BACKGROUND The present invention relates to at least the partial degradation of filter cakes in subterranean formations. More particularly the present invention provides filter cake degradation compositions and methods of degrading filter cakes. In general, filter cakes are residues deposited on the walls of subterranean well bores as a result of various subterranean operations such as drilling, completion, and work-over operations. Such filter cakes are often tough, dense, substantially water insoluble, and usually capable of reducing the permeability of a surface on which they have formed. In general, filter cakes may prevent a fluid used in subterranean operations from being lost into the formation. Filter cakes also may prevent solids from entering the pores of the formation, thus preventing damage to the conductivity of the formation. Eventually, for a subterranean formation or portion of a subterranean formation to produce, the filter cake is often removed from the walls of the well bore. Filter cakes are desirable, at least temporarily, in subterranean operations for several reasons. For instance, a filter cake may be used in a fluid-loss control operation. In such an operation, a filter cake may act to localize the flow of a servicing fluid and minimize undesirable fluid loss into the formation matrix. This is an important function of a filter cake because if too much fluid is lost the conductivity or permeability of the formation may be damaged. A filter cake also may add strength and stability to the formation surfaces on which the filter cake forms. For example, one type of drilling fluid, commonly referred to as a "drill-in fluid," may be used to drill a well bore while minimizing the damage to the permeability of the producing zone. Drill-in fluids may include a fluid-loss additive (e.g., starch) and a bridging agent to block fluid entry into formation pores (e.g., calcium carbonate). Typically, a drill-in fluid forms a filter cake on the walls of the well bore that prevents or reduces fluid loss during drilling, and upon completion of the drilling operation, stabilizes the well bore during subsequent completion operations. The filter cake may be beneficial to other well bore operations, for example, hydraulic fracturing, and gravel packing. In general, filter cakes include bridging agents that block formation pores and fluid- loss additives that, inter alia, bind the bridging agents to the well bore and further inhibit fluids from entering the formation. The fluid-loss additive component of a filter cake generally should form a coherent membrane so that the filter cake maintains its integrity. Although useful, the coherent membrane oftentimes can make it difficult to remove the filter cake from the face of the formation when it is desirable to do so. Typical fluid-loss additives include starches (e.g., xanthan, amylose, and/or amylopectin) and typical bridging agents include salts (e.g., calcium carbonate and/or sodium chloride). Starch is a polysaccharide that comprises monosaccharide units linked by glycosidic bonds, e.g., α-1,4 glucosidic bonds and α-1,6 glucosidic bonds. In addition, filter cakes commonly include drilled solids, weighting agents, and viscosifying polymers that have been used to viscosify fluids used in some subterranean operations. Although some fluids used in well bore operations do not form filter cakes, these fluids may create conditions analogous to those found within filter cakes, e.g., by plugging formation pores. Therefore, the term "filter cake" when used herein also refers to these conditions. Although desirable for a certain amount of time or during a certain operation, to produce the desirable fluids from the formation, at some point the filter cake generally may need to be removed. Accordingly, some subterranean fluids may comprise an additional component that is capable of degrading the fluid-loss additive of the filter cake. Such components include acids, enzymes, and oxidizers. Although enzymes may be useful for degrading the fluid-loss additive component of a filter cake, enzymes may be unstable at certain elevated temperatures like those frequently encountered in some subterranean operations. At sufficiently high temperatures, enzymes can undergo irreversible denaturation (Le., conformational alteration entailing a loss of biological activity). Enzymes also may be intolerant to the salt concentrations commonly found in well bores. In addition, the combination of salt concentration and temperature may cause enzymes to coagulate and precipitate as shown in Figure 1. Typical enzymes often produce this damaging precipitate at the enzyme concentrations, salinity, and temperatures needed to effectively remove the filter cake. This sort of precipitation is particularly problematic with filter cakes because the gelatinous precipitate may clog formation pore throats, which can decrease the permeability of the formation and ultimately reduce production from the formation. SUMMARY The present invention relates to at least the partial degradation of filter cakes in subterranean formations. More particularly the present invention provides filter cake degradation compositions and methods of degrading filter cakes. In one embodiment, the present invention provides a method of degrading a fluid-loss additive component in a portion of a filter cake in a subterranean formation comprising: contacting the fluid-loss additive component with a filter cake degradation composition that comprises a precipitation resistant enzyme, wherein the precipitation resistant enzyme is capable of degrading the fluid-loss additive component; and allowing the filter cake degradation composition to at least partially degrade the fluid-loss additive component in a portion of the filter cake. In one embodiment, the present invention provides a filter cake degradation composition comprising a precipitation resistant enzyme component that will at least partially degrade a portion of a filter cake. The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the embodiments that follows.

