Patent Publication Number: US-7905287-B2

Title: Methods of using a polymeric precipitate to reduce the loss of fluid to a subterranean formation

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to downhole operations, and more particularly to methods of forming a polymeric precipitate in a subterranean formation to reduce the loss of fluid to the formation. 
     BACKGROUND OF THE INVENTION 
     The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art. 
     Natural resources such as oil or gas residing in a subterranean formation can be recovered via the formation of wells that penetrate the formation. In particular, a wellbore is typically drilled down to the formation while circulating a drilling fluid (also known as a drilling mud) through the wellbore. During the drilling process, the drill bit generates drill cuttings that consist of small pieces of shale and rock. The drilling fluid carries the drill cuttings in a return flow stream back to the well drilling platform. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run into the wellbore. The drilling fluid is then usually circulated downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. 
     Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Subsequent secondary cementing operations, i.e., cementing operations occurring after the primary cementing operation, may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas. 
     Unfortunately, the fluids used in such downhole operations may be lost to the subterranean formation while circulating the fluids in the wellbore. In particular, the fluids may enter the subterranean formation via depleted zones, zones of relatively low pressure, naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the drilling fluid, and so forth. As a result, the services provided by the fluids are more difficult to achieve. For example, a problem known as lost circulation may occur in which the circulation of the drilling fluid in the wellbore drops due to it being lost to the formation. Its circulation may eventually become too low to allow for further drilling of the wellbore. Also, a cement slurry may be lost to the formation as it is being placed in the annulus, thereby rendering it ineffective in isolating the adjacent subterranean formation. In particular, the amount of cement slurry may be insufficient to fill the annulus from top to bottom during primary cementing or to fill areas of lost integrity in a pre-existing cement column during secondary cementing. Further, dehydration of the cement slurry may result, compromising the strength of the cement that forms in the annulus. 
     Traditional methods of overcoming the problems described above include sealing the zones through which the fluids can enter the subterranean formation with thixotropic cements, non-aqueous dispersions of clays, sodium silicate solutions in combination with calcium salt sweeps, and fluids containing inert platelets such as mica. However, the presence of such sealants in the formation may block the flow of oil or gas into the wellbore when it is desirable to begin production. Further, those materials may contaminate fresh water produced by the formation ahead of the oil or gas. Unfortunately, the sealants typically cannot be easily removed from the formation before production. A need therefore exists to develop a way to prevent the loss of fluid to the subterranean formation without adversely affecting the production of water, oil, or gas by the formation. 
     SUMMARY OF THE INVENTION 
     Some teachings and advantages found in the present application are summarized briefly below. However, note that the present application may disclose multiple embodiments, and not all of the statements in this Summary section necessarily relate to all of those embodiments. Moreover, none of these statements limit the claims in any way. 
     In embodiments, methods of treating a subterranean formation comprise contacting a polymeric solution and an activator in the subterranean formation, thereby forming a precipitate to at least partially block a flow of a wellbore servicing fluid further into the formation. The wellbore servicing fluid may be, for example, a drilling fluid, a cement composition, a workover fluid, or combinations thereof. The polymeric solution may comprise, for example, a polyvinyl pyrrolidone aqueous solution, and the activator may comprise, for example, a formate brine. In one embodiment, the polymeric solution and the activator are contacted before completing a drilling operation in the wellbore. In another embodiment, they are contacted before completing a primary cementing operation in the wellbore. In yet another embodiment, they are contacted before completing a secondary cementing operation in the wellbore. When desirable, the precipitate may be easily and quickly removed from the subterranean formation by dissolving it in fresh water. 
     In additional embodiments, methods of treating a wellbore that penetrates a subterranean formation comprise: introducing an activator to the wellbore; introducing a polymeric solution in situ with the activator, thereby forming a hard precipitate to at least partially block a flow pathway into the subterranean formation; circulating a wellbore servicing fluid through the wellbore; and dissolving the precipitate to prepare for hydrocarbon production. 
     In more embodiments, methods of drilling a wellbore at least partially through a subterranean formation comprise: introducing an activator to the wellbore; introducing a polymeric solution in situ with the activator, thereby forming a hard precipitate to at least partially block a flow pathway into the subterranean formation; and applying torque to a bit within the wellbore while applying force to urge the bit to extend through the wellbore; and circulating a drilling fluid past the bit to remove cuttings therefrom. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a side plan view of a drilling rig and a wellbore in the earth for recovering oil from a subterranean formation. 
