252Cf-based thermal-neutron cross-section gauge for interpretation of neutron logs for oil exploration

A newly developed neutron absorption cell measures the thermal-neutron absorption cross sections (Σ/sub w/) of formation brines encountered in logging operations. Thermal-neutron decay time logs with pulsed neutron logging systems are important tools of modern oil exploration technology. These logs yield information about oil saturation characteristics of the formation and oil-water contact location, provided the thermal-neutron cross-section behavior of the host formation is known. This cross section is determined to an important degree by that of the formation brine (Σ/sub w/). Presently, Σ/sub w/ is in most cases estimated from the chemically determined salinity of the brine, since actual measurements with a pulsed neutron logging tool are impractical, requiring several hundred gallons of the brine in a large tank. The chemical analyses are typically performed in a laboratory which is remote from the exploration site. With the portable neutron absorption cell, this measurement is made on site on a sample as small as four liters, with a precision of 0.1% in less than 5 minutes. The results of cell analyses on seventeen brine samples from oil fields in the United States and Canada are discussed. The cross sections measured have been compared with cross sections calculated from chemical and atomic absorption analyses. The agreement is within the experimental errors of the chemical analyses in all cases. Importantly, in all cases boron contributed a noticeable amount to the total absorption cross section. This boron contribution is ignored by conventional brine analyses precluding an accurate estimate of oil saturation, especially in the case of expensive log-inject-log measurements of residual oil preceding secondary or tertiary recovery