Method of generating a deep resistivity image in LWD measurements
A pseudo-image is produced by combining deep-reading azimuthally sensitive resistivity measurements with azimuthally insensitive resistivity measurements made by a multiple propagation resistivity tool. This image is useful in reservoir navigation. Features on the azimuthally sensitive measurements such as saddle-points and mid-points are diagnostic of bed geometry relative to the borehole.
1. A method of evaluating an earth formation, the method comprising:
conveying a bottomhole assembly (BHA) into a borehole in the earth formation;
using the BHA for obtaining azimuthally sensitive propagation resistivity measurements; and
combining the azimuthally sensitive measurements with background measurements to provide a resistivity image of the earth formation.
2. The method of claim 1 wherein obtaining the background measurements further comprises using at least one of (i) a propagation resistivity tool, and (ii) a resistivity tool with tilted coils.
3. The method of claim 1 wherein obtaining the azimuthally sensitive propagation resistivity measurements further comprises applying a coordinate transformation.
4. The method of claim 1 wherein the image is indicative of a direction of a bed boundary relative to the borehole.
5. The method of claim 1 wherein combining the azimuthally sensitive measurements and the background measurements further comprises estimating a conductivity contrast at a bed boundary using the background measurements.
6. The method of claim 1 further comprising controlling a drilling direction of the BHA using the image.
7. The method of claim 6 wherein controlling the drilling direction of the BHA further comprises at least one of (i) staying below a shale roof, (ii) staying above an oil-water contact, (iii) identifying an azimuth and a distance of a resistivity boundary in the earth formation, (iv) avoiding a shale lens, and (v) avoiding a calcite stringer.
8. An apparatus configured to evaluate an earth formation, the apparatus comprising:
a bottomhole assembly (BHA) configured to be conveyed into a borehole in the earth formation;
a logging tool in the BHA configured to obtain azimuthally sensitive propagation resistivity measurements;
a processor configured to combine the azimuthally sensitive measurements with background measurements to provide a resistivity image of the earth formation.
9. The apparatus of claim 8 wherein the processor is further configured to use an output of at least one of (i) a propagation resistivity tool, and (ii) a resistivity logging tool to derive the background measurements.
10. The apparatus of claim 8 wherein the image is indicative of a direction of a bed boundary relative to the borehole.
11. The apparatus of claim 8 wherein the processor is further configured to combine the azimuthally sensitive measurements and the background measurements by further estimating a conductivity contrast at a bed boundary using the background measurements.
12. The apparatus of claim 8 wherein the processor is further configured to control a drilling direction of the BHA using the image.
13. The apparatus of claim 12 wherein the processor is further configured to control the drilling direction of the BHA further by at least one of (i) staying below a shale roof, (ii) staying above an oil-water contact, (iii) identifying an azimuth and a distance of a resistivity boundary in the earth formation, (iv) avoiding a shale lens, and (v) avoiding a calcite stringer.
14. A computer readable medium product having stored thereon instructions that when read by at least one processor cause the at least one processor to perform a method, the method comprising:
combining azimuthally sensitive resistivity measurements made by a logging tool conveyed on a bottomhole assembly in a borehole with background measurements to provide a resistivity image of the earth formation.
15. The medium of claim 14 further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EEPROM, (iv) a flash memory, and (v) and optical disk.