Experimental study on enhancing coal-bed methane production by wettability alteration to gas wetness

The development of coalbed methane (CBM) has been challenging as many coal formations have very low permeabilities, which causes the low CBM production. One of the main stimulation techniques to enhance CBM production is large scale fracturing. The high leak-off and the low flow back percentage of the injected fracturing fluids result in poor CBM production. Gas wettability alteration (GWA) approach has been demonstrated successful in sandstone reservoirs. However, few microscopic study or experiments have been conducted in coal samples for GWA in order to enhance CBM production. Additionally, most of the studies in the literature focused mainly on surfactants that reduce the capillary pressure of coal through surface tension instead of wettability. In such cases, the incremental CBM recovery is limited. In this study, we conducted experiments with the fluorocarbon surfactants Perfluorooctyl Methacrylate Monomer-containing Polymethacrylate (PMP) and developed approaches to altering the wettability of coal samples to strong gas-wetness. ESEM (environment scanning electron microscope) observation was used to examine the microscopic mechanism of GWA. The changes in contact angles and spontaneous imbibition rates were used to judge the wettability alteration macroscopically. Experimental results demonstrated that the imbibed water decreased by 59.99%wt and gas-flooding water recovery was enhanced by about 20.31%wt after the wettability of the coal sample was altered from water-to gas-wetness. Furthermore, experimental results from both sandstone and coal samples were compared after gas wettability alteration. Although sandstone sample showed a greater water recovery than that of coal sample, the coal sample demonstrated a greater incremental enhanced recovery.