HIGH RESOLUTION RESISTIVITY EARTH IMAGER
An imaging tool made includes a pad whose curvature is chosen based on the expected range of borehole radius and the pad size so as to maintain the maximum standoff below a desired value. The curvature may be adjusted using fasteners.
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This application is a continuation of U.S. patent application Ser. No. 12/199,278 filed Aug. 27, 2008 the disclosure of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE DISCLOSURE1. Field of the Disclosure
This disclosure generally relates to exploration for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this disclosure relates to a method and apparatus having the flexibility to provide an image of a borehole wall for a wide range of borehole sizes.
2. Background of the Art
Electrical borehole logging is often used to provide images of an electrical property of boreholes. There are two categories of devices typically used as electrical logging devices for imaging boreholes. In the first category (galvanic devices), a measure electrode (current source or sink) is used in conjunction with a return electrode (such as the tool body). A current flows from a current source in the tool to a measure electrode through the earth formation. The current returns to the source via another electrode. The second category relates to inductive measuring tools in which an antenna within the electrical logging tool induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate antenna.
There are several modes of operation of a galvanic device. In one mode, the current at a current electrode is maintained constant and a voltage is measured between a pair of monitor electrodes. In another mode, the voltage of the measure electrode is fixed and the current flowing from the measure electrode is measured.
The galvanic devices are contact devices, in that the measure electrodes generally come in contact with the borehole wall during logging of the wellbore. Such devices are sensitive to the effects of borehole rugosity, borehole size, and the standoff of the pad carrying the measure electrodes. The present disclosure provides an improved electrical logging tool that reduces the effects of the pad offset.
SUMMARY OF THE DISCLOSUREOne embodiment of the disclosure provides a method of generating an image of a resistivity property of an earth formation. In one aspect, a method may include: determining a size of a borehole in which a resistivity imaging instrument is to be used; selecting a radius of curvature of a pad of the resistivity imaging instrument based on a pad size, the size of the borehole, and a selected value for a maximum standoff of the pad; conveying the resistivity imaging instrument into the borehole; using a plurality of sensors on the pad responsive to a formation resistivity to obtain measurements indicative of the resistivity property; and providing the image of the resistivity property of the earth formation using the measurements obtained by the plurality of sensors.
Another embodiment of the disclosure includes an apparatus for providing an image of a resistivity property of an earth formation. In one aspect, the apparatus may include a resistivity imaging instrument configured to be conveyed into a borehole, wherein the he resistivity imaging instrument includes a pad having a radius of curvature that is determined by using a pad size, a borehole size, and a selected value for a maximum standoff of the pad between the pad and inside of the borehole. The apparatus may further include at least one processor configured to: use an output of each of a plurality of sensors on the pad responsive to a resistivity property of the earth formation to provide an image of the resistivity property.
Another embodiment according the disclosure provides a computer-readable-medium for use with an apparatus for providing a resistivity image of an earth formation, wherein the apparatus comprises a resistivity imaging instrument configured to be conveyed into a borehole, the resistivity imaging instrument having a pad with a radius of curvature determined using a pad size, a borehole size, and a defined value for a maximum standoff of the pad; wherein the computer-readable-medium comprises instructions that enable at least one processor to: use an output of each of a plurality of sensors on the pad responsive to a resistivity property of the earth formation to provide an image of the resistivity property.
The present disclosure is best understood with reference to the accompanying figures in which like numerals generally refer to like elements and in which:
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The electronics associated with each pad may be the same as in prior art. Any suitable coupling arrangement including but not limited to those that can withstand the pressure and fluids downhole may be used. The present disclosure does envisage the possibility of using fewer than the full complement of electrodes on the pad. In such cases, the same azimuthal electrode spacing may correspond to a smaller azimuthal separation for a borehole of large radius than for a borehole of a smaller radius. Using a subset of the azimuthal electrodes provides flexibility in the azimuthal resolution of the produced image. Using a subset of the electrodes may also be used to reduce the effective azimuthal pad size when the standoff on the edges of the pad is excessive.
For a borehole of larger radius, there may be gaps in the full 360° image since each pad would provide a smaller azimuthal coverage. Any suitable interpolation technique may be used to fill in the gaps in the image.
