Integrated petrophysical log evaluation for coalbed methane in the Hancheng area, China

摘要

In order to investigate coalbed physical parameters and gas bearing properties, based on coal core experiments, a nonlinear comprehensive model estimating coal properties and gas content is established. Three-dimensional numerical simulation of the dual laterolog response of a coalbed fracture is carried out, and a dual laterolog fast computation model for predicting fracture porosity is established. The results show that the study area mainly developed lean coal and meagre coal. The coalbed is rich in gas. Ash content, volatile dry ash-free basis, true density and fixed carbon show good correlation. The logging responses of the coalbed have distinct characteristics; however, the responses highly depend on coal composition and gas content. The simulated annealing differential evolution (SADE) neural network is adopted to predict coal composition and gas content. Numerical analysis of the dual laterolog responses of the coal fractures shows that as coal matrix resistivity increases, the dual laterolog apparent resistivity increases accordingly. For a coalbed with low-angle fractures, deep and shallow resistivity show little difference; however, for a coalbed with high-angle fractures, its dual laterolog shows an obvious positive difference. Fracture porosity and dual laterolog apparent conductivity presents a linear relationship. According to coal matrix resistivity distribution and fracture development, a fast computation model to predict fracture porosity is established based on the dual laterolog. Field data show that the predicted coal composition and gas content from the SADE algorithm agree with that of coal core analysis, and the calculated fracture porosity based on the dual laterolog matches the coal core's macroscopic description.