Paul W. Caton
Cities Service Company
Tulsa, Oklahoma
ABSTRACT
Borehole gravity meter (BHGM) data are used to obtain densities for meters of formation in comparison to the few centimeters sampled by conventional density logging tools. Precise BHGM density estimates can be obtained from numerous repeated measurements, but due to time considerations, the data are necessarily sparse. Existing analysis procedures identify data precision and errors from density and instrument drift plots; however, these approaches are cumbersome, and several factors affecting the measurements are not always obvious. A new presentation, the “reduced gravity plot,” has been developed which illustrates data precision and errors, instrument drift and tares, and the nonlinear effects due to temperature stabilization.
The plot can be constructed on site, during data collection, to evaluate survey and instrument performance before coming out of the hole.
Constructing a reduced plot is simple: by removing the large depth dependent gravity trend from the data, the measurements can be analyzed with knowledge of instrument mechanics and characteristics. Field applications of the reduced plots were used during two 1980 BHGM surveys, and older data have been reevaluated. The results indicated significant temperature influence on slim-tool BHGM measurements, and the effects due to magnetized sondes and operator reading errors were obvious. Additionally, two to three instrument tares seem common during BHGM surveys. These anomalous effects require adjustment before final densities are calculated. Fortunately, from reduced plot analyses of repeated measurements, anomalous instrument responses can be identified and subsequently removed.