Abstract:
An apparatus for obtaining resistivity images of a borehole includes an array of measure electrodes separated from a pad or the body of the instrument by a guard electrode. The guard electrode is maintained at a slightly lower potential than the pad and the measure electrode is at an intermediate potential thereto. With this arrangement, the current from the guard electrode defocuses the measure current from the measure electrode as it enters the formation, and at greater distances, the current from the measure electrode is refocused by the effect of the current from the pad. This defocusing and refocusing defines a region of investigation away from the borehole wall that is relatively insensitive to borehole rugosity. Circumferential and vertical overlap may be obtained either by the arrangement of the measure electrodes, or by relying on the broadening of the measure beam as it enters the formation.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 60/175,585 filed on Jan. 11, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to explorations for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this invention relates to highly localized borehole investigations employing the introduction and measuring of individual focused survey currents injected toward the wall of a borehole with a tool moved along the borehole. 
     2. Background of the Art 
     Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. In an electrical investigation of a borehole, current from an electrode is introduced in the formation from a tool inside the borehole. There are two modes of operation: in one, the current at the measuring electrode is maintained constant and a voltage is measured while in the second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain constant the voltage measured at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. 
     Techniques for investigating the earth formation with arrays of measuring electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Pat. No. 685,727 to Mann et al. U.S. Patent No. 4,468,623 to Gianzero, and U.S. Pat. No. 5,502,686 to Dory et al.. The Baker patent proposed a plurality of electrodes, each of which was formed of buttons which are electrically joined by flexible wires with buttons and wires embedded in the surface of a collapsible tube. The Mann patent proposes an array of small electrode buttons either mounted on a tool or a pad and each of which introduces in sequence a separately measurable survey current for an electrical investigation of the earth formation. The electrode buttons are placed in a horizontal plane with circumferential spacings between electrodes and a device for sequentially exciting and measuring a survey current from the electrodes is described. 
     The Gianzero patent discloses tool mounted pads, each with a plurality of small measure electrodes from which individually measurable survey currents are injected toward the wall of the borehole. The measure electrodes are arranged in an array in which the measure electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole. The measure electrodes are made small to enable a detailed electrical investigation over a circumferentially contiguous segment of the borehole so as to obtain indications of the stratigraphy of the formation near the borehole wall as well as fractures and their orientations. In one technique, a spatially closed loop array of measure electrodes is provided around a central electrode with the array used to detect the spatial pattern of electrical energy injected by the central electrode. In another embodiment, a linear array of measure electrodes is provided to inject a flow of current into the formation over a circumferentially effectively contiguous segment of the borehole. Discrete portions of the flow of current are separably measurable so as to obtain a plurality of survey signals representative of the current density from the array and from which a detailed electrical picture of a circumferentially continuous segment of the borehole wall can be derived as the tool is moved along the borehole. In another form of an array of measure electrodes, they are arranged in a closed loop, such as a circle, to enable direct measurements of orientations of resistivity of anomalies 
     The Dory patent discloses the use of an acoustic sensor in combination with pad mounted electrodes, the use of the acoustic sensors making it possible to fill in the gaps in the image obtained by using pad mounted electrodes due to the fact that in large diameter boreholes, the pads will necessarily not provide a complete coverage of the borehole. 
     The prior art devices, being contact devices, are sensitive to the effects of borehole rugosity: the currents flowing from the electrodes depend upon good contact between the electrode and the borehole wall. If the borehole wall is irregular, the contact and the current from the electrodes is irregular, resulting in inaccurate imaging of the borehole. A second drawback is the relatively shallow depth of investigation caused by the use of measure electrodes at the same potential as the pad and the resulting divergence of the measure currents. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for obtaining resistivity images of a borehole. The apparatus includes an array of measure electrodes separated from a pad or the body of the instrument by guard electrodes. The guard electrode is maintained at a slightly lower potential than the pad and the measure electrode is at an intermediate potential thereto. With this arrangement, the current from the measure electrode initially diverges as it enters the formation, then converges (focuses) and then finally diverges again to define a depth of investigation. This arrangement makes it relatively insensitive to borehole rugosity. If required, circumferential and vertical overlap may be obtained either by the arrangement of the measure electrodes to provide the overlap, or by relying on the broadening of the measure beam in a region of investigation within the formation. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 (PRIOR ART) is a perspective and block diagram view of a borehole investigating tool with pads; 
     FIG. 2 illustrates an idealized current flow necessary for obtaining proper measurements using the tool of FIG. 1; 
     FIG. 3 illustrates the actual flow of current using the tool of FIG. 1; 
     FIG. 4 is a schematic cross section illustrating the arrangement of electrodes and the flow of currents in an embodiment of the present invention; 
     FIGS. 5A-5D are illustrations of arrangements of electrodes in various embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to gain a proper understanding of the present invention, reference is made to FIGS. 1-5. 
