Abstract:
Method for characterizing subterranean formation is described. One method includes: placing a subterranean fluid into a well extending into at least a portion of the subterranean formation to induce one or more fractures; measuring pressure response via one or more pressure sensors installed in the subterranean formation; and determining a physical feature of the one or more fractures.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/917,659 filed Dec. 18, 2013, entitled “METHOD FOR DETERMINING HYDRAULIC FRACTURE ORIENTATION AND DIMENSION,” which is incorporated herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to hydraulic fracturing. More particularly, but not by way of limitation, embodiments of the present invention include tools and methods for determining hydraulic fracture orientation and dimensions using downhole pressure sensors. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hydraulic fracturing is an economically important stimulation technique applied to reservoirs to increase oil and gas production. During hydraulic fracturing stimulation process, highly pressurized fluids are injected into a reservoir rock. Fractures are created when the pressurized fluids overcome the breaking strength of the rock (i.e., fluid pressure exceeds in-situ stress). These induced fractures and fracture systems (network of fractures) can act as pathways through which oil and natural gas migrate en route to a borehole and eventually brought up to surface. Efficiently and accurately characterizing created fracture systems is important to more fully realize the economic benefits of hydraulic fracturing. Determination and evaluation of hydraulic fracture geometry can influence field development practices in a number of important ways such as, but not limited to, well spacing/placement design, infill well drilling and timing, and completion design. 
         [0004]    More recently, fracturing of shale from horizontal wells to produce gas has become increasingly important. Horizontal wellbore may be formed to reach desired regions of a formation not readily accessible. When hydraulically fracturing horizontal wells, multiple stages (in some cases dozens of stages) of fracturing can occur in a single well. These fracture stages are implemented in a single well bore to increase production levels and provide effective drainage. In many cases, there can also be multiple wells per location. 
         [0005]    There are several conventional techniques (e.g., microseismic imaging) for characterizing geometry, location, and complexity of hydraulic fractures out in the field. As an indirect method, microseismic imaging technique can suffer from a number of issues which limit its effectiveness. While microseismic imaging can capture shear failure of natural fractures activated during well stimulation, it is typically less effective at capturing tensile opening of hydraulic fractures itself. Moreover, there is considerable debate on interpretations of microseismic events and how they relate to hydraulic fractures. Other conventional techniques include solving geometry of fractures as an inverse problem. This approach utilizes defined geometrical patterns and varies certain parameters until numerically-simulated production values matches field data. In practice, the multiplicity of parameters involved combined with idealized geometries can result in non-unique solutions. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0006]    The present invention relates generally to hydraulic fracturing. More particularly, but not by way of limitation, embodiments of the present invention include tools and methods for determining hydraulic fracture orientation and dimensions using downhole pressure sensors. The present invention can monitor evolution of reservoir stresses throughout lifetime of a field during hydraulic fracturing. Measuring and/or identifying favorable stress regimes can help maximize efficiency of multi-stage fracture treatments in shale plays. 
         [0007]    One example of a method for characterizing a subterranean formation includes: placing a subterranean fluid into a well extending into at least a portion of the subterranean formation to induce one or more fractures; measuring pressure response via one or more pressure sensors installed in the subterranean formation; and determining a physical feature of the one or more fractures. 
         [0008]    Another example includes: placing a fracturing fluid down a well of a subterranean formation at a rate sufficient to induce a fracture and a pressure response within the subterranean formation; measuring the pressure response via one or more pressure gauges installed in selected locations within the subterranean formation; and determining a physical feature of the fracture. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which: 
           [0010]      FIG. 1  show configuration of a reservoir monitored by pressure gauges. 
           [0011]      FIG. 2  (middle gauge) and  FIG. 3  (bottom gauge) show poroelastic response of the reservoir in  FIG. 1  subjected to net pressure inside tensile hydraulic fracture. 
           [0012]      FIG. 4  illustrates configuration of downhole wells as described in Example 1. 
           [0013]      FIG. 5  plots pressure response in the fractures and monitor wells of  FIG. 4 . 
           [0014]      FIG. 6  is a close-up view of  FIG. 5  as described in Example 1. 
           [0015]      FIG. 7  is a close-up view of  FIG. 5  as described in Example 1. 
           [0016]      FIG. 8  is a close-up view of  FIG. 5  as described in Example 1. 
           [0017]      FIG. 9  is a close-up view of  FIG. 5  as described in Example 1. 
           [0018]      FIG. 10  illustrates configuration of downhole wells and fractures as described in Example 1. 
           [0019]      FIG. 11  illustrates a model as described in Example 1. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not as a limitation of the invention. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations that come within the scope of the invention. 
