A Quantitatively Determining Gas Saturation Method Using Pulsed-Neutron Element Logging in Tight Gas Reservoirs

Abstract

Tight sandstone and carbonate gas reservoirs are important types of unconventional oil and gas, and their exploration and development are of great significance to increase production and economic development. It is difficult to accurately determine gas saturation for tight gas reservoirs due to their low porosity and permeability, which differs from conventional reservoirs. In the past few decades, pulsed neutron logging technology is one of the most important means to evaluate gas saturation, especially the ratio of gamma-ray counts from pulsed neutron logging tool with long spacing detector was generally used to improve detection sensitivity for quantitatively evaluating gas saturation. However, there are complex formation minerals and pore fluid composition in tight sandstone and carbonate reservoirs, and then it will obviously affect gas saturation evaluation. In the previous study, we introduced a three-detector pulsed neutron consisting of a DT pulsed neutron generator, two He-3 thermal neutron detectors, and a LaBr3 detector located between the two thermal neutron detectors. The logging tool performs elemental measurement. In addition, the formation porosity was calculated according to the information of gamma ray and thermal neutron detected by the three detectors in previous research. In this paper, the thermal-neutron ratio of near to extra-far detector and the lithology factors from elemental yields based on the measuring information from three detector are also used to determine gas saturation and eliminate the influence of lithology. The SuperMC software is employed to build three-dimension model with the pulsed neutron tool and formation which contains different minerals, gas saturation and salinity to obtain the responses of thermal neutron and gamma-ray under complex formation conditions. In addition, the spectrum of the inelastic gamma and capture gamma are used to acquire the yields of elements, such as iron, silicon, potassium, aluminum, magnesium, calcium and carbon. The regression coefficients of different elements are obtained from the relationship between the ratio of thermal neutron count ratio and the ratio of elemental yield of the formation, which is combined with the information of thermal neutron count ratio and elemental yield to build a new interpretation model for gas saturation. The new model eliminates the influence of lithology and improves the accuracy of gas saturation determination. A complex formation containing quartz, calcite, plagioclase, chlorite and pyrite with 12% porosity and 50% gas saturation is evaluated by the conventional interpretation model with 0% gas saturation, while the interpretation model proposed in this paper evaluates 48% gas saturation, and the relative error is reduced from 100% to 4%. Finally, a field example of a tight gas reservoir from Shanxi verifies the accuracy of the new model of gas saturation.