AN INNOVATIVE CASED-HOLE, OIL-SATURATIONMETHOD OF UTILIZING EXCESS CARBON ANALYSIS OF PULSED NEUTRON MEASUREMENTS IN A SILICICLASTIC CENOZOIC FORMATION,LOS LLANOS BASIN, COLOMBIA

摘要

Slim pulsed-neutron logging instruments areextensively used for fluid saturation monitoring incased-holes since the discovery of pulsed-neutronradioactive sources in the early 1950s. The sigmamethod of pulsed-neutron logging was used todetermine water saturation in high-salinity formationwater reservoirs. The carbon/oxygen (C/O) method ofpulsed-neutron logging was extensively used since themid-1970s in low- or mixed-salinity formation waterreservoirs. The C/O method provides salinityindependent,oil/water saturation measurement byanalyzing the ratio of gamma rays generated by theinelastic neutron scattering interaction of fast neutronswith carbon and oxygen present in the formation.However, slow logging speeds are required to achievethe desired statistical accuracy. In the recent past,natural- and neutron-induced gamma-ray spectroscopyinstruments were developed to obtain detailed lithologyand mineralogy of the formation in openhole or casedholeenvironments. These instruments, designed forobtaining lithology and mineralogy of the formation,are considerably larger (approximately 4.75” diameter)than conventional slim pulsed-neutron instruments(typically, 1.7” diameter) for fluid-saturationmonitoring and provide significantly higher elementalsignals, providing an opportunity to identify oil/watersaturation in an efficient manner.Natural- and neutron-induced gamma-ray spectroscopyanalysis helps to identify elements present in theformation, which define the lithology and mineralogyof the formation. It also identifies the amount of carbonpresent in the reservoir, including the formation and thefluid in the porosity. Total organic carbon (TOC)analysis assigns the portion of the measured carbonsignal to the mineralogy of the formation and identifiesexcess carbon, which is expected to be present in theformation porosity. The analysis is combined with anindependent source of porosity to identify the oilsaturation of the formation. Additional informationsuch as oil density and fraction of carbon in oil can helpto reduce uncertainty. Pulsed-neutron spectroscopyanalysis is impacted by the presence of casing andconcrete. For the characterization of attenuation andcontribution effects of casing and concrete on spectralgamma-ray measurements, proprietary correctionalgorithms are developed using a combination oflaboratory and modeled tool measurements standards.This paper presents the validity of this novel techniqueby comparing the results of excess carbon analysisbasedoil saturation with the conventional C/O methodbasedoil saturation in Cenozoic formation in the LosLlanos basin of Colombia. The results of these twoindependent methodologies matched very closely in thisheavy-oil high porosity sandstone reservoir. The basinis a low-salinity reservoir, due to which theconventional openhole resistivity based saturationanalysis or pulsed-neutron sigma measurement basedsaturation analysis are not effective.