Abstract:
A method for identifying fractures from measurements made by a multi-axial electromagnetic induction tool in a wellbore traversing subsurface formations includes determining a value of a fracture orientation indicator from in line components of the multi-axial electromagnetic inducion measurements mode transverse to a tool axis, and parallel to the tool axis. The tool axis is sub-stantially parallel to a bedding plane of the subsurface formations. A value of a vertical fracture indicator is determined using the in line components of the multi-axial electromagnetic induction measurements made transverse to the tool axis, and parallel to the tool axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/916042 filed on Dec. 13, 2013, the contents of which are incorporated herein for all purposes. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates generally to the field of subsurface formation fracture evaluation. More specifically, the disclosure relates to techniques for detecting fractures having a planar orientation substantially perpendicular to a wellbore using multi-axial electromagnetic induction well logging instruments. 
         [0003]    A multi-axial electromagnetic induction well logging instrument such as a triaxial electromagnetic induction well logging instrument sold under the trademark RT SCANNER, which is a trademark of Schlumberger Technology Corporation, Sugar Land, Texas, measures 9-component apparent conductivity tensors (σm(i, j, k), i, j=x, y, z) at a plurality of receiver spacings from a transmitter, wherein each spacing is represented by the index k.  FIG. 2  schematically illustrates such a tri-axial tool  10  and the component tensor measurement C. The instrument  10  may include one or more multi-axial electromagnetic transmitters T disposed on the instrument  10 , and have one or more multi-axial electromagnetic receivers (each receiver usually consisting of a main receiver RM and a balancing or “bucking” receiver RB to attenuate direct induction effects) at one or more axially spaced apart positions along the longitudinal axis z of the tool  10 . The RT SCANNER instrument uses triaxial transmitters and receivers, wherein the transmitters and receivers have three, mutually orthogonal coils having magnetic dipole axes oriented along the tool axis z and along two other mutually orthogonal directions shown at x and y. The instrument measurements in the present example may be obtained in the frequency domain by energizing the transmitter T with a continuous wave (CW) alternating current having one or more discrete frequencies (using more than one discrete frequency may enhance the signal-to-noise ratio). However, measurements of the same information content may also be obtained using time domain signals through a Fourier decomposition process by energizing the transmitter T with one or more types of transient currents. This is a well known physics principle of frequency-time duality. Voltages induced in each coil of one of the receivers RM/RB is shown in the tensor C represented by the voltage V with a two letter subscript as explained above representing the axis (x, y or z) of the transmitter coil used and the axis of the receiver coil (x, y or z) used to make the particular voltage measurements. The voltage measurements in tensor C may be processed to obtain the described apparent conductivity tensors. Subsurface formation properties, such as horizontal and vertical conductivities (σh, σv) or their inverse, horizontal and vertical resistivities (Rh, Rv), relative dip angle (θ) and the dip azimuthal direction (Φ), as well as borehole/tool properties, such as drilling fluid (mud) conductivity (σmud), wellbore diameter (hd), tool eccentering distance (decc), tool eccentering azimuthal angle ( 104  ), all affect the measurements of voltages used to determine the conductivity tensors. 
         [0004]      FIG. 3A  illustrates a top view, and  FIG. 3B  shows an oblique view of an eccentered tool  10  in a wellbore  12  drilled through an anisotropic formation F with a non-zero dip angle (θ). Eccentering of the tool  10  is shown by decc and the azimuthal angle of the dip azimuth is represented by (ψ). The tool  10  eccentering azimuthal angle is shown by ψ.  FIG. 3C  shows vertical and horizontal conductivity determinable with the tool of  FIGS. 3A and 3B  with reference to a dip angle between formation layering and a wellbore (and corresponding tool) longitudinal axis. The above description is to provide a frame of reference to understand an example method according to the present disclosure. 
         [0005]    Using a simplified model of layered anisotropic formation traversed obliquely by the wellbore  12 , the response of the conductivity tensors depends on the above eight parameters in a very complex manner. The effects of the wellbore and instrument orientation and position on the measured conductivity tensors may be very large even in wellbores having substantially electrically nonconductive fluid therein, e.g., oil base mud (OBM). Through one of several known inversion techniques the above wellbore and formation parameters can be calculated and borehole effects can be removed from the measured conductivity tensors to determine values of horizontal and vertical resistivities (Rh, Rv), relative dip angle (θ) and the dip azimuthal direction (Φ). 
