Abstract:
A method and device for monitoring a subsoil zone, wherein a plurality of receivers are arranged on a surface of the soil or near said surface, straight above a geological zone to be monitored, comprising the following steps: generating a set of reference seismic data; recording seismic data by means of said receivers; correlating the seismic data recorded with the reference seismic data; comparing each trace of the correlated data, with correlated traces located in a vicinity of said trace, in order to evaluate a similarity of each correlated trace with the adjacent correlated traces; and, detecting a microseismic event occurring in the subsoil zone by analysing said similarity. The method and device enables real-time monitoring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority, under 35 U.S.C. §119, to French Patent Application No. 0953487, filed on May 27, 2009. 
       TECHNICAL FIELD 
       [0002]    The invention relates to a method for monitoring a subsoil zone by detecting microseismic events. 
         [0003]    It is applicable, for example, to method of monitoring stimulated fracturing performed in a well. 
       BACKGROUND OF THE INVENTION 
       [0004]    Stimulated fracturing operations are intended to increase the productivity of a hydrocarbon reservoir working well. These operations consist of injecting a high-pressure fluid into a layer of the subsoil where the reservoir is located. The injection of the fluid produces microfractures in the layer. This technique makes it possible to increase the permeability of the reservoir by favoring hydrocarbon circulation via the microfractures to the well. 
         [0005]    However, these operations require continuous monitoring of the reservoir so as, on one hand, to monitor the progress of the fracturing operation and, on the other, stop the operations when the fracturing is sufficient. 
         [0006]    Known monitoring techniques make use of the fact that the microfractures generated in the layer induce micro-earthquakes which are propagated and can be detected by seismic receivers. 
         [0007]    International Patent Publication No. WO 2008/033797 describes a monitoring method wherein the seismic receivers are arranged on the soil surface. The subsoil zone to be monitored in divided into a plurality of cells. An expected travel time between each cell and each receiver is then calculated using a subsoil velocity model. The seismic signals received by the receivers and recorded by the recorder (referred to as “traces”) are corrected to account for the differences in expected travel times between one cell and each of the receivers, and the sum calculated. The summed data (referred to as “source” data) are analyzed to detect the presence of a possible (or “triggering”) event characterized by a large amplitude and high energy parameters. The monitoring method described in WO 2008/033797 does not enable rapid data processing, and is not suitable for real-time subsoil monitoring. 
         [0008]    U.S. Pat. No. 7,391,675 describes a real-time monitoring wherein the seismic receivers are arranged in a well, which may be either the producing well or another well. The method consists of migrating the seismic data recorded to perform continuous mapping of the subsoil, on the basis of a predictive travel time model of the waves P and S in the subsoil, and detecting a time of a location corresponding to a maximum coalescences associated with the occurrence of an event. One drawback of the method of U.S. Pat. No. 7,391,675 is that it is necessary to install receivers inside a well, which complicates the installation and increases the cost. In addition, if the receivers are installed in the injection well, the well tends to vibrate under the effect of this injection, which raises the noise level on the receivers positioned in this well. It is not always possible to have access to another well. 
         [0009]    Furthermore, the following documents describe the use of a perforation shot to calibrate a velocity model: 1) SPE  115722 , Denver, 21-24 Sep. 2008, “Velocity Calibration for Microseismic Monitoring: Applying Smooth Models With and Without Perforation Timing Measurements”, Pei et al., and 2) EAGE Workshop on Passive Seismic, Limasol, 22-25 Mar. 2009, A-13, “Dual Treatment Monitoring with Horizontal Receiver Array”, Michaud et al. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention is directed to a method and device for monitoring a subsoil zone by detecting microseismic events. It is applicable, for example, to methods of monitoring stimulated fracturing performed in a well. 
         [0011]    One embodiment of the invention is a monitoring method enabling rapid detection of a microseismic event. 
         [0012]    According to one embodiment, there is a method for monitoring a subsoil zone by using a plurality of seismic receivers arranged on the soil surface or near said surface, comprising the following steps: generating a set of reference seismic data; recording seismic data using said receivers; correlating the seismic data recorded with the reference seismic data; comparing each correlated data trace with correlated traces located in a vicinity of said trace, in order to evaluate a similarity of each correlated trace with the adjacent correlated traces; and, detecting a microseismic event occurring in the subsoil zone by analyzing said similarity. 
