MULTIDETECTOR PULSED-NEUTRON TECHNOLOGY FOR LOW-POROSITY RESERVOIR—INTERPRETATION METHODOLOGY

摘要

Formation evaluation through casing is becoming more prevalent with the availability of multidetector pulsed-neutron tools (MDPNTs) from all major service companies. Monte Carlo modeling is important for not only optimizing nuclear tool designs but also for providing completion-specific petrophysical interpretations. Laboratory measurements provide key benchmarks for Monte Carlo modeling for validating liquid-saturated rock as well as gas-saturated rock models.This paper presents a very-low-porosity gas saturation evaluation example for two limestone formations in Oklahoma. Data for saturation analysis were collected using a three-detector pulsed-neutron tool. The well was drilled through the following formations, beginning from the top depth: Union Valley limestone, Caney shale, Woodford shale, and Viola limestone. Union Valley limestone and Viola limestone are evaluated in this paper for gas saturation. Both formations have total porosity ranging from 1.5 to 2.5 pu. The total porosity used in the analysis is from the measurements obtained with nuclear magnetic resonance (NMR) tools.When evaluating gas saturation in very-low porosity formations, the dynamic range of measurement can be limited for differences between raw measurements for 100% gas and 100% water. To provide a quantitative saturation analysis in low-porosity rock with total porosities less than 3 pu, a service provider adapted field-specific optimizations to the gas-saturation evaluation workflow. This modification accounts simultaneously for formation and borehole variations from the input properties to the computer model. During qualitative analysis of raw data, one limestone formation indicated low or no gas, while the second indicated possible accumulation of gas. When the modified saturation workflow was applied, during quantitative analysis, the gas volume was calculated for both formations. In summary, the benchmark for accurately modeling gas responses provided important validation for the new interpretation workflow of MDPNTs. The detailed petrophysical analysis provided reasonable saturation results for rocks of similar mineral composition. A consistent petrophysical workflow was applied through the interpretation. This paper also recommends best practices through collaboration with operators for detailed reservoir knowledge.