         [0006]    The formation parameters (vertical and hortrizontal conductivities, dip and dip azimuth) may be displayed substantially in real-time (as computed by a processor near the wellbore, see  FIG. 1A  and  FIG. 1B ) to help make various decisions related to the drilling and completion of the wellbore. The resistivites (the inverse of conductivities) of the subsurface formations determinable by a tool such as illustrated in  FIG. 2  are known in the art to be used, for example, to delineate low resistivity laminated hydrocarbon bearing formations. The dip and dip azimuth are known to be used to map the structure of the formations in a scale much finer than that provided by, e.g., surface reflection seismic. 
         [0007]    One of the important items of information that may affect the drilling and completion decisions of any particular wellbore is whether the wellbore has traversed significant fractured zones. Fractures may occur in some formations due to tectonic forces acting over geological time. Fractures can also be induced in some formations by the drilling operation. Large fracture systems can sometimes be a principal factor that enables economically useful production of oil and/or gas from a particular wellbore. Large fracture systems traversed by a wellbore could also cause loss of drilling mud. Accordingly, knowing the location of the fracture zone and the fracture plane orientation can significantly improve the drilling and completion decision. 
         [0008]    Fractures with large planar extent, even if very thin, filled with non-conductive fluid, such as connate oil and/or oil based drilling fluid may block the induced current in the formation resulting from electromagnetic induction effects of energizing the transmitter T on the tool and could produce significant anomalies in the inverted formation parameters compared with those from the same formation without fractures. The size of such anomalies may depend on the formation resistivities (Rh, Rv), the size of the fracture plane, and the relative dip and azimuth between the fracture plane and the layering structure of the formation, among other factors. If the fracture plane is nearly parallel to the layering structure of the formation, the effects of the fracture on measurements made by an instrument such as shown in  FIG. 2  may be relatively small. On the other hand, if the fracture plane is perpendicular to the layering structure of the formation, the effect of the fracture may dominate the response of the tool. A fracture system often encountered by wellbores is that of substantially horizontal layered formations with vertical fractures. Accordingly, techniques for characterizing such fractures using multi-axial (e.g., tri-axial) electromagnetic induction measurements may be useful in this regard. 
       SUMMARY 
       [0009]    This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
         [0010]    A method according to one aspect for identifying fractures from measurements made by a multi-axial electromagnetic induction tool in a wellbore traversing subsurface formations includes determining a value of a fracture orientation indicator from in line components of the multi-axial electromagnetic induction measurements made transverse to a tool axis, and parallel to the tool axis. The tool axis is substantially parallel to a bedding plane of the subsurface formations. A value of a vertical fracture indicator is determined using the in line components of the multi-axial electromagnetic induction measurements made transverse to the tool axis, and parallel to the tool axis. 
         [0011]    Other aspects and advantages will be apparent from the description and claims which follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Certain embodiments are described below with reference to the following figures: 
           [0013]      FIG. 1A  shows an example wireline conveyed multi-axial electromagnetic well logging instrument disposed in a wellbore drilled through subsurface formations. 
           [0014]      FIG. 1B  shows an example well logging instrument system that may be used during wellbore drilling. 
           [0015]      FIG. 2  shows an illustration of a multi-axial (e.g., triaxial) induction array measurement devices (transmitter and receivers) at a given spacing between the transmitter and each receiver. 
           [0016]      FIG. 3A  shows schematically a top view of an eccentered multi-axial induction tool in a wellbore passing through an anisotropic formation at a relative dip angle. 
           [0017]      FIG. 3B  shows an oblique view of the eccentered tool shown in  FIG. 3A . 
           [0018]      FIG. 3C  shows vertical and horizontal conductivity determinable with the tool of  FIGS. 3A and 3B  with reference to a dip angle between formation layering and a wellbore (and corresponding tool) longitudinal axis. 
           [0019]      FIG. 4  shows schematically a triaxial induction well logging tool moving through a substantially horizontal wellbore that intersects a substantially vertical fracture. 
           [0020]      FIG. 5  shows an example computer system. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1A  shows an example multi-axial electromagnetic well logging instrument  30 . The measurement components of the instrument  30  may be disposed in a housing  111  shaped and sealed to be moved along the interior of a wellbore  32 . The well logging instrument  30  may, in a form hereof, be of a type sold under the trademark RT SCANNER, which is a trade mark of Schlumberger Technology Corporation, Sugar Land, Tex. 
         [0022]    The instrument housing  111  may contain at least one multi-axial electromagnetic transmitter  115 , and two or more multi-axial electromagnetic receivers  116 ,  117  each disposed at different axial spacings from the transmitter  115 . The transmitter  115 , when activated, may emit a continuous wave electromagnetic field at one or more selected frequencies. Shielding (not shown) may be applied over the transmitter  115  and the receivers  116 ,  117  to protect the antenna coils which are deployed near the outer layer of the tool. The detectors  116 ,  117  may be multi-axis wire coils each coupled to a respective receiver circuit (not shown separately). Thus, detected electromagnetic energy may also be characterized at each of a plurality of distances from the transmitter  115 . 