BRIEF DESCRIPTION OF THE FIGURES A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings. The patent or application file contains at least one figure executed in color. Copies of this patent or patent application publication with color figure(s) will be provided by the Office upon request and payment of the necessary fee. FIGURE 1 illustrates an embodiment of a precipitated enzyme. FIGURE 2 illustrates a graph of the change in permeability possible if using certain methods of the present invention. FIGURE 3 illustrates a graph of the change in permeability with a comparative test sample. While the present invention is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown in the figures and are herein described. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DESCRIPTION OF EXEMPLARY EMBODIMENTS The present invention relates to at least the partial degradation of filter cakes in subterranean formations. More particularly the present invention provides filter cake degradation compositions and methods of degrading filter cakes. In general, the present invention provides filter cake degradation compositions and methods of degrading the fluid-loss additive components of filter cakes. In certain embodiments, the methods of the present invention degrade at least a portion of the fluid-loss additive component of a filter cake in a subterranean formation. The term "degrade," as used herein, refers to at least a partial degradation of the fluid-loss additive component of the filter cake, e.g., by hydrolysis. In certain embodiments, the methods of the present invention also may comprise degradation of bridging agents from a filter cake in a subterranean formation. In certain exemplary embodiments, the methods of the present invention compromise the integrity of the filter cake to a degree at least sufficient to allow any pressure differential between formation fluids and the well bore to induce flow from the formation. The filter cake degradation compositions of the present invention comprise precipitation resistant enzymes. Suitable precipitation resistant enzymes should be capable of hydrolyzing starch and should be resistant to precipitation under conditions sometimes found in subterranean well bores, e.g., elevated temperatures. Precipitation resistant enzymes suitable for use in the methods of the present invention generally catalyze the hydrolysis of the fluid-loss additive component of a filter cake, e.g., by chemically removing any of the linkages between the monomers of a starch molecule. In certain embodiments, the precipitation resistant enzymes include hydrolase enzymes of enzyme classification (E.C.) number 3.2, according to the Recommendations of the Nomenclature Committee of the International Union of Biochemistry on the Nomenclature and Classification of Enzymes. In certain embodiments of the present invention, glycosidase enzymes (E.C. 3.2.1) may be used. In certain exemplary embodiments, the precipitation resistant enzymes include α-amylase enzymes (E.C. 3.2.1.1), β-amylase enzymes (E.C. 3.2.1.2), glucan 1,4-α-glucosidase enzymes (E.C. 3.2.1.3), or combinations thereof. Examples of suitable precipitation resistant enzymes that are commercially available, include, but are not limited to, Liquezyme® X (Novozymes A/S of Bagsaerd, Denmark) and Optisize® HT (Genencor International, Palo Alto, California). The precipitation resistant enzymes of the present invention should resist precipitation in temperatures ranging from about 1O0C (5O0F) to about 15O0C (3270F) and pHs ranging from about 2 to about 11. In addition, the precipitation resistant enzymes should resist precipitation at salt concentrations of up to at least about 2.5 molar; and may resist precipitation at salt concentrations up to at least 5 molar. The term "salt" refers to salts of monovalent cations and anions. A person of ordinary skill in the art, with the benefit of this disclosure, will recognize how the valency of the salt will affect molarity and ionic strength. In certain exemplary embodiments, the precipitation resistant enzymes of the filter cake degradation compositions of the present invention are capable of degrading starch without precipitation in saturated brines (e.g., sodium chloride) at a temperature up to about at least 900C. The precipitation resistant enzymes may be present in the compositions of the present invention in an amount sufficient to degrade at least a desired portion of a filter cake. In some exemplary embodiments, the precipitation resistant enzymes may be present in an amount in the range of from about 10 kilo novo units (KNU) to about 150 KNU. One KNU is defined as the quantity of enzyme which degrades 4.87 grams of starch (Merck, soluble amylum, Erg. B6, Batch No.: 6380528), at pH 5.6, and at a temperature of 370C. The filter cake degradation compositions of the present invention may be used in any form including a solid, a liquid, an emulsion, or a combination thereof. The precipitation resistant enzymes in the compositions of the present invention also may be used as, or with, encapsulated particles, particles that are impregnated on a carrier, solids, liquids, emulsions, or mixtures thereof. The filter cake degradation compositions may be designed to have a delayed effect on a portion of a filter cake, for instance, when the process will involve a long pump time and consequently it is necessary to delay the enzymatic action of the precipitation resistant enzymes. Examples of delayed forms include encapsulated embodiments and solid embodiments. If immediate enzymatic action is desired, a liquid form may be preferable, e.g., in an aqueous solution. In certain embodiments of the present invention, the precipitation resistant enzymes in the filter cake degradation compositions may be spray- dried, freeze-dried, or the