         [0013]    In some embodiments, the reference data generation step comprises the following sub-steps: activating an isotropic point source near the subsoil zone to be monitored; recording the data generated by said source; and, filtering said data to obtain a set of reference data. In some embodiments, the filtering step comprises the following: determining an RMS (Root Mean Square) velocity which maximizes the sum of the traces contained in the reference data; normalizing the amplitudes of the different traces to compensate for the differences in amplitudes between traces due to differences in source-receiver distance; and, filtering the normalized data to reduce noise contained therein. 
         [0014]    In some embodiments, the reference data is obtained using a propagation model in the deep and superficial layers. 
         [0015]    In some embodiments, the comparison step comprises the following: for each trace contained in the correlated seismic data, correlating said trace with a sum of the traces located in the vicinity of said trace; and, calculating a semblance parameter as a function of the correlated seismic data obtained in sub-step. In some embodiments, the step of correlating comprises calculating a sample at time  0 . In some embodiments, the step of calculating a sample at time  0  comprises: multiplying each trace contained in the correlated seismic data by the sum of the traces located in a vicinity of said trace; and, convoluting the result of this multiplication by a rectangular filter. 
         [0016]    In some embodiments, the microseismic event detection step includes a comparison of the semblance parameter with a threshold value. 
         [0017]    In some embodiments, the microseismic event detection step includes an analysis of the statistical distribution of the semblance parameter value. 
         [0018]    In another aspect of the present invention, there is a device for monitoring a subsoil zone, comprising: a plurality of seismic receivers arranged on the surface or in the vicinity of said surface; a recording unit which receives the signals transmitted by the receivers and writes same in a seismic data file; and, seismic data processing means programmed to perform the steps of 1) generating a set of reference seismic data, 2) recording seismic data using said receivers, 3) correlating the seismic data recorded with the reference seismic data, 4) comparing each correlated data trace with correlated traces located in a vicinity of said trace, in order to evaluate a similarity of each correlated trace with the adjacent correlated traces, and, 5) detecting a microseismic event occurring in the subsoil zone by analyzing said similarity; to detect microseismic events occurring in the subsoil zone to be monitored. 
         [0019]    In another aspect of the present invention, there is a method for monitoring a fracturing operation in a well by means of a plurality of seismic receivers arranged about the well on the soil surface or near said surface, comprising the following steps: generating a set of reference seismic data following a perforation shot in the well; recording seismic data using said receivers; correlating the recorded seismic data with the reference seismic data; comparing each correlated data trace with correlated traces located in a vicinity of said trace, in order to evaluate the similarity of each correlated trace with the adjacent correlated traces; and, detecting a microseismic event occurring in the subsoil zone by analyzing said similarity. 
         [0020]    The method of the present invention offer the advantage of enabling real-time processing of the seismic data acquired. 
         [0021]    According to another embodiment, there is a method for monitoring a fracturing operation in a well using a plurality of seismic receivers arranged about the well on the soil surface or in the vicinity of said surface, comprising the following steps: generating a set of reference seismic data following a perforation shot in the well; recording the seismic data with said receivers; correlating the seismic data recorded with the reference seismic data; comparing each correlated data trace with correlated traces located in a vicinity of said trace, in order to evaluate a similarity of each correlated trace with the adjacent correlated traces; and, detecting a microseismic event occurring in the subsoil zone by analyzing said similarity. 
         [0022]    In another embodiment, there is a device for monitoring a subsoil zone, comprising: a plurality of seismic receivers arranged on the surface or in the vicinity of said surface, a recording unit which receives the signals transmitted by the receivers and writes same in a seismic data file; seismic data processing means programmed to perform the steps of the method as defined above, to detect microseismic events occurring in the subsoil zone to be monitored. 
         [0023]    In addition to the fact that it enables real-time processing, the method and device of the present invention uses a perforation shot in the well to generate reference seismic data. This offers the advantage of not requiring the use of a velocity model. Indeed, the seismic data acquired following a perforation shot serve somewhat to characterize the subsoil zone and replace the use of the velocity model. 