         [0023]    The instrument housing  111  maybe coupled to an armored electrical cable  33  that may be extended into and retracted from the wellbore  32 . The wellbore  32  may or may not include metal pipe or casing  16  therein. The cable  33  conducts electrical power to operate the instrument  30  from a surface  31  deployed recording system  70 , and signals from the receivers  116 ,  117  may be processed by suitable circuitry  118  for transmission along the cable  33  to the recording system  70 . The recording system  70  may include a computer as will be explained below for analysis of the detected signals as well as devices for recording the signals communicated along the cable  33  from the instrument  30  with respect to depth and/or time. 
         [0024]    The well logging tool described above can also be used, for example, in logging-while-drilling (“LWD”) equipment. As shown, for example, in  FIG. 1B , a platform and derrick  210  are positioned over a wellbore  212  that may be formed in the Earth by rotary drilling. A drill string  214  may be suspended within the borehole and may include a drill bit  216  attached thereto and rotated by a rotary table  218  (energized by means not shown) which engages a kelly  220  at the upper end of the drill string  214 . The drill string  214  is typically suspended from a hook  222  attached to a traveling block (not shown). The kelly  220  may be connected to the hook  222  through a rotary swivel  224  which permits rotation of the drill string  214  relative to the hook  222 . Alternatively, the drill string  214  and drill bit  216  may be rotated from the surface by a “top drive” type of drilling rig. 
         [0025]    Drilling fluid or mud  226  is contained in a mud pit  228  adjacent to the derrick  210 . A pump  230  pumps the drilling fluid  226  into the drill string  214  via a port in the swivel  224  to flow downward (as indicated by the flow arrow  232 ) through the center of the drill string  214 . The drilling fluid exits the drill string via ports in the drill bit  216  and then circulates upward in the annular space between the outside of the drill string  214  and the wall of the wellbore  212 , as indicated by the flow arrows  234 . The drilling fluid  226  thereby lubricates the bit and carries formation cuttings to the surface of the earth. At the surface, the drilling fluid is returned to the mud pit  228  for recirculation. If desired, a directional drilling assembly (not shown) could also be employed. 
         [0026]    A bottom hole assembly (“BHA”)  236  may be mounted within the drill string  214 , preferably near the drill bit  216 . The BHA  236  may include subassemblies for making measurements, processing and storing information and for communicating with the Earth&#39;s surface. The bottom hole assembly is typically located within several drill collar lengths of the drill bit  216 . In the illustrated BHA  236 , a stabilizer collar section  238  is shown disposed immediately above the drill bit  216 , followed in the upward direction by a drill collar section  240 , another stabilizer collar section  242  and another drill collar section  244 . This arrangement of drill collar sections and stabilizer collar sections is illustrative only, and other arrangements of components in any implementation of the BHA  236  may be used. The need for or desirability of the stabilizer collars will depend on drilling conditions. 
         [0027]    In the arrangement shown in  FIG. 1B , the components of multi-axial induction well logging instrument may be located in the drill collar section  240  above the stabilizer collar  238 . Such components could, if desired, be located closer to or farther from the drill bit  216 , such as, for example, in either stabilizer collar section  238  or  242  or the drill collar section  244 . 
         [0028]    The BHA  236  may also include a telemetry subassembly (not shown) for data and control communication with the Earth&#39;s surface. Such telemetry subassembly may be of any suitable type, e.g., a mud pulse (pressure or acoustic) telemetry system, wired drill pipe, etc., which receives output signals from LWD measuring instruments in the BHA  236  (including the one or more radiation detectors) and transmits encoded signals representative of such outputs to the surface where the signals are detected, decoded in a receiver subsystem  246 , and applied to a processor  248  and/or a recorder  250 . The processor  248  may comprise, for example, a suitably programmed general or special purpose processor. A surface transmitter subsystem  252  may also be provided for establishing downward communication with the bottom hole assembly. 
         [0029]    The BHA  236  can also include conventional acquisition and processing electronics (not shown) comprising a microprocessor system (with associated memory, clock and timing circuitry, and interface circuitry) capable of timing the operation of the accelerator and the data measuring sensors, storing data from the measuring sensors, processing the data and storing the results, and coupling any desired portion of the data to the telemetry components for transmission to the surface. The data may also be stored downhole and retrieved at the surface upon removal of the drill string. Power for the LWD instrumentation may be provided by battery or, as known in the art, by a turbine generator disposed in the BHA  236  and powered by the flow of drilling fluid. The LWD instrumentation may also include directional sensors (not shown separately) that make measurements of the geomagnetic orientation or geodetic orientation of the BHA  236  and the gravitational orientation of the BHA  236 , both rotationally and axially. 