like. In certain embodiments, cells capable of producing the precipitation resistant enzymes that have been lyophilized may provide the precipitation resistant enzymes. In certain embodiments, the precipitation resistant enzymes of the present invention may be provided, inter alia, in a purified form, in a partially purified form, as whole cells, as whole cell lysates, or any combination thereof. One of ordinary skill in the art with the benefit of this disclosure will be able to determine the appropriate form for a given application. In certain embodiments of the present invention, the filter cake degradation compositions of the present invention may comprise other additives, including, but not limited to, glycerol, bactericides, microbiocides, surfactants, chelating agents, foaming agents, and the like. With the benefit of this disclosure, one of ordinary skill in the art will recognize when such additives may be useful in a given application. In certain embodiments, the filter cake degradation compositions of the present invention may comprise agents designed to remove or dissolve bridging agents in a filter cake. Examples of such agents include, but are not limited to, complexing agents {e.g., salts of ethylenediaminetetraacetic acid, a salt thereof, or other chelating agents), organic acids, or acid precursors (e.g., diethylene glycol diformate, glycerol diacetate, and glycerol triacetate). Some organic acids of this type may react with the bridging agents (e.g., acid-soluble bridging agents like calcium carbonate) and, in the presence of a conjugate base, may form a buffered system with a pH of about 4 or greater. Similarly, in the case of the acid precursors, which can produce organic acids in situ, since the acid is produced very slowly, the pH may stay in a range where precipitation resistant enzymes are active (e.g., in the range of from about 4 to about 5.5). In certain embodiments, the filter cake degradation compositions of the present invention may be used in conjunction with agents designed at least to partially remove a bridging agent component of the filter cake. For example, a strong acid, such as hydrochloric acid or hydrofluoric acid, may be used in a two-stage, sequential process. Such a process may involve treatment of the filter cake with a filter cake degradation composition of the present invention and then treatment of the filter cake with the strong acid. Thus, inactivation of the precipitation resistant enzyme at the resultant low pHs created by a strong acid may be avoided. In embodiments where the bridging agent is water soluble, e.g., a salt, the bridging agent may be removed with fresh water or water undersaturated with respect to the water- soluble bridging agent. The filter cake degradation compositions of the present invention may be contacted with a filter cake to degrade at least a portion of the filter cake using any method. For instance, the filter cake degradation compositions may be incorporated in a clean-up fluid. The term "clean-up fluid" refers to any fluid introduced into a subterranean formation for the purposes of facilitating the degradation of a filter cake. In certain embodiments, the filter cake degradation compositions of the present invention are internally incorporated in a servicing fluid, externally applied to a servicing fluid, or any combination thereof. The term "servicing fluid" refers to any fluid suitable for use in subterranean operations. Examples of servicing fluids, include, but are not limited to, drill-in fluids, fracturing fluids, and gravel packing fluids. For applications such as, e.g., fracturing and gravel packing, the precipitation resistant enzyme may be incorporated internally in the fluid or onto a particulate used in the process. In one embodiment, the filter cake degradation compositions of the present invention may be pumped to the location of the treatment zone at a rate sufficient to introduce sufficient precipitation resistant enzymes to at least partially degrade the fluid-loss additive component in a portion of a filter cake. To achieve certain beneficial effects of the present invention, the filter cake degradation compositions of the present invention may be shut in the formation for a time sufficient to at least partially degrade the fluid-loss additive component of a filter cake. This shut-in-time may be affected by the activity and/or concentration of the precipitation resistant enzyme and/or by the environmental conditions of the well bore, such as temperature, pH, and the like. If necessary, the pH of the treatment fluid may be adjusted through the use of acids, bases, or buffers. One of ordinary skill in the art with the benefit of this disclosure will recognize the conditions that might affect the requisite shut-in time needed to achieve a desired result. An example of a method of the present invention is a method of degrading a fluid-loss additive component in a portion of a filter cake in a subterranean formation comprising: contacting the fluid-loss additive component with a filter cake degradation composition that comprises a precipitation resistant enzyme, wherein the precipitation resistant enzyme is capable of degrading the fluid-loss additive component; and allowing the filter cake degradation composition to at least partially degrade the fluid-loss additive component in a portion of the filter cake. An example of a composition of the present invention is a filter cake degradation composition comprising a precipitation resistant enzyme component that will at least partially degrade a portion of a filter cake. To facilitate a better understanding of the present invention, the following examples of specific embodiments are given. In no way should the following examples be read to limit, or to define, the scope of the invention.

