Experimental research on supercritical CO2 enhanced coalbed methane recovery in different rank coals

Due to the difference in geological genesis and pore fissure structure,the permeability and the effect of enhanced coal bed methane recovery in different rank coals are variable. To study the effect of supercritical CO2 en- hanced CH4 recovery in different rank coals,some experiments were conducted in weakly caking coal,gas coal,1 / 3 coking coal and anthracite,respectively. Four coal specimens in the dimensions of 100 mm×100 mm×200 mm were in- vestigated at the temperature of 50 ℃ and the confining stress of 25 MPa. Supercritical CO2 injection pressure was maintained at 10. 0 MPa. The experimental results showed:① The pores configuration and porosity are different for the four different rank coals. The main pore type in the weakly caking coal is ink-bottle pore,anthracite is narrow -slit pore,while the pore types of gas coal and 1 / 3 coking coal are wedge pore or parallel plate pore. Anthracite and weakly caking coal have relatively high pore surface areas of 259. 651 0 m2 / g and 154. 066 9 m2 / g respectively,while gas coal and 1 / 3 coking coal have relatively low pore surface areas of 71. 235 9 m2 / g and 41. 420 1 m2 / g respective- ly. ② Coal permeability was affected by many factors such as coal structure,coal rank,fracture, coal composition, etc. ,due to ore-forming geological environment and tectonic activity. Permeabilities of four tested coal samples de- creased with the coal rank,and the permeabilities in weakly caking coal,gas coal,1 / 3 coking coal and anthracite were 4． 58×10-18 ,2． 75×10-18 ,0. 91×10-18 and 0. 05×10-18 m2 ,respectively at 10 MPa injection pressure. CO2 injection a- mounts decreased with coal rank,which were 18. 13,6. 45,5. 01 and 0. 78 mol in the weakly caking coal,gas coal,1 / 3 coking coal and anthracite,respectively. ③ The results of CH4 recovery rate and CO2 sequestrated volume show that gas coal is better than 1 / 3 coking coal,weakly caking coal and anthracite. The effect of supercritical CO2 enhanced CH4 recovery of gas coal and 1 / 3 coking coal with wedge pore or parallel plate pore,low surface area and medium per- meability is relatively good,which is a comprehensive expression of pore configuration,porosity and permeability in dif- ferent rank coals.