Abstract:
An apparatus for estimating at least one parameter of interest of an earth formation includes a first sub and a second sub positioned along the conveyance device. The first sub and the second sub cooperate to generate at least one main component measurement and only the second sub is configured to generate at least one cross-component measurement. A method includes conveying a first sub and a second sub along a wellbore formed in the earth formation using a conveyance device, using the first sub and the second sub to generate at least one main component measurement, and using only the second sub to generate at least one cross-component measurement.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The disclosure is related generally to the field of electrical resistivity well logging methods. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    To obtain hydrocarbons such as oil and gas, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. Boreholes may be drilled vertically, but directional drilling systems are often used for drilling boreholes deviated from vertical and/or horizontal boreholes to increase the hydrocarbon production. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, tool azimuth, tool inclination. Also used are measuring devices such as a resistivity-measuring device to determine the presence of hydrocarbons and water. Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, reasonably determine the petrophysical properties of the formation and the fluids therein. 
         [0003]    The present disclosure is directed resistivity tools that provide enhanced operation and functionality. 
       SUMMARY OF THE DISCLOSURE 
       [0004]    In one aspect, the present disclosure provides an apparatus for estimating at least one parameter of interest of an earth formation. The apparatus may include a first sub and a second sub positioned along the conveyance device. The first sub and the second sub cooperate to generate at least one main component measurement and only the second sub is configured to generate at least one cross-component measurement. 
         [0005]    In another aspect, the present disclosure provides a method for estimating at least one parameter of interest of an earth formation. The method may include conveying a first sub and a second sub along a wellbore formed in the earth formation using a conveyance device, using the first sub and the second sub to generate at least one main component measurement, and using only the second sub to generate at least one cross-component measurement. 
         [0006]    Examples of certain features of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood and in order that the contributions they represent to the art may be appreciated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a detailed understanding of the present disclosure, reference should be made to the following detailed description of the embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein: 
           [0008]      FIG. 1  shows a schematic of a drilling system using a resistivity tool according to one embodiment of the present disclosure; 
           [0009]      FIG. 2  shows a schematic close up of a resistivity tool according to one embodiment of the present disclosure; and 
           [0010]      FIG. 3A-D  show various embodiments of resistivity tools according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    This disclosure generally relates to exploration for hydrocarbons involving electromagnetic investigations of a borehole penetrating an earth formation. In aspects, the present disclosure provides a “looking deep” azimuth resistivity tool formed on a single sub or joint of a tool string with a medium spacing. This configuration reduces the complexity associated with synchronization of signals. Embodiments of the present disclosure may be implemented with relatively less engineering work and without a significant loss of penetration depth. One illustrative arrangement uses two subs for transmitters and receivers that can be arranged in a controllable larger spacing (e.g., greater than 10 meters) for looking deep (i.e., radially outward from the longitudinal axis of the tool a distance 10 meters or greater) and also looking “ahead of the bit” or axially along the trajectory of the wellbore. 
         [0012]    Referring now to  FIG. 1 , there is shown an exemplary drilling system  20  suitable for use with the present disclosure. As is shown, a conventional rig  22  includes a derrick  24 , derrick floor  26 , draw works  28 , hook  30 , and swivel  32  A conveyance device such as a drillstring  38  which includes drill pipe section  40  and drill collar section  42  extends downward from rig  22  into a wellbore  44 . In other embodiments, at least some of the conveyance device may include a non-rigid carrier such as coiled tubing. Drill collar section  42  preferably includes a number of tubular drill collar members which connect together, including a measurement-while-drilling (MWD) subassembly including a number of sensors and cooperating telemetry data transmission subassembly, which are collectively referred to hereinafter as “MWD system  46 ”. The drill string  38  further includes a drill bit  56  adapted to disintegrate a geological formation and known components such as thrusters, mud motors, steering units, stabilizers and other such components for forming a wellbore through the subterranean formation  14 . Other related components and equipment of the system  20  are well known in the art and are not described in detail herein. The MWD system  46  may include a resistivity tool  60 , which is shown in greater detail in  FIG. 2 . 
