Patent Publication Number: US-9423524-B2

Title: Oil-based mud imager with a line source

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application Ser. No. 61/321,554 filed on Apr. 7, 2010. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The disclosed disclosure is related to downhole well investigation methods and, in particular, to measuring a resistivity property of an earth formation from a borehole containing non-conductive mud. 
     2. Description of the Related Art 
     A big challenge in oil-based mud imaging tools using electrodes is to reduce or correct the standoff effects. This is due to the fact that changes in tool standoff from the borehole wall produce variations in the current in the electrodes that mask any current variation in the formation being measured. When the standoff is large, current flow between the electrodes also become important. Capacitive coupling has been used for conveying currents into the formation, but at the high frequencies needed for capacitive coupling, the capacitance between the electrodes becomes a serious problem. 
     The present disclosure addresses this problem using a line source for conveying current into the formation. 
     SUMMARY OF THE DISCLOSURE 
     One embodiment of the present disclosure is an apparatus configured to evaluate an earth formation. The apparatus includes: a carrier including a line source of electric current configured to be conveyed in a borehole; and at least one pair of electrodes proximate to a wall of the borehole, a difference in potential between a first one of the at least one pair of electrodes and a second one of the at least one pair of electrodes being indicative of a resistivity property of the earth formation. 
     Another embodiment of the disclosure is a method of evaluating an earth formation. The method includes: activating a line source of electric current conveyed on a carrier in a borehole; providing a first signal from a first one of at least one pair of electrodes proximate to a wall of the borehole indicative of a potential of the first one of the electrodes and a second signal from a second one of the at least one pair of electrodes indicative of a potential of the second one of the electrodes; and using a processor for estimating a resistivity property of the earth formation using the first signal and the second signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For detailed understanding of the present disclosure, references should be made to the following detailed description of an exemplary embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein: 
         FIG. 1  shows an exemplary imaging tool suitable for use with this disclosure suspended in a borehole; 
         FIG. 2 a    shows a detail of one embodiment of the present imaging tool suspended in a borehole; 
         FIG. 2 b    shows a detail of a pad of the imaging tool; 
         FIG. 3 a    shows an illustration of one embodiment of the present imaging tool suspended in a borehole with the line source on a pad; 
         FIG. 3 b    shows a detail of a pad of the imaging tool of  FIG. 3   a;    
         FIG. 3 c    shows an alternative embodiment of a pad of the imaging tool of  FIG. 3 a    having an additional line source; 
         FIGS. 4 a - d    shows different configurations of a line source including toroidal coils on a conductive member; 
         FIGS. 5 a -5 b    show an embodiment of the disclosure wherein only vertical measurements are made; and 
         FIG. 6  shows a modeled response of the tool configuration of  FIG. 5  as a function of bed thickness and tool standoff. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
       FIG. 1  shows an imaging tool  10  suspended in a borehole  12 , that penetrates earth formations such as  13 , from a suitable cable  14  that passes over a sheave  16  mounted on drilling rig  18 . By industry standard, the cable  14  includes a stress member and seven conductors for transmitting commands to the tool and for receiving data back from the tool as well as power for the tool. The tool  10  is raised and lowered by draw works  20 . Electronic module  22 , on the surface  23 , transmits the required operating commands downhole and in return, receives data back which may be recorded on an archival storage medium of any desired type for concurrent or later processing. The data may be transmitted in analog or digital form. Data processors such as a suitable computer  24 , may be provided for performing data analysis in the field in real time or the recorded data may be sent to a processing center or both for post processing of the data. Some of the data processing may also be done by a downhole computer. Novel aspects of the imaging tool of the present disclosure are discussed next. 
       FIG. 2 a    shows an exemplary imaging tool  200  of the present disclosure disposed in a borehole  201 . Three intervals of the formation  211 ,  213  and  215  are shown. The logging tool  200  includes a conductive member  203  that is shown with two toroidal coils  221 ,  223  spaced a distance ‘L’ apart. For an MWD implementation, the conductive member may be a drill pipe while for a wireline implementation, the conductive member may be a metal rod inside a non-conducting mandrel. For the purposes of the present disclosure, the logging tool  200  may be referred to as a carrier. The term “carrier” is also intended to include a bottomhole assembly conveyed on a drilling tubular for MWD implementation. 
     A sensor pad  207  shown in  FIG. 2 b    is positioned in proximity to a wall of the borehole. For the present disclosure, the term “proximate” includes “in contact with” as well as “close to.” The term “close to” in the context of the present disclosure is intended to mean that the sensor pad  207  is close enough to make measurements of formation resistivity as discussed further below. The sensor pad may be referred to as a contact member and includes an array of electrodes  209 . The array of electrodes may be a two-dimensional array as shown in  FIG. 2 b   . The sensor pad is coupled to the carrier by a coupling member such as extension rod  225 . 
     Of interest are potential differences indicated by ΔV 1 , ΔV 2 , ΔV 3  and ΔV 4  that may be measured between adjacent electrodes in the 2-d array of electrodes responsive to activation of the coil(s)  221 ,  223 . Those knowledgeable in the art and having benefit of the present disclosure would recognize that the arrangement of the coil(s)  221 ,  223  and the conductive member  203  forms a line source of current. The combination of the line source and a pair of electrodes may be viewed as a three terminal device. It should be noted that the current distribution of the line source as illustrated is not the same as that of an axially oriented transmitter, i.e., a dipole antenna. 
     From the measured potential differences, the following quantities indicative of the resistivity property of the earth formation may be calculated: 
     