Referring now to
The embodiments presented herein relate to galvanic devices in which the current in current electrodes is used for resistivity imaging. The principles and methods described above may also be used for galvanic devices in which voltages across pairs of electrodes are used. The principles and method described above are equally applicable to induction devices in which small antenna loops are used to measure the magnetic field produced by induced currents flowing in the formation. For this reason, the term sensor is intended to include both electrodes and sensing antennas.
The description above has been in the context of a wireline conveyed imaging pad. An implementation for a measurement-while-drilling is also possible. Drilling is carried out by a drillbit on a bottomhole assembly (BHA) conveyed on a drilling tubular. In particular, there are at least two situations in which the pad curvature may need to be adjusted to match the curvature of the borehole. The first situation where a pad mounted imaging device may be used is when drilling is done using an oversized drillbit. The use of an extendable pad has been discussed, for example, in U.S. Pat. No. 5,242,020 to Cobern, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. The second situation is when there is an axial separation between the drillbit and the imaging pad and the borehole has caved in or been washed out.
Implicit in the processing of the data is the use of a computer program implemented on a suitable machine readable medium that enables one or more processors to perform the acquisition 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.
While the foregoing disclosure is directed to certain embodiments of the disclosure, various modifications will be apparent to those skilled in the art. It is intended that all such variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.
Claims
1. A method of estimating a resistivity property of an earth formation, the method comprising:
- determining a size of a borehole in which a resistivity imaging instrument is to be used, the resistivity imaging instrument including a pad having a face configured to have an adjustable radius of curvature;
- adjusting the radius of curvature of the pad face using at least the determined borehole size;
- conveying the resistivity imaging instrument into the borehole;
- measuring a parameter indicative of the resistivity property using a plurality of sensors on the pad; and
- imaging the resistivity property using the measurements obtained by the plurality of sensors.
2. The method of claim 1, wherein the radius of curvature is adjusted by also using a selected value for a maximum standoff of the pad.
3. The method of claim 2 further comprising specifying the value of maximum standoff based on a simulation result.
4. The method of claim 1 wherein the plurality of sensors comprise electrodes and the measurements comprise currents.
5. The method of claim 1 wherein the plurality of sensors comprise antennas responsive to a magnetic field produced by a current in the formation.
6. The method of claim 1 further comprising conveying the resistivity imaging instrument into the borehole as part of a string of logging instruments on a wireline.
7. The method of claim 1 wherein the imaging is performed using an output of a subset of the plurality of sensors.
8. An apparatus for providing an image of a resistivity property of an earth formation, the apparatus comprising:
- a resistivity imaging instrument including: a pad, the pad including a face having an adjustable radius of curvature; a plurality of sensors responsive to a resistivity property of the earth formation; and
- at least one processor configured to use an output of each of plurality of sensors on the pad to provide an image of the resistivity property.
9. The apparatus of claim 8, wherein the adjustable radius of curvature of the pad reduces a stand off at one of: (i) a center of the pad, and (ii) a location away from the center of the pad.
10. The apparatus of claim 8, wherein the radius of curvature is adjusted at one of: (i) a single location on the pad face, and (ii) a plurality of locations on the pad face.
11. The apparatus of claim 8 wherein the plurality of sensors comprise electrodes and the output of the sensors comprise currents.
12. The apparatus of claim 8 wherein the plurality of sensors comprise antennas responsive to a magnetic field produced by a current in the formation.
13. The apparatus of claim 8 further comprising a wireline configured to convey the resistivity imaging instrument into the borehole.
14. The apparatus of claim 8 further wherein the pad is extendable from a bottomhole assembly conveyed on a drilling tubular.
15. A non-transitory computer-readable medium product accessible to a processor, the computer-readable medium including instructions which enable the processor to use measurements made by a resistivity imaging instrument to image a resistivity property of a formation, the resistivity imaging instrument including a pad having a face with an adjustable radius of curvature and a plurality of sensors responsive to the resistivity property of the earth formation.
16. The non-transitory computer-readable medium product of claim 15 further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EAROM, (iv) a flash memory, and (v) an optical disk.
Type: Application
Filed: Aug 2, 2012
Publication Date: Nov 29, 2012
Applicant: BAKER HUGHES INCORPORATED (Houston, TX)
Inventors: Stanislav W. Forgang (Houston, TX), Randy Gold (Houston, TX), Peter J. Nolan (Houston, TX), Carlos A. Yanzig (Houston, TX)
Application Number: 13/565,533
International Classification: G01V 3/18 (20060101); G01V 3/26 (20060101);