         [0021]    Recently, horizontal well developments in unconventional plays have increasingly utilized multiple downhole gauges to monitor pressure and temperature variations during both stimulation and production phase. For example, pressure variations may be observed by the monitor/offset wells during hydraulic fracturing operations during almost every stage. These pressure responses can range from just a couple psi to over a thousand psi. Modeling the geomechanical impact of a propagating fracture can demonstrate that almost all observed pressure responses do not represent a hydraulic communication between the fracture and the monitoring well. Instead a poroelastic response to the mechanical stress is introduced during the fracturing process. 
         [0022]    When a stress load is applied to a fluid-filled porous material, the pressure inside the pores will increase in response to it (squeezing effect). The incremental pore pressure is then progressively dissipated until equilibrium is achieved. In a shale formation, diffusion can be so slow that excess pressure is maintained throughout the stimulation phase. As a result, the pressure response captured by the downhole gauges is directly proportional to stress perturbation induced by tensile deformation taking place during the propagation of a hydraulic fracture. 
         [0023]    After building a geomechanical model of a propagating tensile fracture in a poro-linear-elastic material, we were able to match the pressure response of one fracturing stage and estimate the height, length, and orientation of the hydraulic fracture. At the end of stage, the downhole gauge features a pressure fall-off that represents the closing of the induced fracture, as the fracturing fluid leaks off into the formation. By simulating the leak-off process, we were able to calculate the effective permeability of the formation after it has been stimulated, often referred to as the SRV permeability. When applied to different field cases, this technology has been able to identify differences in height growth and stimulated permeability between a slickwater and a hybrid completion. 
         [0024]    Poroelastic Response Analysis is showing tremendous potential in narrowing down the uncertainties of multi-stage fracture treatments in unconventional plays. Among its many advantages, it is based on simple well-established physical models (linear-poro-elasticity), it is much less sensitive to rock heterogeneities than pressure transient analysis, each stage can be matched separately, and the noise to signal ratio is small. Also, unlike microseismic which captures shear failure events in natural fractures, this technology directly measures the dilation of the actual hydraulic fracture. 
         [0025]    The present invention provides tools and techniques for characterizing a subterranean formation subjected to stimulation. More specifically, the present invention evaluates dimensions and orientations of fractures induced during hydraulic fracturing using pressure response information gathered downhole in one or more wells (e.g., active, offset, monitoring). Length, height, vertical position, and orientation of hydraulic fractures can be evaluated by relating pressure variations measured downhole to actual fracture dilation. Use of multiple pressure sensors (in a single well or in multiple wells) allows fracture geometry to be triangulated during the entire propagation phase. 
         [0026]    As opposed to some conventional methods (e.g., microseimic analysis), the present invention is a direct characterization of hydraulic fractures. The present invention may also be extensively implemented in multi-stage, multi-lateral horizontal wells and dramatically improve characterization of stimulated reservoirs. Such improvements could impact numerous aspects of production forecasting, reserve evaluation, field development, horizontal-well completions and the like. Uncertainty present in downhole pressure measurements are generally low and provide high signal to noise ratios. Other advantages will be apparent from the disclosure herein. 
       Pressure Monitoring During Hydraulic Fracturing 
       [0027]    A subterranean formation undergoing stimulation (e.g., hydraulic fracturing) experiences stress and subsequently responds to that stress. In terms of pressure within the subterranean formation, a response can be the result of one or more of: interference mechanism (e.g., hydraulic communication, stress interference), perturbation (pressure, mechanical), measurement itself (direct or indirect), and the like. A careful analysis of pressure response can provide information about the fracture (e.g., length, orientation), fracture network (e.g., connectivity, lateral extent), and formation (e.g. native, stimulated permeability; natural fractures; stress anisotropy, heterogeneity). 
         [0028]    As used herein, the term “poroelastic response” refers to a phenomenon resulting from an increased fluid pressure caused by, for example, an applied stress load (“squeezing effect”) in a fluid-filled porous material. A poroelastic response differs from a hydraulic response, which results from a direct fluid pressure communication between the induced fracture and a downhole gauge. Typically, this applied stress load results in incremental increase in pore pressure, which is then progressively dissipated until equilibrium is reached (“drained response”). During hydraulic fracturing, squeezing effect is achieved when net fracturing pressure causes tensile dilation (“squeezing effect”) in propagating fractures. However, in a typical shale formation, diffusion is negligible and excess pressure is maintained in pore(s) (“undrained response”) throughout the stimulation phase. 