       
         
           
             
               
                 
                   
                     
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       FIGS. 3 a -3 c    shows an alternate embodiment of the disclosure in which the line source is formed by a longitudinal metal rod with at least one toroidal coil  301  on the back of the sensor pad. The same quantities given by eqn. (1) can be measured. Due to the shorter length of the metal rod  301  compared to the rod  203 , the standoff effects are larger, but the azimuthal resolution is improved. In an optional embodiment, and additional line source (transverse)  301 ′ may be provided. When measurements are made responsive to activation of the longitudinal line source and the transverse source, it is possible to accurately determine the orientation of dipping beds and anisotropy. This is discussed in U.S. Pat. No. 7,365,545 to Itskovich et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. 
     Turning now to  FIGS. 4 a -4 d   , different line sources comprising a conductive member  203  and one or more toroidal coils  401  are illustrated. The more the number of toroidal coils, the better will be the line source. 
       FIGS. 5 a -5 b    show an embodiment of the disclosure wherein only vertical measurements are made. As shown in  FIG. 5 b   , two rows of electrodes are used. Consequently, only first differences in the vertical direction, denoted by V z  are made in contrast to the plurality of differences possible with the electrode configuration of  FIGS. 2 and 3 . 
     Modeling results with two receiver electrodes are used to obtain voltage difference and shown in  FIG. 6 . The spacing between two toroidal coils is 4 m. The pad and electrodes are expanded circumferentially to conform to a 2.5D forward modeling engine. A multiple layer model with different layer thickness from 0.5″ to 4″ (1.25 cm to 10 cm) is used to check the vertical resolution of the design. Also, different standoffs ranging from 0.125 in to 0.5 in. (3.125 mm to 12.7 mm) is used to check the standoff effects for the design. The results at 10 MHz are shown in  FIG. 6  and show a good resolution of bed thickness even for a standoff of 12.7 mm. 
     The present disclosure has been made with respect to a wireline implemented device. It may also be adapted for an MWD embodiment using as the carrier a bottomhole assembly conveyed on a drillstring or coiled tubing. It may also be implemented for use on a slickline. It should be noted that for an MWD application, the contact member may be a stabilizer or a steering rib. 
     The device may be used to measure any resistivity property of the earth formation. This includes resistivity, conductivity, permittivity and dielectric constant. 
     The operation of the transmitter and receivers may be controlled by the downhole processor and/or the surface processor. Implicit in the control and 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 machine readable medium may include ROMs, EPROMs, EAROMs, Flash Memories and Optical disks. 
     While the foregoing disclosure is directed to the specific embodiments of the disclosure, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.