Coalbed methane （CBM） is an important type of unconventional gas. Commercial development of CBM in America has been very successful since the 1980s. The CBM industry in Australia and Canada has developed rapidly during the last decade. Commercial development of CBM in China started in the 1990s, and has made great progress. The geological theory of CBM in China has achieved great advancement in genesis, occurrence and accumulation. On the aspect of CBM genesis, five CBM genetic types （primary biogenic gas, secondary biogenic gas, thermal degradation gas, pyrolysis gas and mixed gas） are identified by studying the geochemical characteristics of CBM, and a tracing indicator system is established. The discovery of secondary biogenic gas in medium-high rank coal reservoirs has widened the potential of CBM resources. On the aspect of CBM occurrence, the gas adsorption regulation under combined action of temperature and pressure is revealed by conducting adsorption experiments of different coal ranks under varying temperature and pressure conditions. Besides, by applying the adsorption potential theory in CBM research, the adsorption model under combined action of temperature and pressure is established. The new model can predict CBM resources accurately, and overcome the limitation of the traditional Langmuir model which uses just a single factor to describe the adsorption characteristics of deep buried coal. On the aspect of CBM accumulation, it is proposed that there are three evolutionary stages during CBM accumulation, including gas generation and adsorption, unsaturated gas adsorption, gas desorption-diffusion and preservation. Controlled by tectonic evolution, hydrodynamics and sealing conditions, CBM tends to be regionally enriched in synclines. Advances in geological theory of CBM in China can not only improve understanding of natural gas, but also provide new ideas for further exploration of CBM.
Song YanLiu ShaoboZhang QunTao MingxinZhao MengjunHong Feng
Objective More than 60%of China＇s coalbed methane（CBM）resources are distributed in medium-high rank coalbearing basins.Exploration practices have confirmed that the medium-high rank CBM enrichment and high yield zones are mainly distributed in the slope areas of coal basins and are significantly jointly controlled by gas content and permeability of coal seams.
The pool-forming mechanism of coalbed methane has its own characteristics.In this paper, based on studies on the typical coal-bearing basins in China,it is pointed out that the reservoir formation of medium- to high-rank coalbed methane has experienced three critical stages：the coalbed methane generation and adsorption stage,the coalbed adsorption capacity enhancement stage,and the coalbed methane desorption-diffusion and preservation stage.The regional tectonic evolution, hydrodynamic conditions and sealing conditions play important roles in the stage of coalbed methane desorption-diffusion and preservation.Medium- to high-rank coalbed methane has three types of enrichment models,that is,the most favorable,the relatively favorable,and the unfavorable enrichment models.
SONG YanZHAO MengjunHONG FengLIU ShaoboQIN ShengfeiWANG Hongyan
