J. E. Hall, J. Tittman,
Schlumberger Well Services, Houston, Texas
H. N. Edmundson
Schlumberger-Doll Research, Ridgefield, Connecticut
INTRODUCTION
Traditionally, the resistivity of mud filtrate and formation water has been measurable at the wellsite. The familiar filtrate cell is one of the most consistently used pieces of equipment on the logging truck. Unless logs are being run in an oil-base mud, a meaningful log interpretation absolutely requires the systematic measurement of Rmf, not to mention, of course, Rm and Rmc.
This has not been the case with the running and interpretation of pulsed-neutron decay-time logs.
The true value of the capture cross-section of a fluid, meaning Smf, Sw or a mixture of both, is
frequently obscured by the need for borehole and diffusion corrections. It is usually obtained from
crossplots made in formations assumed to be clean and water-bearing. Perturbed greatly by small amounts of thermal neutron absorbers--boron, gadolinium, etc., the capture cross-section of the water in the invaded zone often remains a substantial uncertainty in pulsed-neutron log interpretation.
Previously, the only reliable method of determining the capture cross-section of a fluid required a
detailed knowledge of its chemical analysis. This requires great care and, in time, at least a week.
Concentrations of the strong absorbers are very small, a few parts per million typically. Thus, they
must be measured with great sensitivity. Because of the proven usefulness of quantitative
interpretation of pulsed-neutron logging, the need became clear for a simple measuring technique to overcome these complications.