         FIG. 1B  is a detailed view of a section of the wellbore depicted in  FIG. 1A , showing a precipitate blocking a flow pathway into the subterranean formation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A polymeric solution and an activator may be contacted in a subterranean formation in such a manner as to form a hard precipitate in the formation. As used herein, a “polymeric solution” is defined as a solution comprising a polymer, an oligomer, or combinations thereof. As used herein, an “activator” is defined as a material that, when present in a sufficient amount, can cause the polymer and/or oligomer to precipitate out of the solution, wherein the activator may be a liquid in which the polymer and/or oligomer is insoluble. It is understood that “subterranean formation” encompasses both areas below exposed earth or areas below earth covered by water such as sea or ocean water. The resulting polymeric precipitate is typically impermeable to fluid and thus may form a barrier in the formation that at least partially blocks one or more flow pathways through which a fluid could undesirably migrate into the formation. Examples of such flow pathways include natural or induced fractures, depleted zones, zones of relatively low pressure, weak zones having fracture gradients exceeded by the hydrostatic pressure of the fluid being used to service the wellbore, perforations formed by a perforating gun, and combinations thereof. 
     The polymeric solution and the activator typically reach the subterranean formation by pumping them down a wellbore that penetrates the formation. The presence of the precipitate barrier in the formation allows fluids (known as wellbore servicing fluids) to be circulated or squeezed in the wellbore for the purpose of servicing the wellbore without being concerned that a substantial amount of the fluid could flow into and be lost to the formation.  FIGS. 1A and 1B  illustrate how the precipitate forms a barrier to at least partially reduce the flow of fluid far into a subterranean formation.  FIG. 1A  depicts a wellbore  10  that has been drilled in the earth such that it penetrates a subterranean formation. An oil rig  30  positioned near the surface of the earth  20  may be used to recover oil from the formation. A pipe  40 , e.g., a drill pipe or tubing, may extend down through wellbore  10  for delivering fluid to and/or from the wellbore.  FIG. 1B  illustrates a section  50  of wellbore  10  in detail. As illustrated in  FIG. 1B , a precipitate  60  forms a solid coating across a flow pathway  70  such as a crack within wellbore  10  and thereby seals flow pathway  70 . The larger the quantity of polymer solution pumped into wellbore  10 , the thicker the coating formed therein. In an embodiment, the coating may have a thickness ranging from about 1 millimeter to about 10 millimeters. Also, from about 0.25 to about 0.5 gram/cubic centimeter of precipitate  60  may extend across flow pathway  70 . Precipitate  60  is desirably impermeable to fluid and thus inhibits fluid from passing through pathway  70  and deeper into the adjacent subterranean formation. Thus, precipitate  60  serves to reduce the loss of such fluid to the formation, allowing it to be primarily retained within wellbore  10  as it is pumped therethrough. 
     It is believed that the coating formed by the precipitate will be effective in cracks that are both larger and smaller than the coating thickness near the wall of the wellbore. That is, cracks within the subterranean formation usually decrease in size at greater depths within the formation. Thus, the polymer solution may be pumped into cracks larger than the ensuing coating thickness to a depth where the cracks become sufficiently narrow to allow the coating to plug the entire crack. 
     In an embodiment, a spacer separates the polymeric solution and the activator as they are being pumped downhole. As used herein, “spacer” is defined as a fluid that prevents the polymeric solution and the activator from interacting with each other as they pass down the wellbore and that is inert with respect to the polymeric solution and the activator. Examples of suitable fluids to use as spacers include natural hydrocarbons, synthetic hydrocarbons, surfactants, glycols, fresh water, and combinations thereof. The amount of the spacer employed in this embodiment may range from about 1 to about 5 barrels or alternatively may be about 2 barrels. In one embodiment, the polymeric solution, the spacer, and the activator are sequentially pumped to the formation. In another embodiment, the activator is pumped first and the polymeric solution is pumped behind the spacer. Within the subterranean formation, the polymeric solution and the activator are no longer separated and thus contact each other, resulting in the formation of the precipitate. 