         [0024]    The correlation of the seismic data recorded during the fracturing operation with the reference data produces correlated data showing the presence of a potential microseismic event. 
         [0025]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Other features and advantages will emerge from the description hereinafter, which is purely illustrative and non-limitative, and must be read with reference to the appended figures. For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
           [0027]      FIG. 1  represents schematically an example of an installation to perform stimulated fracturing operations in a working well; 
           [0028]      FIG. 2  is a diagram representing schematically the steps of a first phase of the monitoring process, enabling the acquisition of reference data; 
           [0029]      FIG. 3  represents schematically raw reference seismic data, as recorded following a drilling blast; 
           [0030]      FIG. 4  represents schematically, the reference seismic data after correction of the estimated propagation time; 
           [0031]      FIG. 5  represents schematically, the reference seismic data, after filtering; 
           [0032]      FIG. 6  represents schematically the steps of a second phase of the monitoring process, enabling detection of microseismic events; 
           [0033]      FIG. 7  represents schematically raw seismic data, as recorded during stimulated fracturing operations; 
           [0034]      FIG. 8  represents schematically correlated data obtained by correlating recorded seismic data with the reference seismic data; 
           [0035]      FIG. 9  represents schematically a recording device and a proximity zone determined about a given seismic trace; 
           [0036]      FIG. 10  is a diagram representing for each recorded trace, a trace similarity parameter with the traces recorded in the associated proximity zone. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    In  FIG. 1 , the installation is deployed on a hydrocarbon (gas or oil) production site  1 . The site  1  comprises a subsoil zone, comprising geological layers  11 ,  12 ,  13 . The layer  13  is a layer containing hydrocarbons. The site  1  comprises a well  14  drilled through the geological layers to the layer  13 . 
         [0038]    The installation also comprises a fracturing device  2  and a monitoring device  3 . The fracturing device  2  comprises a fluid injection column  21  extending into the well  14  and a pumping unit  22  positioned on the surface  15  of the subsoil zone. The injection column  21  comprises a lower end  211  provided with openings  212  and an upper end  213  equipped with a well head  214  connected to the pumping unit  22 . 
         [0039]    The pumping unit  22  comprises a pump  221  suitable for injecting a high-pressure fracturing fluid  222  into the layer  13  via the injection column  22 . The fracturing fluid typically consists of a mixture of a liquid (for example, water) and particles (for example, sand). 
         [0040]    The fracturing fluid is injected via the pump  221  into the column  21  via the well head  214 , circulates in the column  21  and is discharged from the column via openings  212  created by well casing perforations. The fracturing fluid enters the subsoil layer  13 , inducing fracturing of the layer  13 , i.e. the appearance of cracks inside the layer  13 . The particles contained in the fracturing fluid are forced into the cracks and remain in place in the cracks when the fluid pressure is released, thus forming a permeable network enabling hydrocarbon to flow in the layer  13  to the well  14 . 
         [0041]    The monitoring device  3  comprises a network  31  of receivers (geophones), a recording unit  32  and a processing unit  34 . The receiver network  31  comprises a plurality of receivers  33  arranged on the soil surface  15  or in the vicinity of said surface. The receivers  33  may be arranged on the nodes of a grid as in  FIG. 9 , but may be arranged in any other configuration. 
         [0042]    The soil movements detected by the receivers  33  are converted into electric voltages transmitted to the recording unit  32  to be recorded therein. The recordings obtained represent the seismic data (or traces). 
         [0043]    The processing means  34  include calculation means programmed to perform the steps of the monitoring method using the seismic data. Examples of such processing means is a computer of any kind or the equivalent such as a microprocessor. Other examples known to those of skill in the art can also be used. 
         [0044]    The monitoring method is performed in two successive phases: a first phase (preparatory phase) consisting of generating the reference data, prior to the stimulated fracturing operation, and a second phase (monitoring phase) enabling real-time detection of microseismic events during the stimulated fracturing operation. 
         [0045]      FIG. 2  is a diagram representing schematically the step of a possible implementation of the first phase of the monitoring process. 
