EXPERIMENTAL SIMULATION OF THE GEOCHEMICAL EFFECTS WITH CO₂ INJECTION AND CH₄ ENHANCED RECOVERY FOR DEEP COAL SEAMS

CO₂ storage and CH₄ enhanced recovery(CO₂-ECBM) of deep coal seams is considered as a prospective application technique in the near future. With super critical CO₂ being injected into coals via surface wells, a series of complicated interactions will take place between fluid of CO₂ - H₂O and coal hosting CH₄, that result in stress and strain effect characterized by coal matrix swelling, CO₂ replacing CH₄ since preferential absorption, and geochemical effect induced by geochemical reaction between H₂CO₃ and minerals in coal and featured by dissolution and eluviation. Both influence on dynamic change of permeability and other properties of coal seams and are dominant mechanisms deciding on efficiency of CO₂-ECBM. Compared with the former, research on chemical effect is limited.

On the experimental platform developed by our own, a device of geochemical effect simulation, in case of coal samples from Qinshui basin, the most significant one for coalbed methane development in China, the profound research on geochemical effect has been done by simulating CO₂-ECBM processes under the stratum conditions recently. It focused on SCCO₂-H₂O-coal rock reaction processes, micro-mechanism, mineral variation and element migration, porosity and permeability changing, mechanical properties changing, etc. The authors conclude the following: Chemical effect exerts substantial influence on CO₂-ECBM by modifying mineral matter occurrence, pore structure and permeability and strength of coal; mineral modification and element migration are in light of the certain regulation, depending on mineral categories and occurring forms, element nature and distribution and relationship between minerals and elements; porosity and permeability of coal seam will be improved to some extent, corresponding to mineral matter being dissolved out; the mechanical strength of coal rock decreases, while its fragility get better; chemical effect is of long term and positive for CO₂-ECBM efficiency.