     The polymeric solution and the activator may be displaced into the wellbore before or during the circulation/squeezing of a wellbore servicing fluid in the wellbore. In an embodiment, they may be pumped downhole after the discovery that at least a portion of the wellbore servicing fluid is being lost to the subterranean formation. At this point, the circulation/squeezing of the wellbore servicing fluid in the wellbore may be terminated to allow for the formation of the precipitate within the formation. Once the precipitate has been formed, the circulation/squeezing of the fluid in the wellbore may then be resumed. The formation of the precipitate may occur within 1 minute of the initial contact between the polymeric solution and the activator. However, typically one waits for a period of in a range of from about 30 minutes to about 4 hours before passing the wellbore servicing fluid into the wellbore again. 
     In one embodiment, the precipitate may be used to reduce a pre-completion loss to a subterranean formation. As used herein, a “pre-completion loss” refers to a loss of fluid to a subterranean formation that occurs before the wellbore is completed, i.e., before the annulus of the wellbore has been sealed by a sealant composition, e.g., a cement composition. In an embodiment, the pre-completion loss is the loss of a drilling fluid during its circulation through a wellbore both during and after the drilling of the wellbore. In another embodiment, the pre-completion loss is the loss of a cement slurry (or other type of sealant composition) during primary cementing. 
     In another embodiment, the precipitate may be used to prevent a post-completion loss to the subterranean formation. As used herein, a “post-completion” loss refers to a loss of fluid to a subterranean formation that occurs after the wellbore is completed, i.e., after the annulus of the wellbore has been sealed by a sealant composition, e.g., a cement composition. In an embodiment, the post-completion loss may be that of a secondary sealant composition such as a cement composition. By way of example, the secondary sealant composition may be placed in one or more permeable zones present in the wellbore using a squeeze technique known in the art. The permeable zones may extend, for example, through the wall of a conduit positioned in the wellbore, a sealant/cement column in the annulus of the wellbore, a microannulus of the wellbore, or combinations thereof. Examples of those permeable zones include a fissure, a crack, a fracture, a streak, a flow channel, a void, and combinations thereof. In yet another embodiment, the post-completion loss may be the loss of a fluid, e.g., a workover fluid, during a production enhancement operation, e.g., a workover operation. 
     In certain situations, it may be desirable to remove the polymeric precipitate from the subterranean formation. For instance, the precipitate may be removed to prepare the formation for the production of oil, gas, and/or water. That is, the precipitate may be removed to prevent it from blocking the flow of such natural resources from the formation. It may also be removed to avoid contaminating any water produced by the formation. In an embodiment, the precipitate may be removed by dissolving it in a fluid in which it is soluble such as fresh water, thereby providing a simple and inexpensive way to eliminate the precipitate. For example, the precipitate may be removed by water that is initially produced by the formation, or alternatively, it may be removed by water that is intentionally pumped into the wellbore. Examples of compounds that may be employed to dissolve the precipitate include but are not limited to fresh water and brines such as halide brines, e.g., sodium chloride brine. 
     The methods described above for reducing the loss of a wellbore servicing fluid to the subterranean formation utilize a polymeric solution and an activator. The volume ratio of the polymeric solution to the activator may be in a range of from about 50:50 to about 70:30. The polymeric solution comprises a polymer and a fluid in which the polymer has a relatively high solubility. In embodiments, the polymeric solution may also include but is not limited to lost circulation materials, weighting agents such as barite, acid soluble particles, and combinations thereof. An example of a suitable polymeric solution includes polyvinylpyrrolidone (PVP) in water, which is commercially available from ISP Technologies, Inc. under the tradename of VIVIPRINT 540 solution (10% PVP by weight of the aqueous solution) and from Halliburton Energy Services, Inc. under the tradename of PERFORMATROL polymer (10% PVP by weight of the aqueous solution). The amount of the polymer in the polymeric solution may be a range of from about 1% to about 90%, alternatively from about 5% to about 50%, by weight of the polymeric solution. Examples of suitable activators include formate brines (e.g., potassium formate, sodium formate, and cesium formate), acetate brines (e.g., potassium acetate, sodium acetate, and cesium acetate), oxalate brines (e.g., potassium oxalate, sodium oxalate, and cesium oxalate), halide brines (e.g., zinc bromide), and combinations thereof. In an embodiment, the amount of the potassium formate in the activator may be in a range of from about 10% to about 78%, or alternatively from about 50% to about 78%, by weight of the total formate brine. 
