Investigation on log responses of bulk density and thermal neutrons in coalbed with different ranks

Abstract

Density and neutron logs play an important role in the exploration of coalbed methane (CBM) reservoirs. However, the study of these two log responses of coalbeds is deficient. Based on laboratory data in the published literature and field logs of CBM reservoirs from several coal basins in China, this paper focuses on acquiring and analyzing variations in bulk density and thermal neutrons of coal with different ranks. Two new methods are introduced to correct the effect of ash on field logs to obtain the log values of ash-free coal. The corrected coalbed density logs are in accordance with the density of coal samples in the laboratory. Then, hydrogen indices of coals with different ranks are simulated, and the corrected neutron logs are well verified by simulated values. After obtaining the variations of density and neutrons of coalbeds with different ranks, the behaviors of density and neutron logs are analyzed and discussed. Based on the laboratory data and the simulated and corrected field logs, the specific values of density and neutron porosity for ash-free coals with higher ranks are listed. Furthermore, a crossplot of density versus neutron logs is proposed to identify coal ranks, which has been successfully applied to various CBM fields of several basins in China.