         [0029]    At the end of stimulation, induced fractures progressively close as fracturing fluids leak-off into the formation, thus “un-squeezing” the rock. This in turn leads to a decrease in the downhole gauge poroelastic response. The rate of change in the poroelastic response depends on how fast fracturing fluid leaks off the induced fractures, which is directly related to the permeability of the stimulated rock located in the vicinity of the hydraulic fracture (often referred to as Stimulated Reservoir Volume or SRV). During hydraulic fracturing, poroelastic response can result from variations in tensile dilation both during hydraulic fracture propagation and closure. 
         [0030]      FIG. 1  illustrates a sample configuration of pressure sensors installed downhole. As shown, this setup features a monitor well  10  with two pressure gauges (middle gauge  20  and bottom gauge  30 ). The middle gauge  20  is located above a first fracture  40  (“7192H”) is located approximately 600 feet laterally from the monitor well  10 . The bottom gauge  30  is located below 7192H fracture but above fracture  50  (“7201H”) which is located approximately 700 feet laterally from the monitor well  10 . The poroelastic response as measured by the pressure gauges has been plotted versus time in  FIGS. 2  (middle gauge) and  3  (bottom gauge). Sharp vertical spikes (e.g., line between dotted lines in  FIG. 3 ) shown in  FIGS. 2 and 3  is largely due to tensile fracture dilation caused by a net pressure increase when fracturing fluid is introduced. Pressure relaxation (e.g., signal portion after the dotted lines in  FIG. 3 ) is largely due to fracture closure resulting from fluid leaking off into stimulated reservoir. Typically, a small-scale poroelastic response ranges from several psi&#39;s to several hundred psi&#39;s although pressure changes above 1000 psi&#39;s can be observed. A poroelastic response can propagate and be detected by pressure sensors located thousands of feet away from the propagating fracture. By analyzing pressure data, propagation as well as characteristics (e.g., length, height, orientation) of a hydraulic fracture can be tracked during each stage of a fracturing process. 
         [0031]    Poroelastic response analysis can be aided by a coupled hydraulic fracturing and geomechanics model used to synthetically recreate the poroelastic response to the mechanical stress perturbation caused by displacement of fracture walls (dilation) during hydraulic fracture propagation. When a stress load is applied to a fluid-filled porous material, the pressure inside the pores will increase in response to it (“squeezing effect”). Incremental pore pressure is then progressively dissipated until equilibrium is reached. In shale formations, diffusion is typically so slow such that excess pressure is maintained throughout the stimulation phase. As a result, pressure response captured by downhole pressure sensors is directly proportional to stress perturbation induced by tensile deformation taking place during propagation of a hydraulic fracture. The pressure signal detected by downhole pressure sensors may be synthetically calculated using a numerical model. An example of a suitable numerical model utilizes Symmetric Galerkin Boundary Element Method (SGBEM) and also applies Finite Element Method (FEM) in order to simulate stress interference (including poroelastic response) induced by hydraulic fracture propagation. The SBGEM is used to model fully three-dimensional hydraulic fractures that interact with complex stress fields. The resulting three-dimensional hydraulic fractures can be non-planar surfaces and may be gridded and inserted inside a bounded volume to allow the application of FEM calculations. 
         [0032]    Once geometry information has been determined, it can then be entered as input in a reservoir simulator for, among several things, production forecasting, reservoir evaluation, and the like. The geometry information can also influence field development practices such as, but not limited to, well spacing design, infill well drilling, and completion design. 
         [0033]    At time-step levels, local aperture predicted by the hydraulic fracture simulation can be applied as a boundary condition for the FEM to calculate a perturbed stress field around a dilated fracture. The poroelastic response to the propagation of the hydraulic fracture can then be monitored at specific points of the reservoir, corresponding to location of pressure sensors installed in offset/monitor wells. Numerical models may be used to generate type-curves that can be used to interpret the pressure signal from downhole pressure sensors using graphical methods similar Pressure Transient Analysis. Alternatively or additionally, the measured pressure signals may also be matched to the model by varying its input parameters. 
         [0034]    The following examples of certain embodiments of the invention are given. Each example is provided by way of explanation of the invention, one of many embodiments of the invention, and the following examples should not be read to limit, or define, the scope of the invention. 
       Example 1 
       [0035]    In this Example, pressure gauges were installed downhole and monitored during multi-stage hydraulic fracturing of horizontal wells in a shale formation located in Eagle Ford Formation located near San Antonio, Tex. 