     Modifications to the Foregoing Embodiments 
     In an alternative embodiment, the activator may also serve as the solvent in which the polymer is dissolved. The polymer may be coated with a material that prevents it from initially contacting the activator/solvent. Thus, the polymer and the activator may be concurrently pumped downhole without forming a precipitate. The coating may eventually dissolve such that the activator/solvent contacts the polymer and thus forms the precipitate. The period of time required for the coating to completely dissolve may be sufficient to allow the polymer to be pumped into a crack or void in a subterranean formation. As a result, the formation of the precipitate is time delayed. 
     EXAMPLES 
     The invention having been generally described, the following examples are given as particular embodiments of the invention and to demonstrate the practice and advantages thereof. It is understood that the examples are given by way of illustration and are not intended to limit the specification or the claims to follow in any manner. 
     Example 1 
     Several samples of a PERFORMATROL solution (i.e., the PVP solution) were used in this example. About 16 lbs/gal of barite weighting agent were added to each PERFORMATROL solution sample. The PERFORMATROL solution samples remained pumpable after the addition of the barite weighting agent. 
     Various potassium formate brines (i.e., the activator) having different densities were combined with respective PERFORMATROL solution samples. The volume ratio of the potassium formate to the PERFORMATROL solution in each sample was 50:50. The densities of the potassium formate brine were 9.8 lb/gal, 10.3 lb/gal, 10.8 lb/gal, 11.7 lb/gal, and 12.2 lb/gal. These particular densities of the potassium formate brines caused a relatively hard PVP precipitate to form. It is believed that potassium formate brines having densities lower than 9.5 lb/gal will not form such a precipitate. 
     As shown in Table 1 below, various properties of the precipitate formed using the 13.2 lb/gal potassium formate brine were determined. For each mL of PERFORMATROL solution used, approximately 0.4 mL of the potassium formate brine was required to precipitate all of the PVP. The precipitate formed within 1 minute of contacting the PERFORMATROL solution with the formate brine. The sealing capability of the precipitate was determined by placing it on a slotted disk and then placing the disk in a filtration apparatus. A drilling mud was then placed on the disk and an attempt was made to filter the mud through the precipitate by applying a pressure of 500 psi across the disk. The precipitate withstood a 500 psi differential such that no mud could pass through the precipitate at this pressure. The thickness of the seal was measured as ⅛ inch. The precipitate was also heated on a hotplate to determine the temperatures at which it is stable. The precipitate exhibited no apparent decomposition at temperatures less than or equal to 250° F. 
                                 TABLE 1                       Precipitate Properties                          Sealing Capability   500 psi           Thickness   ⅛ inch           Rate of formation   &lt;1 min.           Temperature Stability   ≦250° F.                        
In addition, the lengths of time required to dissolve the precipitate by contacting it with fresh water (placing it in about 100 mL), non-aqueous drilling muds, and halide brines were compared, as presented in Table 2 below. It is noted that ACCOLADE drilling mud is commercially available from Halliburton Energy Services, Inc.
 
                                 TABLE 2                       Fluid   Time to dissolve                          ACCOLADE base oil   Never dissolved           10.5 lb/gal ACCOLADE mud   Never dissolved                                 Freshwater   10   minutes           Sodium Chloride brine (10 ppg)   15   minutes           Sodium Bromide brine (12.5 ppg)   2   hours           Calcium Chloride brine (11.6 ppg)   12   hours           Calcium Bromide brine (14.2 ppg)   24+   hours                        
Based on Table 2, the precipitate can be dissolved relatively quickly with freshwater or sodium chloride brine. Further, it may also be removed at a relatively slow rate with calcium chloride brine or calcium bromide brine. It is desirably insoluble in the ACCOLADE drilling mud.
 
     Example 2 
     The procedure followed in Example 1 was repeated with different activators. More specifically, the potassium formate brines were replaced with sodium formate brines, cesium formate brines, and zinc bromide brines. All of those brines caused the PVP to precipitate out of solution. 
     While preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. 
     Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are a further description and are an addition to the preferred embodiments of the present invention. The discussion of a reference herein is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.