Method of processing data from a dual detector LWD density logging instrument coupled with an acoustic standoff measurement
A logging-while-drilling density sensor includes a gamma ray source and at least two NaI detectors spaced apart from the source for determining measurements indicative of the formation density. An acoustic caliper is used for making standoff measurements of the NaI detectors. Measurements made by the detectors are partitioned into standoff bins. An adaptive spine and rib method uses the standoff measurements to obtain density estimates. The method of the invention may also be used with neutron porosity logging devices.
1. A method of logging of an earth formation comprising:
(a) conveying a logging tool into a borehole in the earth formation and rotating the tool therein;
(b) making a plurality of long spaced (LS) and short spaced (SS) measurements of a parameter of interest of the earth formation using nuclear sensors on the logging tool over a time interval during continuing rotation of the tool;
(c) determining a standoff associated with each of said LS and SS measurements; and
(d) using an adaptive spine and rib method for determining from said plurality of LS and SS measurements and their associated standoffs, an estimate of the parameter of interest;
wherein the adaptive spine and rib method further comprises:
(e) defining a plurality of standoff bins,
(f) determining a first estimate of density at zero standoff, and
(g) representing compensated density of each said standoff bin as a function of said first estimate and the standoffs of said plurality of standoff bins.
2. The method of claim 1 wherein determining said standoff further comprises using a caliper on the logging tool.
3. The method of claim 2 wherein said caliper is an acoustic caliper.
4. The method of claim 2 wherein said caliper comprises an acoustic sensor azimuthally aligned with said nuclear sensors.
5. The method of claim 1 wherein the parameter of interest comprises a bulk density of the earth formation.
6. The method of claim 1 wherein the parameter of interest comprises a porosity of the earth formation.
7. The method of claim 1 wherein said function is a polynomial function of said standoffs.
8. The method of claim 1 wherein determining said estimated bulk density comprises minimizing a statistic of error in said functional representation.
9. The method of claim 8 wherein said statistic of error is a Chi-squared (χ 2 ) function.
10. The method of claim 8 wherein minimizing said statistic of error further comprises a filtering of parameters of said functional representation.
11. The method of claim 10 wherein said filtering is an exponential filtering.
12. The method of claim 1 wherein said first estimate is obtained using a common spine and rib.
13. An apparatus for logging while drilling of a borehole in an earth formation comprising:
(a) a drill collar conveyed in the borehole;
(b) at least two nuclear sensors carried by the drill collar which make measurements of a parameter of interest of the earth formation;
(c) a caliper on the drill collar which measure a standoff of the at least two sensors relative to a wall of the borehole; and
(d) a processor which uses
(i) measurements made by the at least two nuclear sensors, and
(ii) an adaptive spine and rib method,
and determines therefrom an estimate of the parameter of interest corrected for standoff of the sensors;
wherein the processor uses the adaptive spine and rib method by further:
(e) defining a plurality of standoff bins,
(f) determining a first estimate of density at zero standoff, and
(g) representing a compensated density of each said standoff bin as a function of said first estimate and the standoffs of said plurality of standoff bins.
14. The apparatus of claim 13 wherein the at least two nuclear sensors are selected from the group consisting of (i) gamma ray detectors, and, (ii) neutron detectors.