         [0036]      FIG. 4  shows a configuration of active (Koopmann C 1 ) and offset (Burge A 1 , Koopman C 2 ) wells and monitoring wells (MW 1 , MW 2 ) used in this Example. Pressure gauges ( 100 ,  110 ,  120 ,  130 ) were installed in two of the wells (Koopmann C 1  and Burge A 1 ) as well as both monitoring wells (MW 1  and MW 2 ). Initial stages of the multi-stage hydraulic fracturing process start at toe end of the horizontal wells while each subsequent fracturing stage starts closer and closer to heel end of the horizontal well. As illustrated, hydraulic communication between the monitoring wells and Koopmann C 1  is present during various fracturing stages  70 ,  80 , and  90 . 
         [0037]      FIG. 5  plots pressure response recorded by the pressure gauges as a function of time. Koopmann C 1  and Burge A 1  were subjected to multiple fracturing stages. Dotted line in  FIG. 5  clearly denotes a time when Koopman C 1  fracturing has ended and just prior to when Burge A 1  fracturing began. Referring to  FIG. 5 , the large pressure signals in the monitor wells (MW 1  and MW 2 ) mirror the large pressure changes in the active well (Koopman C 1 ) but not in the offset well (Burge A 1 ). This confirmed that MW 1  and MW 2  were in hydraulic communication These pressure responses are on the order ˜1000 psi or greater (vertically-oriented ellipticals in  FIG. 5 ). 
         [0038]    With the exception of few instances of direct hydraulic communication, pressure signatures may be attributed to poroelastic response to mechanical perturbations induced during reservoir stimulation. As shown in  FIGS. 5 and 6 , pressure responses ranging from ˜100 to ˜1000 psi (horizontally-oriented ellipticals) were observed in Burge A 1  and MW 2  respectively. Referring to  FIG. 6 , there is a slightly delay in the pressure response following commencement of fracturing stage. It is believed that compressed fluid column in the Burge A 1  offset well can leak-off back into the formation, thereby providing diagnostic information on formation permeability. As shown in  FIG. 6 , a rapid pressure increase was seen after the delay, followed by slower pressure decay after fracture injection. This pressure response is likely a poroelastic response to stress interference. There are at least two types of stress perturbations (poroelastic and mechanical) that can create stress interference which, in turn, induces poroelastic response. Typically, poroelastic response to mechanical perturbation is much larger (orders of magnitude) than its response to poroelastic perturbation. Poroelastic responses are generally characterized by short response time combined with small magnitude of pressure signal. The pressure response is observed following almost every fracturing stage regardless of treatment distance to monitor or offset well (i.e., non-localized phenomenon). Small pressure responses ranging from ˜1 to ˜100 psi can also be observed as shown in  FIG. 7  (Koopman C 1 ),  FIG. 8  (MW 1 ), and  FIG. 9  (MW 2 ). The dotted line in  FIGS. 6-9  indicate start of each fracturing stage and correlate well with changes in small pressure response.  FIG. 10  shows a revised configuration of active, offset, and monitoring wells with predicted fractures  200  based on the collected pressure response data. 
         [0039]    Two methods were developed to calculate the fracture dimensions and orientations based on the measured poroelastic response. One methods called dynamic analysis, uses a geomechanical finite element code to simulation the dynamic evolution of the poroelastic response as the induced fracture propagates into the shale reservoir. Dyanamic analysis can analyze the whole pressure profile as captured by the downhole gauges in an offset well. The fracture properties are obtained as a typical inverse problem by matching the numerically simulated poroelastic response to the one measured in the field. Dynamic analysis allows improved, stage-by-stage, induced fracture characterization (e.g., fracture length, SRV permeability, multiple fracs/stage). 
         [0040]    A second method, called static analysis, only uses the magnitude of the poroelastic response. An analytical model was developed (see equations) that express the static poroelastic response as a function of the relative position of the downhole gauge to the induced fracture. The inverse problem is then solved to find the combination of induced fracture height, orientation, and vertical position that matches the measured poroelastic responses. 
         [0041]    Poroelastic response to changes in volumetric stress: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    Referring to  FIG. 11 , stresses in the vicinity of a semi-infinite fracture for undrained deformations (Sneddon, 1946): 
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         [0000]    The undrained Poisson&#39;s ratio can be expressed as a function of drained elastic and poroelastic properties: 
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         [0000]    The final expression for the poroelastic response to a dilated semi-infinite fracture is: 
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         [0042]    Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents. 
       REFERENCES 
       [0043]    All of the references cited herein are expressly incorporated by reference. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication data after the priority date of this application. Incorporated references are listed again here for convenience:
   1. Sneddon, I. N. 1946. The Distribution of Stress in the Neighborhood of a Crack in an Elastic Solid.  Proceedings, Royal Society of London  A-187: 229-260.