     FIG. 1 (Prior Art) shows a pad  30  and array  70  of ten circular measure electrodes  36  as closely spaced as possible in a multiple number of rows  74 ,  76 . The electrodes  36  are surrounded by insulator rings  44 . Measure electrodes  36  are mounted flush on pad  30  whose surface  42  is conductive. Insulators  44  electrically isolate measure electrodes  36  from the conductive pad surface  42  while in the operation of a pad  30  the electrical potential of the several measure electrodes  36  and conductive surface  42  is effectively the same. The currents flowing from each of the electrodes  36  is indicative of the resistivity of the formation in contact with the electrode. The pad  30  or the body of the instrument (not shown) acts as an effective electrode. 
     FIG. 2 shows a schematic cross section of a resistivity array device including a measure electrode. Shown is a device  100  in contact with a borehole wall  106  with a formation  102 . The device includes a pad of which portions  101   a ,  101   b  are shown, along with a measure electrode  103  and gaps  107  electrically separating the measure electrode  103  from the pad. For simplifying the illustration, the connections between a source of electrical current and the measure electrode  103  and the pad  101   a ,  101   b  are not shown. Also shown within the formation  102  are idealized current paths of electrical current  109   a ,  109   b  and  109   c  from the measure electrode  103  and current paths of the electrical current  111   a ,  111   b , . . .  111   f  from the pad. For the idealized situation shown the current flow is directly into the formation and the flow lines are parallel for a distance indicated by  105  after which the current paths start diverging. As those versed in the art would know, the current will ultimately return back to the source through a return electrode (not shown). The distance from the borehole wall  106  to  105  is the depth of investigation of the tool. The magnitude of the current flowing from the measure electrode  103  is indicative of the electrical conductivity of the formation adjacent to the measure electrode  103  up to the depth of investigation. In the prior art array resistivity devices, the currents from the individual measure electrodes are measured to give an indication of the conductivity of the formation adjacent to the electrodes. 
     Turning now to FIG. 3, the real situation is indicated. Due to the geometry of the pads, the conductive portion of the tool body in the vicinity of the pads and the geometry of the electrically isolating section of the tool between the pad section and the aforementioned return electrode, the electrical current from pad and the measure electrodes diverges quite rapidly. The is indicated by the paths  109   a ′,  109   b ′ and  109   c ′ of current from the measure electrode  103  and the rapid divergence of the currents  111   a ′. . .  111   f  from the pad. As a result of this, the current flowing from the electrode  103  depends upon the conductivity of the formation in a first region in contact with the measure electrode as well as portions of the formations surrounding this first region. This reduces the resolution of the instrument. In addition, the current paths will be effected by irregularities in the borehole wall as indicated by  106 ′. Both of these effects (the divergence and the effects of rugosity) can give erroneous indications of formation conductivity. 
     These two problems are addressed in the present invention by use of a beam focusing technique. This technique causes the measure beam (current from the measure electrode) to be defocused as it leaves the instrument and then refocus again approximately ¼ to ½ inch (.625 to 1.25 cm.) away from the pad carrying the measure electrode. This desensitizes the image to the effects of borehole wall rugosity. This is described below. 
     Turning now to FIG. 4, a schematic cross section of the measure electrode of the present invention is shown. For illustrative purposes, a single measure electrode  215  is shown. This measure electrode is flanked by a primary guard electrode  203   a ,  203   b  that separate the measure electrode  215  from the pad or body of the instrument  201   a ,  201   b . The measure electrode  215  is maintained at a voltage V that is higher than the voltage V−ε of the guard electrodes  203   a ,  203   b  but less than the voltage V+δ of the pad  201   a ,  201   b . In a typical application, the voltage V is 5 volts while the values of δ and ε are of the order of 500 μV. These are the relative voltages when electrical current is flowing out of the measure electrodes. 
     In an alternate embodiment of the present invention, the arrangement of the electrodes is unchanged but the current flows into the measure electrodes. In such an arrangement, the magnitudes of the voltages are the same as discussed above. This configuration is not discussed further but would be a straightforward variation of the discussion here on current flowing out of the measure electrodes. 