         [0030]    While the description that follows is based on measurements made from a tool such as the RT SCANNER tool described with reference to  FIG. 2  in which each of the transmitter and receivers comprises three, mutually orthogonal induction coils with one coil being aligned with the tool&#39;s longitudinal axis, it is to be understood that for purposes of defining the scope of the disclosure, any induction well logging instrument with multi-axial transmitter(s) and receiver(s) having magnetic dipole axes along other directions and in other than three magnetic dipole axis elements (e.g., coils) per transmitter or receiver may be used provided that for each such transmitter and receiver it is possible to resolve three mutually orthogonal components of the transmitted electromagnetic field and the received electromagnetic field and where such resolved components are susceptible to either or both mechanical (physically embodied) or mathematical rotation to any selected coordinate system, e.g., Cartesian or cylindrical. 
         [0031]      FIG. 4  is a schematic of a substantially vertical fracture  90  and a triaxial induction tool  10  as explained above disposed in a nearly or actually horizontal wellbore  12 A that penetrates the fracture  90 . x, y, z denote the three orthogonal directions of the magnetic moment of the triaxial transmitter(s) and receivers on the triaxial induction tool  10 . For purposes of explaining an example method according to the present disclosure, the z-direction is in line with the tool and the wellbore axes. The x-direction is assumed to be pointed upward or in the top-of-the-hole direction. The y-direction is co-planar with the x-direction and follows the right-hand rule of the standard Cartesian coordinate system. The background formation is assumed to be of uniform composition and is electrically anisotropic. Here, isotropic formation may be considered as a subset of anisotropic formation for which the horizontal and vertical resistivities have equal value (Rh=Rv). The fracture  90  plane is assumed to be much larger than the diameter of the well logging tool  10 . It will be appreciated by those skilled in the art that using a logging while drilling system such as explained with reference to  FIG. 1B  may enable the system operator to orient the transmitter(s) and receivers on the well logging tool so that their orientation is along the directions explained above. It is also possible to use some or all of the nine component tensor measurements to resolve certain measurement components from the 9 component measurement tensor as will be explained below. The tool&#39;s axis is assumed to be substantially perpendicular to the long dimension of the fracture plane. The z-axis of the tool may intersect the fracture plane at any arbitrary angle, which angle may be determined as explained below. 
         [0032]    As an example, selected components for fracture detection in the present example horizontal well configuration may be σyy, σzz, and σyy_45. Here, σyy_45 is the σyy component of the measured apparent conductivity tensor mathematically rotated 45 degrees around the x-axis. In the present context, a measurement made using a transmitter and a receiver with their magnetic moments oriented in the same direction may be referred to as an “in-line” measurement. Correspondingly, when the transmitter direction is different from the receiver direction, such measurement may be referred to as a “crossline” measurement. 
         [0033]    Using these above signal components, it is possible to derive two indicators for detection of vertical fractures (VFIND for vertical fracture indicator) and the fracture strike orientation (FOI for fracture orientation indicator) using the following expressions: 
         [0000]        FOI= 0.5*tan −1 [(2 *σyy _45 −(σ zz+σyy ))/(σ yy−σzz )]  (1)
 
         [0000]        VFIND=ABS (0.5*(σ yy−σzz )/[δ+cos(2* FOI )])  (2)
 
         [0034]    The parameter ∂ in equation (2) is a very small constant used for the purpose of preventing the denominator from being zero. ABS( )is the function symbol for taking absolute value of the expression within the parentheses. In the case where measurements are made with the x-axis transmitter(s) and receivers oriented other than vertically, and correspondingly the y-axis transmitter(s) and receiver oriented other than horizontally, well known trigonometric relationships may be used to determine the σyy_45, σzz and σyy measurement components used in the two above equations. 
         [0035]    FOI in equation (1) is a fracture orientation indicator. It indicates the strike direction of the fracture, which in the present example may be defined as the angle subtended between the fracture plane and the wellbore/tool axis or z direction. If the geomagnetic or geodetic orientation of the logging tool axis is determined, the FOI may be referenced to geomagnetic and/or geodetic direction. 
