. Another use is to combine the microgels with a conventional fracturing fluid to limit leakoff of the fracturing base fluid. This would improve the fracturing efficiency and the resultant formation permeability. The various properties including monolayer absorption, relative permeability effects originating from the hydrophilic nature of the microgel, and the deformability to plug off pores, make the microgels an excellent fluid loss agent for hydraulic fracturing. 
         [0028]    Fluid loss control agents in accordance with embodiments of the invention may be prepared in any form conventionally used in the art. For example, these agents may be prepared in solid, powder, or granule form ready to be formulated before use, and they may be prepared in pre-weighed dosage for convenient use. Alternatively, these agents may be mixed with a fluid to form highly viscous fluids, referred to as “fluid loss control pills” or “kill pills.” These fluid loss control pills may be made into a solution or suspension before use, or they may be added to the well fluids in a wellbore to form the fluid loss control fluid in situ. Alternatively, these agents may be pre-formulated into a solution ready to use. 
         [0029]    Embodiments of the invention may have one or more of the following advantages. A fluid loss control system of the invention has the advantage of simple field handling and simple mixing; it does not require hydration, nor shear degradation. Furthermore, it uses no solids. Using such a fluid loss control system, water permeability may be reduced in both proppant pack and matrix. 
         [0030]    Since the microgels form a monolayer when adsorbed on rocks, a large quantity will not be required. The microgel fluid loss control system in accordance with embodiments of the invention may be made to have controllable adsorption on the rocks, by adjusting the carrier fluid salinity. At higher salinity the microgels are absorbed on the rock; at lower salinity they are desorbed. The microgels are very stable in water, even at high temperatures. It has been found that they are stable for over a month at 300° F. (149° C.). 
         [0031]    Oil or gas production will cause the microgels to shrink against the formation walls. Therefore, fluid loss control agents of the invention will cause little or no permeability damage to the formation when hydrocarbons are flowing. These microgels will have good fluid loss control when applied in the preflush and/or first fracture stages. In addition, they are relatively inert due to the crosslinked nature of these gels. 
         [0032]    Although the invention has been described primarily in terms of fracturing compositions and methods, the compositions may be used as well in other oilfield treatments in which fluids come into contact with formations. For example, fluids containing microgels of the invention and formation-dissolving chemicals such as acids, acid precursors such as ammonium bifluoride and polylactic acid and similar compounds (also called latent acids), and chelating agents such as EDTA, HEIDA and HEDTA or their salts, or mixtures of these agents, may be used in acidizing and acid fracturing. Most microgels of the invention are stable in acid at most treatment conditions (this can readily be determined by simple laboratory experiments). They limit leakoff of live or spent acid, and, when stable in acid, after the treatment they limit water production. The microgels of the invention can also be used to limit lost circulation in drilling and cementing. 
         [0033]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.