EXAMPLES Methods and Procedures In general, the test method for assessing enzyme activity generally involved addition of a known amount of an enzyme concentrate to a solution containing a standard amount of a standard starch in solution. The course of the resulting reaction is then followed by testing for the presence of starch with an iodine test solution. The starch was judged to have been consumed when coloration due to a complex formed between starch and iodine was no longer observed as compared to a colored glass standard. Reaction conditions were controlled to a pH of 5, a temperature of 37°C, and included a trace concentration of calcium (-0.0003 molar). The enzyme assay was performed as follows. 5 milliliters iodine solution B was pipetted into at least 5 test tubes per sample, which were placed in a water bath at 400C. 20 milliliters starch solution was pipetted into a large test tube. The pH was checked to ensure a pH of 5. 5 milliliters of calcium chloride solution was then added to the test tube. The test tube was warmed to 400C before adding an enzyme solution. An amount of enzyme solution was gradually added to the mixture and mixed. The reaction was allowed to proceed at 4O0C. At suitable intervals, 1 milliliter of the reaction mixture was removed and added to the test tubes containing 5 milliliters iodine solution B. Each tube was shaken briefly and color checked to determine the presence of starch. Iodine solution A was made as follows. 22 grams of potassium iodine were dissolved in approximately 60 milliliters of demineralised water in a 500 milliliter volumetric flask. 11 grams of iodine were dissolved in the flask, which was then filled to the mark with demineralised water. Iodine solution B was made as follows. 80 grams of potassium iodine was weighed out and added to a 2,000 milliliter volumetric flask. Then 8 milliliters of iodine solution A was added and the flask was filled to the mark with demineralised water. A stock salt solution was made as follows. 9.36 grams NaCl, 69 grams KH2PO4 and 4.8 grams Na2HPO4 were weighed out and poured into a 1,000 milliliter volumetric flask, which was then filled to the mark with demineralized water. The pH of the solution was checked, and when necessary, adjusted using HCl or NaOH as appropriate to reach a pH of 5.2. A starch solution was made as follows. An equivalent amount of 6.95 grams of dry matter content starch was added to demineralized water to a volume of 1,000 milliliters. The percentage dry matter content (DM %) of the starch was analyzed at 1050C (water determination at 1050C). The starch was suspended in 100 milliliters of demineralised water. The starch solution was then transferred quantitatively while stirring to a beaker containing 200 milliliters boiling demineralised water. The solution was boiled for approximately 30 seconds. The solution was transferred quantitatively to a 1,000 milliliter volumetric flask and cooled to room temperature. The pH was adjusted to 5 with HCl or NaOH as appropriate. The solution was then made up to the mark with demineralised water. A calcium chloride solution was prepared as follows. 0.82 grams of CaCl2 in 100 milliliter solution of demineralised water. The pH was then adjusted to 5 with HCl or NaOH as appropriate. An enzyme solution was prepared as followed. Enzyme concentrates were dissolved in 100 milliliters deionised water and diluted to the degree necessary to yield a measurable rate (e.g., 5 to 20 minutes) in the test procedures described. The pH was then adjusted to 5 with HCl or NaOH as appropriate. A test method for precipitation of enzymes based on salinity was conducted as follows. Three test brine solutions were made up by combining fresh water, saturated sodium chloride brine (density 1.2 kilograms per liter) and sodium chloride brine at 50% saturation (density 1.2 kilograms per liter). An enzyme sample at a moderately high concentration and at a low concentration were added to the brine and then heated to 900C. The appearance of the solutions was monitored for formation of a precipitate. Simulation of the tendency of a precipitated enzyme to block porous media was tested by noting the rate of flow through fine filter paper and core flow studies using sandstone of low permeability. A pore blockage test using fϊlterpaper was conducted as follows. A steel cell of diameter 5 centimeters and length 150 centimeters was fitted with filter paper having a pore dimension of 2.7 microns. The cell was then filled with 100 milliliters of water, sealed and pressurized to 100 pounds per square inch. The rate of water discharge through the filterpaper was timed in seconds to measure the initial injectivity of water through the filterpaper. Enzyme solutions of known concentrations were prepared in brine solutions and either heated and allowed to cool before being tested or tested without prior heating. The cell was then filled with 100 milliliters of the enzyme solutions, sealed, and pressurized to 100 pounds per square inch and the rate of discharge was timed. Next, water was injected through the filter paper as described above to check whether any precipitated enzyme creates a lasting permeability reduction in the filterpaper. A pore blockage test using a test core was conducted as follows. An enzyme solution was injected into a test core at ambient temperature. The temperature was then raised to 930C to induce precipitation. The direction of flow into the core was reversed to assess whether any precipitation occurring inside the core affected the permeability of the core. Specifically, Berea sandstone core plugs were cut, dried, and vacuum saturated in 1.2 specific gravity NaCl brine. A core plug was then mounted in the permeameter and sealed with 500 pounds per square confining pressure. The temperature was increased to 2000F while maintaining the confining pressure. Soltrol® 170, an isoparaffin solvent commercially available from Chevron Phillips Chemical Company, The Woodlands, Texas, was flowed through the core in the production direction until a stable permeability was measured (Ki). Ten pore volumes of the 0.5% v/v enzyme in a NaCl brine solution having a specific gravity of 1.2 was flowed through the core in the injection direction. The core was shut in and held for 24 hours at 2000F. Flow of Soltrol® 170 was resumed in the production direction and continued until the permeability reached a stable value (Kf). One example of a precipitation resistant enzyme suitable for use in the methods of the present invention, Liquizyme® X (commercially available from Novozymes A/S, Bagsaerd, Denmark), was compared to comparative test samples of other enzymes. The comparative test samples were: Termamyl® 120L (commercially available from Novozymes A/S, Bagsaerd, Denmark); Ban® (commercially available from Novozymes A/S, Bagsaerd, Denmark); and Nervanase® BT2 (commercially available from Rhodia Food Ltd, Cheshire, United Kingdom). The activities per gram of the various enzyme samples tested as quoted by suppliers and estimated according to the method outlined above are summarized in Table 1. Table 1