         [0036]    VFIND in equation (2) is a vertical fracture indicator. VFIND is primarily a function of the following parameters:
   FA—fracture aperture   FD—fracture displacement   FW—fracture width   FH—fracture height   Rh—formation horizontal resistivity   Rv—formation vertical resistivity   Dip—the dip angle of the anisotropy   
 
         [0044]    For a given value of Rh, Rv, and Dip, VFIND becomes an indicator of the size of the fracture (FW×FH), FA, and FD. The 9-component electromagnetic induction measurements from each of a plurality of multi-axial receivers may be processed by a zero-D model inversion to obtain background formation information. By way of example only, one type of zero-D inversion process is described in Wu, P., Wang, G., and Barber, T.,  Efficient hierarchical processing and interpretation of triaxial induction data in formations with changing dip , paper SPE 135442 presented at the SPE Annual Technical Conference and Exhibition, Florence, Italy, Sep. 19-22, 2010. The zero-D inversion may output, at each depth index n, formation horizontal resistivity, vertical resistivity, dip angle, and dip azimuth (Rh n , Rv n , Dip n , and Az n , respectively). The foregoing values may be computed when the value of VFIND falls below a selected threshold, i.e., when the tool is far enough away from any vertical fractures to have a substantial effect on the component tensor measurements. For purposes of defining the scope of the present disclosure, it is believed that having the tool longitudinal axis (z axis) subtend an angle of at most about 30 degrees with respect to the orientation of the long dimension of the fracture plane will still provide useful results. 
         [0045]    The foregoing computations may be performed on a computer system such as one shown in the processor at 248 in  FIG. 1B , or in the surface unit  70  in  FIG. 1A . However, any computer or computers may be used to equal effect.  FIG. 5  depicts an example computing system  100  in accordance with some embodiments for carrying out example methods such as those explained above. The computing system  100  can be an individual computer system  101 A or an arrangement of distributed computer systems. The computer system  101 A includes one or more analysis modules  102  that are configured to perform various tasks according to some embodiments, such as the tasks described above with reference to  FIG. 4 . To perform these various tasks, an analysis module  102  executes independently, or in coordination with, one or more processors  104 , which is (or are) connected to one or more storage media  106 . The processor(s)  104  is (or are) also connected to a network interface  108  to allow the computer system  101 A to communicate over a data network  110  with one or more additional computer systems and/or computing systems, such as  101 B,  101 C, and/or  101 D (note that computer systems  101 B,  101 C and/or  101 D may or may not share the same architecture as computer system  101 A, and may be located in different physical locations, e.g. computer systems  101 A and  101 B may be on a ship underway on the ocean, in a well logging unit disposed proximate a wellbore drilling, while in communication with one or more computer systems such as  101 C and/or  101 D that are located in one or more data centers on shore, other ships, and/or located in varying countries on different continents). 
         [0046]    A processor can include a microprocessor, microcontroller, processor module or subsystem, programmable integrated circuit, programmable gate array, or another control or computing device. 
         [0047]    The storage media  106  can be implemented as one or more non-transitory computer-readable or machine-readable storage media. Note that while in the embodiment of  FIG. 5  storage media  106  is depicted as within computer system  101 A, in some embodiments, storage media  106  may be distributed within and/or across multiple internal and/or external enclosures of computing system  101 A and/or additional computing systems. Storage media  106  may include one or more different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; optical media such as compact disks (CDs) or digital video disks (DVDs); or other types of storage devices. Note that the instructions discussed above can be provided on one computer-readable or machine-readable storage medium, or alternatively, can be provided on multiple computer-readable or machine-readable storage media distributed in a large system having possibly plural nodes. Such computer-readable or machine-readable storage medium or media is (are) considered to be part of an article (or article of manufacture). An article or article of manufacture can refer to any manufactured single component or multiple components. The storage medium or media can be located either in the machine running the machine-readable instructions, or located at a remote site from which machine-readable instructions can be downloaded over a network for execution. 
         [0048]    It should be appreciated that computing system  100  is only one example of a computing system, and that computing system  100  may have more or fewer components than shown, may combine additional components not depicted in the embodiment of  FIG. 5 , and/or computing system  100  may have a different configuration or arrangement of the components depicted in  FIG. 5 . The various components shown in  FIG. 5  may be implemented in hardware, software, or a combination of both hardware and software, including one or more signal processing and/or application specific integrated circuits. 
         [0049]    Further, the steps in the methods described above may be implemented by running one or more functional modules in information processing apparatus such as general purpose processors or application specific chips, such as ASICs, FPGAs, PLDs, SOCs, or other appropriate devices. These modules, combinations of these modules, and/or their combination with general hardware are all included within the scope of protection of the invention. 
         [0050]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.