         [0013]      FIG. 2  shows one embodiment of resistivity tool  60  in accordance with the present disclosure. The tool  60  may be configured for deep azimuthal investigation by operating at a low frequency. As used herein, a low frequency may be a frequency at or below 500 KHz. As used herein, a “deep” investigation is an investigation of the formation at least ten meters radially away from the wellbore. The resistivity tool  60  includes a first sub  62  and a second sub  64 . The first sub  62  and the second sub  64  cooperate to generate at least one main component measurement and the second sub is configured to generate at least one cross-component measurement. The main component may be one of: (i) a co-axial component, and (ii) a co-planar component. 
         [0014]    The subs  62 ,  64  may be separated by unrelated equipment  55 . By unrelated, it is meant that the equipment does not operationally interact with the receivers and transmitters of the subs  62 ,  64  (e.g., emit or detect signals associated operation of the subs  62 ,  64 ). The term “sub” refers to a unitary body of oil field well equipment and may be a tool string, a housing, support, frame, enclosure, or carrier. In some conventions, a standard sub may have a length of 30 feet or a length of 10 meters. In one sense, a “sub” is sufficiently functionally and structurally integral to enable onboard equipment share the same electronic components; e.g., a clock for synchronizing measurements. 
         [0015]    The first sub  62  may include one or more Z-transmitters  66 , e.g., a transmitter coil directed along the “co-axial” of the sub  62 . The second sub  64  may include one or more X-transmitters  68  and one or more Z-receivers  70 . The X-transmitters  68  and the Z-receivers  70  may be disposed toward the opposing ends of the sub  64  to maximize the axial space separating these two components. In one illustrative configuration, the spacing may be six or more meters. However, this spacing and relative positioning is small enough to allow a synchronizing circuit  72 , which may include a clock  74 , to provide a synchronizing signal for X-transmitters  68  and one or more Z-receivers  70 . The Z-transmitter and the Z-receiver are on different subs and may be separated by an axial distance of ten meters or more. 
         [0016]    The transmitters  66 , 68  may be placed with their normals substantially orthogonal to each other, in the order shown. The transmitters  66 ,  68  induce magnetic fields in two spatial directions. The letters (“X,” “Z”) indicate an orthogonal system substantially defined by the directions of the normals to the transmitters  66 ,  68 . The z-axis is chosen to be substantially parallel to the longitudinal axis of the tool  60 , while the x-axis is in a perpendicular direction lying in the plane transverse to the longitudinal axis. The receivers  70  are aligned along the orthogonal system defined by the transmitter normals. The orientation of the transmitters and receivers remain fixed with respect to the tool  60 . The multi-component tool in horizontal configuration is sensitive to the anisotropic formation and tool location as well as the rotation of the tool  60  around its axis. 
         [0017]    The first sub  62  and the second sub  64  cooperate to generate co-axial measurements. This is possible by using the Z-transmitter of the first sub  62  and the Z-receiver of the second sub  64 . By “co-axial,” it is meant measurement of the “ZZ” component of a magnetic field. In one configuration, differential axial measurements of the Z transmitter induced magnetic field that have been taken by the closely spaced Z receivers may be used to eliminate the need for synchronization to estimate the ZZ component. The respective receivers may be used to determine an axial signal as follows: Amplitude ratio=A R2 /A R1  (Phase difference=Ø R2 −Ø R1 ). The second sub  64  is configured to generate cross-component measurements. This is possible by using the X-transmitter and the Z-receiver of the second sub  64 . By “cross-component,” it is meant measurements of the “ZX” component of the magnetic field. Thus, the first sub  62  is not used to generate a cross-component measurement. These measurements may be in the frequency domain. 
         [0018]    Referring now to  FIGS. 1 and 2 , during use, the drilling system  10  forms the wellbore  44  by rotating the drill string  38 . At the same time, the resistivity tool  60  rotates while taking resistivity measurements of the formation being traversed by the drill string  38 . The first sub  62  and the second sub  64  cooperatively generate co-axial measurements while the second sub  64  generates cross-component measurements. Advantageously, a single clock  74  associated with the second sub  64  may be used to synchronize the cross-component measurements generated by the second sub  64 . Further, the first sub  62  and the second sub  64  can be arranged at a much larger spacing for a differential main component measurement to enhance looking deeper. 
