A NEW BOREHOLE-COMPENSATED NEUTRON-GAMMA POROSITY MEASUREMENT OPTIMIZED FOR CASED-HOLE FORMATION EVALUATION

摘要

Neutron-porosity measurement is important forquantifying reservoir assets and optimizingproduction. The neutron-gamma porositymeasurement is common for cased-hole logging.Since pulsed-neutron logging is the primarydownhole technology for current residualhydrocarbon monitoring, it is cost effective to addporosity. Another benefit is the deep penetrationfrom the 14MeV neutron, i.e., larger depth ofinvestigation; however, its measurement sensitivityand large borehole effect are well-knownlimitations. These limitations may lead to largepetrophysical uncertainties.This paper introduces a new neutron-gammaporosity algorithm and test results. The newalgorithm combines the inelastic count rates andcapture count rates from a standard pulsed-neutronlogging operation. The inelastic count rates arecorrected for the capture background to optimizethe measurement sensitivity. A detailed study wascompleted to optimize the capture count rates.Through this optimization, the capture count ratematches the net inelastic count rate for the boreholeeffects. The borehole-compensated ratio, betweenthe net inelastic count rate and the total capturecount rate, has been characterized for the newporosity algorithm.The study originated from developing acomprehensive understanding by integrating abroad set of underlying nuclear data, includingneutron slowing down length, scattering crosssection, and activated gamma ray yields.Comprehensive understanding of the particletransport led to the new borehole compensationconcept. This paper presents the detailed results to illustrate the borehole compensation improvementsby more than 20%. The parameters of study coveredprimary environmental factors, such as boreholesize, formation water salinity, and borehole fluidsalinity. Depth of investigation results are derivedfrom a large nuclear modeling database. Forpractical petrophysical applications, lithologycurves are shown for this new porositymeasurement. A detailed error analysis comparesresults from different porosity algorithms andconcludes the significant improvement of themeasurement sensitivity to be within 1.5 pu for theentire range from tight to porous rocks.In summary, this paper introduces a new neutrongammaporosity measurement. This newmeasurement has been optimized by integratingnuclear tool physics and a large set of data. Erroranalysis was conducted for tight and porous rocks.The improved 1.5-pu measurement sensitivity andintrinsic borehole compensation leads to significantuncertainty reduction for petrophysicalapplications.