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Numerical simulation on gas reservoir identification using element gamma spectrum well logging
ZHANG Feng1,2, ZHAO Liang1, ZHANG Quanying1,2, LI Xianghui3, DI Yongzhou3
(1.School of Geosciences in China University of Petroleum(East China), Qingdao 266580, China;2.Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China;3.Isotope Research Institute of Henan Academy of Sciences Company Limited, Zhengzhou 450000, China)
Abstract:
Using Monte Carlo method, we build simulation models under different borehole and formation conditions to simulate the response of R (the ratio of inelastic gamma counts and capture gamma counts) and gas saturation.Meanwhile, the influence of R under different borehole and formation environment conditions are also simulated.This method is further applied to a gas well in Xinjiang Oil field to evaluate gas saturation. The results validate the effectiveness of this method. The simulation results show that both inelastic and capture gamma counts increase in the same porosity, while the value of R decreases. When formation porosity increases, inelastic gamma counts decrease while gas saturation is less than a certain value (about 35%). The trendis opposite when gas saturation is greater, capture gamma counts decrease while the value of R increases.Various factors, such as lithology, formation water salinity, borehole fluid types, borehole size and drilling mud invasion can affect the accuracy of R. Therefore, these factors should be corrected. The effect of the borehole fluid types is not significant. When formation porosity is confirmed, the higher of formation water salinity and the bigger of the borehole diameter are, the greater value of R becomes. In addition, R is larger in limestone formation than in sandstone formation. When the depth of mud invasion is less than the detection depth, the value of R increases with the increasing mud invasion depth. High salinity formation water, big borehole size and deep mud invasion are beneficial for gas saturation evaluation.
Key words:  gas saturation  element gamma spectrum well logging  counts ratio of inelastic and capture gamma ray  Monte Carlo simulation