Joint mining compatibility of superposed gas-bearing systems:A general geological problem for extraction of three natural gases and deep CBM in coal series

Three gases in coal series,including coalbed methane (CBM),tight sandstone gas and shale gas,are impor- tant unconventional gas resources,but their large scale mining has not been realized in China. Six basic geological characteristics of the“three gases”,on the one hand,provide a superior gas source and preservation condition,and,on the other hand,result in outstanding co-mining compatibility,which originates from the superposed coexistence of mul- tiple sets of fluid pressure systems and could not been resolved with conventional measures. Key geological controls of the compatibility among the superposed gas-bearing system (SGS) are represented as the compatibility from the fluid energy difference among the systems and from the difference of the mechanical properties,pore and permeability of va- rious reservoirs inside the system. It is considered that the sequence stratigraphic framework,fluid energy system and rock mechanics properties are three key factors affecting the compatibility. Foundation for the “three gases”co-explora- tion and co-mining is to understand profoundly the relevant geological controls,and the main approach for the optimiza- tion and innovation of co-mining technology is to expound deeply the geological dynamics during the joint mining, which need to take the compatibility as a constraint condition for the full release of the production capacity in SGS. To this end,the future investigation of the compatibility should be focused on some aspects,such as the high-resolution recognition of key strata and fluid energy system,the quantitative characterization of the compatibility,and the geolog- ical units and model of the “three gases” development.