Studies on main factor coupling and its control on coalbed methane accumulation in the Qinshui Basin

摘要

There are many factors controlling coalbed methane (CBM) accumulation. So, it is fundamental for CBM exploration and development to identify these factors and investigate their control mechanism. Through statistical analysis of experimental and field test data, the basin tectonic evolution, sedimentary environments and hydrodynamic conditions are comprehensively analyzed, and the gas control mechanism of three main factors (structure, sedimentation and hydrodynamic force) is discussed. Results show that the tectonic evolutions complicated the process of CBM reservoir formation. Multi-episode tectonic stress fields with different features reform the physical properties of coal seams that make areas with high structural principal stress difference convert to high permeability reservoir areas. The mudsitone cap rock develops well in shallow water delta system. The coal seams are thick and distributed stably in deltaic plain swamp, and their abundant vitrinite, micropores and strong adsorption capacity are favorable for CBM accumulation. Appropriate hydrodynamic condition may carry secondary biological gas into the gas reservoir to supply gas source, and it is also the main force of methane carbon-isotope fractionation during CBM drainage and production. The methane carbon-isotope fractionation is available to identify the yield stability of CBM well.