Occurrence phase and gas-bearing evolution of deep coalbed methane

Deep coalbed methane has achieved a significant breakthrough in daily production of a single well and large-scale commercial development, demonstrating the enormous resource potential. The deep coalbed methane became an important supplement to China’s natural gas production. However, the geological and development characteristics of deep coalbed methane under the influence of high temperature, high pressure, and high crustal stress are significantly different from those under shallow burial conditions. At present, there is still a lack of systematic research on key fundamental issues such as the evolution of deep coalbed methane reservoirs, enrichment modes, transformation mechanisms, and evaluation of development effects. Based on the exploration and development data of Carboniferous-Permian deep coalbed methane in the North China, this study focuses on exploring the occurrence and evolution characteristics of deep coalbed methane, revealing the coupling relationship between the gas content and burial depth, identification methods for occurrence phase states, and their evolutionary patterns. The main scientific issues and future development trends of deep coalbed methane from geological exploration to development evaluation were discussed in order to promote new breakthroughs in coalbed methane industry. The results showed that: The gas content testing based on pressure-preserved coring and wire-line coring and characteristics of production curve are important means to identify the phase state of deep CBM. Among them, the isothermal adsorption experiment combined with wire-line coring is effective methods. The gas content and occurrence state of deep coal seams are jointly controlled by internal factors (such as coal rank, maceral and pore system) and external factors (such as temperature, pressure and preservation conditions). The burial depth is the basis for the enrichment of free gas in deep coal seams, and good preservation conditions are the key to the enrichment of free gas in deep coal seams. The middle-high rank coal in the Carboniferous-Permian in North China has undergone a complex burial evolution process. Four types of evolution models for the occurrence of coalbed methane, considering temperature, pressure, and tectonic activities, have been proposed. The differential structural evolution experienced by coalbed methane reservoirs is an important factor that leads to significant differences in the gas bearing characteristics of deep coal seams in different blocks or even within the same block. The evolution pattern of deep burial and weak structural transformation intensity is conducive to the enrichment of free gas and is a favorable zone for deep coalbed methane exploration and development. Deep coalbed methane has both “conventional” and “unconventional” attributes. Currently, there is still a weak understanding of the fundamental scientific issues related to the accumulation, enrichment, transformation, and development of deep coalbed methane. We should focus on solving the key fundamental problems of the evolution and developed effects of deep coalbed methane, in order to promote new breakthroughs in China’s coalbed methane industry.