         [0019]    Embodiments of the present disclosure may also be configured to use two subs to measure “XX” components and use one sub to measure only the cross-components of the magnetic field. Thus, a common clock may be used for all the cross-component measurements. Additionally, embodiments of the present disclosure may include sub configurations wherein the transmitters and receivers are arranged such that the first sub has two transmitters (either X or Z) while the second sub has at least one receiver (either X or Z) and at least one transmitter (either Z or X or Y). Thus, the second sub may use not only one cross-component but also other cross-components and main components. Illustrative non-limiting variants are discussed below in connection with  FIGS. 3A-3D . These embodiments all include a first sub  62 , a second sub  64 , and a synchronization circuit  72  that has a clock  74 . In all these embodiments, the transmitters and receivers are arranged such that synchronization is not needed for the measurements using both subs  62 ,  64 . Rather, the measurements using both subs  62 ,  64  are differential measurements. The measurements wherein synchronization is used are made using only the sub  64 . It should be understood that the terms “first” and “second” are used merely for ease of discussion. This terminology is not intended to limit the number of subs or to identify a particular spatial orientation for the subs. 
         [0020]    Referring to  FIG. 3A , the first sub  62  may include a Z-transmitter  66 . The second sub  64  may also include X-receivers  80  in addition to a Z-receiver  70  and the X-transmitter  68 . Thus, the second sub  64  may be configured to make measurements of the “XX” magnetic component in addition to the cross-component measurements. 
         [0021]    Referring to  FIG. 3B , the first sub  62  includes one X-transmitter  68  and the second sub  64  has two X-receivers  80  and one Z-transmitter  66 . Optionally, this embodiment may also include an additional Z-receiver  70 , a Y-receiver  82 , or Y-transmitter  84  to make ZZ, YZ, or XY, YY, ZY components. In this embodiment, the differential measurements are made using the two X-receivers  80  on the second sub  64 . The cross-component measurements may be made using the Z-transmitter  66  and the X-receivers  80 . 
         [0022]    Referring to  FIG. 3C , the first sub  62  has two Z-transmitters  66  and the second sub  68  has one Z-receiver  70  and one X-transmitter  68 . Optionally, this embodiment may also include an additional X-receiver, a Y-receiver  82 , or Y-transmitter  84  to make XX, YX, or XY, YY, ZY measurements. In this embodiment, the differential measurements are made using the two Z-transmitters  66  on the first sub  62  and the Z-receiver  70  on the second sub  64 . The cross-component measurements may be made using the X-transmitter  68  and the Z-receiver  70 . 
         [0023]    Referring to  FIG. 3D , the first sub  62  has two X-transmitters  68  and the second sub  64  has one X-receiver  80  and one Z-transmitter  66 . Optionally, this embodiment may also include an additional Z-receiver  70  or Y-receiver  82  or Y-transmitter  84  to make ZZ, YZ, or XY, YY, ZY measurements. In this embodiment, the differential measurements are made using the two X-transmitters  68  on the first sub  62  and the X-receiver  80  on the second sub  64 . The cross-component measurements may be made using the Z-transmitter  66  and the X-receiver  80 . 
         [0024]    Implicit in the processing of the data is the use of a computer program implemented on a suitable machine readable medium that enables the processor to perform the control and processing. The term processor as used in this application is intended to include such devices as field programmable gate arrays (FPGAs). The machine readable medium may include ROMs, EPROMs, EAROMs, Flash Memories and Optical disks. As noted above, the processing may be done downhole or at the surface, by using one or more processors. In addition, results of the processing, such as an image of a resistivity property, can be stored on a suitable medium. 
         [0025]    While the foregoing disclosure is directed to the one mode embodiments of the disclosure, various modifications will be apparent to those skilled in the art. It is intended that all variations be embraced by the foregoing disclosure.