Is Accurate Gas Saturation Behind Pipe Feasible with PNC Measurements?

Abstract

Abstract Pulsed neutron measurements are commonly used to locate gas behind casing and quantify steam saturation, but do not always yield desired results. Several parameters are utilized to identify gas and one parameter, the thermal neutron capture cross section, Sigma, is used to compute steam saturation. In the paper we report a mixed experience in identifying gas with these techniques, across fields, tools and vendors. Some parameters have worked well in some cases but have performed poorly in others. The uncertainty in steam saturation, computed using Sigma, is greater than those previously reported elsewhere. Modeling offers insight into the mixed results. It appears that in some cases the PNC-derived Sigma may yield erroneous steam saturation for a variety of reasons, including uncertainties in the input parameters and possibly an inherent nonlinear transport effect that increases as steam saturation increases. An alternative approach based on PNC pseudo-porosity is explored. Calibration of cased-hole tools in gas reservoirs, generic and local, open-hole baseline data and core analysis of complex rocks are essential. Currently, these are either nonexistent or infrequent. Introduction Pulsed neutron capture (PNC) technique, initially used to compute water saturation in high- salinity reservoirs (for example, Dewan, et al., 1973) and in log-inject-log experiments to determine residual oil saturation, was extended to locate gas (Blackburn and Brimage, 1978). It is increasingly being utilized to locate gas in complex conditions and quantify steam saturation in steam floods. In addition, inelastic data, normally used to compute oil saturation, are being used to complement PNC techniques to detect gas. Recent applications in complex conditions include:identification of gas caps to optimize perforation decisions and reduce production of associated gas in West Africa (Badruzzaman, et al., 1997),monitoring steam-chest growth and estimation of steam saturation in steam floods in California and Indonesia (Badruzzaman, et al., 1998; Harness et al., 1998; Zalan, et al., 2003.),locating shallow gas hazards in Gulf of Thailand (Pathanakitchakarnjaroen, et al., 2005) andlocation of pay and identification of swept gas zones in Gulf of Mexico. The technique is being considered to monitor CO2 sequestration in Australia. In addition to PNC techniques, inelastic counts have been utilized as an independent validation of gas behind pipe. Success with pulsed neutron (PN) techniques, involving either the capture or inelastic interactions, has been mixed. Location of gas cap to reduce production of associated gas has generally been successful in Nigeria. Monitoring steam growth with PNC techniques has been successful in steam floods in California and Indonesia. Quantification of steam saturation has been very accurate in California while less so in Indonesia. Locating shallow gas hazards in the Gulf of Thailand has been ambiguous. Pay identification in Gulf of Mexico has been usually clear, but not always.