The exemplary precipitation resistant enzyme, Liquizyme X, and comparative test

samples were tested using the method to determine enzyme precipitation described above.

The precipitation tendencies of the comparative enzyme samples were determined at 20°C

and 95°C and in different concentrations of sodium chloride in the brine carrier. Table 2

shows the thermal precipitation potential of the Liquizyme® X and comparative test samples.

Table 2

The data in Table 2 exemplify the stability of precipitation resistant enzymes in

sodium chloride brine. The comparative samples all produced precipitates.

The exemplary precipitation resistant enzyme, Liquizyme® X, and comparative test

samples were tested using the methods to determine pore blockage using the filter paper

method as described above. Table 3 shows the effect of temperature on enzyme precipitation

based on injection through filterpaper. Table 3

The results in the Table 3 demonstrate that the comparative test samples tend to

precipitate when heated. In the case of the Termamyl® 120L and the Nervanase® BT2 there

is evidence that a precipitate capable of reducing the permeability of filterpaper is produced

even when the solvent is fresh water. Additionally, Table 3 demonstrates that the

concentration of sodium chloride in the carrier brine has a marked impact on the precipitation

tendency. In the case of the Nervanase® BT2, the precipitation in saturated sodium chloride

was severe. Table 3 also shows that the exemplary precipitation resistant enzyme,

Liquizyme® X, is resistant to precipitation over the entire sodium chloride concentration

range tested.

The effect of precipitation on return permeability was determined using the pore

blockage method using a test core as described above. The Termamyl® 120L (60 KNU/100

mL) comparative test sample was compared to the exemplary precipitation resistant enzyme

Liquizyme® X (100 KNU/100 mL) in 5.27M sodium chloride brine. Upon heating to 9O0C

the Termamyl® 120L solution developed an obvious precipitate whereas the solution of

Liquizyme® X did not. Termamyl® 120L demonstrated the potential to be more damaging to

permeability than a precipitation resistant enzyme. Figure 2 illustrate the return of permeability when Liquizyme® X is used in an exemplary composition and method of the present invention. Following the injection of 10 pore volumes of the Liquizyme® X sample and a static aging period of 24 hours at 9O0C only a mean volume of 20 pore volumes of oil produced through the core was required to restore permeability to 100% of the original when flow was recommenced in the production direction. In the case of the Termamyl® 120L comparative test sample, over the same time span return permeability was only 33% of the original after a flow of more than 400 pore volumes through the core as shown in Figure 3. Thus, the tests carried out with the sandstone core demonstrate that the precipitation resistant enzymes are less damaging to the core's permeability. [0001] Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.