     The lower potential of the primary guard  203   a ,  203   b  causes the current from the measure electrode  215  to diverge as it leaves the measure electrode. The more divergent the beam, the less the sensitivity to that element of the length of the measure beam. As the measure beam progresses further into the formation, the higher potential of the pad  201   a ,  201   b  causes the beam from the measure electrode  215  and the guard  203   a ,  203   b  to be pinched in, thereby increasing the sensitivity from that portion of the measure beam. Since the greatest divergence of the measure beam is closest to the borehole wall, the beam is quite insensitive to the effects of borehole rugosity. At large distances from the borehole wall such as  205   b , the measure beam again diverges. The region between  205   a  and  205   b  includes the region of greatest sensitivity of the tool. The region of greatest sensitivity is roughly defined by the portion between  205   c  and  205   b  where the diameter of the measure beam is small, with the smallest value being attained at a distance between  205   c  and  205   b . By suitable adjustment of the electrical potentials, it is possible to obtain a measure beam having a diameter within the sensitive region that is larger than or smaller than the diameter of the measure electrodes. The electrodes and the pad are connected to sources of electrical current to maintain the desired voltages and, as in prior art devices, the current from the measure electrode is indicative of formation conductivity. 
     One advantage of the present invention over prior art devices is the ability to make accurate measurements in irregular boreholes. As may be seen in FIG. 4, there is no requirement of physical contact between the entire measure electrode and the formation: all that is necessary is good electrical contact, even through any intervening borehole fluid. Due to the defocusing of the beam near the measure electrode, the present device is relatively insensitive to the presence of borehole fluid between portions of the electrode and the formation. Similarly, the invention also functions properly when there is incomplete physical contact between the guard electrode and the formation. 
     In the context of the present invention, the word “pad” is used here only for convenience and the invention would work equally well if the body of the instrument is used instead of the pad for the purpose of carrying measure electrodes and/or injecting currents into the formation as described below. Those versed in the art would recognize that the pad in the present invention does not actually have to contact the borehole wall. The primary function of the pad is to focus the measure beam after the defocusing caused by the guard electrode. This can be accomplished by a tool body that is not in actual contact with the borehole wall almost as well as by a pad in contact with the borehole wall. Accordingly, use of the word “pad” hereinafter and particularly in the claims is intended to include the body of the instrument as well. 
     Various configurations of measure electrodes, guard electrodes and pads may be used in the invention. FIG. 5 a  illustrates an array of measure electrodes  315   a ,  315   b ,  315   c  . . . set within a substantially rectangular guard electrode  303  with gaps  307   a  (that contain insulating material therein). The guard electrode  303  is separated from the pad or body  301  by a substantially rectangular insulating gap  307   b . In one embodiment of the invention, the spacing between the measure electrodes is selected as in the Gianzero patent to provide overlap in azimuth and depth, i.e., the diameter D of the measure electrode is greater than the horizontal spacing d 1  of the electrodes  315   b ,  315   c  in adjacent rows and the vertical spacing d 2  between the rows of electrodes. In another embodiment of the invention, the electrodes do not have this azimuthal an and vertical overlap , but due to the broadening of the measure beam discussed above in reference to FIG. 4, overlap in azimuth and borehole depth of the region of investigation is obtained. 
     FIG. 5 b  depicts another arrangement of electrodes in the present invention. In this, the measure electrodes  415  are concentric with the guard electrodes  403 . Insulating gaps  407  and  419  are also indicated. As with the embodiment discussed above with reference to FIG. 5 a , the measure electrodes may or may not have overlap in azimuth and depth. When the measure electrodes themselves do not overlap, the broadening of the measure beam provides overlap of measurements at the depth of investigation. 
     FIG. 5 c  illustrates an arrangement of measure electrodes  515  separated from a guard electrode  519  by insulating gap  503 . The guard electrode  503  is, in turn, separated form the pad by insulating gap  519 . 
     FIG. 5 d  illustrates a configuration of the measure electrodes measure electrodes  615  are arranged in two groups with insulating gaps  607 . As in FIG. 5 c , the guard electrode  603  is separated from the pad by another gap  619 . 
     In another embodiment of the invention (not shown), a secondary guard electrode located between the pad and the guard electrode is used to provide the focusing of the beam in the region of investigation. 
     While the foregoing disclosure is directed to the preferred embodiments of the invention, 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.