         [0046]    According to a first step  41 , a perforation shot performed in the well to create openings  212  is used, as specified above, as a source of seismic waves. Such a source may be characterized as an isotropic point source (explosive source) located at a point (referred to as “perforation point”), in the vicinity of the subsoil zone to be monitored. The coordinates {x, y and z} of this source are generally known. Knowledge of the specific time of the explosion is not necessary for the proper operation of the process. The subsoil zone to be monitored is the zone of the layer  13  and surrounding layers where microseismic events will be likely to occur due to the injection of fracturing fluid in the layer  13 . At the time of the perforation shot, a seismic wave occurs which is propagated in the layers of the subsoil to the receivers  33 . 
         [0047]    According to a second step  42 , the receivers  33  generate seismic signals or “traces”. The set of traces generated by the receivers form raw reference seismic data (data represented in  FIG. 3 ). These raw reference seismic data are recorded in the recording unit  32 . 
         [0048]    According to a third step  43 , the raw reference seismic data are pre-processed by the processing unit  34 . This third step  43  comprises several sub-steps  431  to  433  for filtering the raw reference seismic data. 
         [0049]    According to a first sub-step  431 , the processing unit  34  determines an RMS (Root Mean Square) velocity which maximizes the sum of the traces contained in the seismic data. 
         [0050]    The determination of the RMS velocity makes it possible to correct the seismic data to compensate for the differences in wave travel time to reach the various receivers. The corrected data are reduced to a common reference, which is the perforation point (corrected data represented in  FIG. 4 ). 
         [0051]    According to a second sub-step  432 , the processing unit  34  normalizes the amplitudes of the various traces to compensate for the differences in amplitudes between the traces, due to the differences in source-receiver distances. 
         [0052]    The normalization sub-step  432  consists of dividing the amplitude of each trace by an RMS (Root Mean Square) amplitude, the RMS amplitude being defined as the root mean square of the amplitudes of the trace samples. 
         [0053]    According to a third sub-step  433 , the processing unit  34  filters the data to reduce the noise contained therein. The data filtering may include so-called “mute”, F−K (Frequency−Wavenumber) filtered, F−X (Frequency−Trace number) filtering, median filtering or subtraction operations. 
         [0054]    According to a fourth step  44 , the filtered reference seismic data (filtered data represented in  FIG. 5 ) are stored in the memory of the processing means  34  in the form of a file referenced: 
         [0000]      Perfo 0 (t,Rc) 
         [0055]    where t refers to the propagation time and Rc the receiver in question. 
         [0056]    In some cases, records capable of providing reference data of sufficient quality are not available. In this case, it is possible to replace the first reference data recording and filtering phase by a reconstruction of these data using methods known to those of skill in the art of seismic processing (DIX formula, ray tracing, finite difference model, etc.) which all assume knowledge of a propagation model consisting of propagation parameters (velocities, attenuations, static corrections) in the zone above the zone to be monitored. 
         [0057]    The reference seismic data obtained using the first phase of the method will be used during the second subsequent real-time monitoring phase of the fracturing operation. 
         [0058]      FIG. 6  represents schematically the steps of the second phase of the monitoring process, enabling detection of microseismic events occurring during the fracturing operation, i.e. during the injection of fracturing fluid in the subsoil. 
         [0059]    According to a first step  51 , the receivers  33  convert the subsoil movement into electrical signals transmitted to the recording unit  32  to form the basic microseismic data file represented in  FIG. 7  and referenced: 
         [0000]      D(θ,Rc) 
         [0060]    where θ refers to the calendar time and Rc to the receiver in question. 
         [0061]    According to a second step  52 , the basic seismic data are correlated with the reference seismic data. 
         [0062]    The correlated seismic data obtained (data represented in  FIG. 8 ) are recorded in the form of a file referenced: 
         [0000]        CC (θ,Rc)= D (θ,Rc)         Perfo 0 ( t,Rc ) 
         [0063]    where           refers to the correlation product. 
         [0064]    The correlated seismic data are in the form of a wavelet signaling the possible presence of a microseismic event, said wavelet having the following properties: the arrival times corresponding to the maximum amplitudes (maximum energies) do not vary much from one receiver to adjacent receivers; the phase distortions created by the superficial layer WZ (Weathered Zone) are reduced or eliminated by the correlation operation; the polarity of the wavelet is spatially stable but may change according to the observation zone. 
         [0065]    According to a third step  53 , the similarity of each correlated trace with the adjacent correlated traces is evaluated. This third step  53  comprises several sub-steps  531  to  533 . 
         [0066]    According to a first sub-step  531 , each seismic data trace CC(θ,Rc) is correlated with the sum of the traces contained in a vicinity of said trace. 
         [0000]    
       
         
           
             
               
                 C 
                 0 
               
                
               
                 ( 
                 
                   θ 
                   , 
                   Rc 
                 
                 ) 
               
             
             = 
             
               
                 CC 
                  
                 
                   ( 
                   
                     θ 
                     , 
                     Rc 
                   
                   ) 
                 
               
               ⊗ 
               
                 S 
                  
                 
                   ( 
                   
                     θ 
                     , 
                     Rc 
                   
                   ) 
                 
               
             
           
         
       
       
         
           where 
         
       
       
         
           
             
               S 
                
               
                 ( 
                 
                   θ 
                   , 
                   Rc 
                 
                 ) 
               
             
             = 
             
               
                 ∑ 
                 
                   Rc 
                   ∈ 
                   V 
                 
               
                
               
                   
               
                
               
                 CC 
                  
                 
                   ( 
                   
                     θ 
                     , 
                     Rc 
                   
                   ) 
                 
               
             
           
         
       
     
         [0067]    where S(θ) is the sum of the traces contained in a vicinity V, the vicinity V being defined as a disk having a radius Rd centered on the trace in question. 
         [0068]      FIG. 9  represents the distribution of the receivers about the well and a vicinity V having a radius Rd centered on the trace in question. 
         [0069]    This correlation step makes it possible to compare each trace with the close environment thereof, by eliminating the polarity inversion problem between the traces. Indeed, the correlation operation demonstrates a similarity between the traces despite the presence of inverted polarity peaks (whereas a summing would result in the traces canceling each other out). This step is performed in time windows. The best results are obtained when the windows fully overlap except for a single sample. For each correlation, it is only necessary to calculate a restricted number of points about the time  0 , or even the single sample at the time  0 . 
         [0070]    According to a second sub-step  532 , a semblance parameter S 1 (θ) is calculated a sum of filtered seismic data C 1 (θ, RC): 
         [0000]    
       
         
           
             
               
                 S 
                 1 
               
                
               
                 ( 
                 θ 
                 ) 
               
             
             = 
             
               
                 ∑ 
                 Rc 
               
                
               
                   
               
                
               
                 
                   C 
                   1 
                 
                  
                 
                   ( 
                   
                     θ 
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                     Rc 
                   
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         [0071]    This sum may be a simple sum, a weighted sum (referred to as “diversity stack”) or a median. 
         [0072]    The semblance parameter S 1 (θ) measures the overall similarity of the CC traces with the surrounding CC traces. After normalization, this parameter is an indicator of the presence of a microseismic event. 
         [0073]      FIG. 10  is a diagram representing the value of the semblance parameter S 1 (θ) as a function of the calendar time θ. In this diagram, the presence of a peak indicating the occurrence of a microseismic event in the time window in question is observed. 
         [0074]    According to a fourth step  54 , the processing unit  34  detects a microseismic event on the basis of the semblance parameter. 
         [0075]    According to a first option, the processing unit  34  detects the occurrence of a microseismic event when a local peak of the semblance parameter is greater than a predefined threshold value (set to 10 in  FIG. 10 ). 
         [0076]    According to a second option, the processing unit  34  detects the occurrence of a microseismic event when the ratio between a local peak of the semblance parameter and the RMS amplitude of the corresponding sample is greater than a predefined threshold value. 
         [0077]    According to a third option, the processing unit  34  detects the occurrence of a microseismic event when the ratio between a local peak of the semblance parameter and a mean value of the local peaks, an RMS value or a median of the local peaks is greater than a predefined threshold value. 
         [0078]    According to a fourth option, the processing unit  34  performs a statistical search in the local peak curve. For example, the processing unit  34  searches for a discontinuity in an ordered sequence of peaks. 
         [0079]    The monitoring method described, while it is of particular value for monitoring a stimulated operation, is not limited to this application and is applicable more generally for monitoring a subsoil zone. 
         [0080]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.