Nitrogen is often injected into coalbed methane (CBM) reservoirs to enhance the recovery by maintaining reservoir pressure and elevating permeability. Its effectiveness as a stimulant relies on impacts to the pore structure and related adsorption capacity of the coal reservoir. We quantify these changes in high volatility bituminous coal from Xinjiang, China, in response to nitrogen injection. Changes in pore size distribution (PSD) were characterized by high-pressure mercury injection (HPMI), low-pressure nitrogen gas adsorption (LP-N2GA), and carbon dioxide gas adsorption (LP-CO2GA). Corresponding changes in post-treatment adsorption capacity were measured by low-field nuclear magnetic resonance (LF-NMR) and isothermal adsorption. The specific pore volume and surface area of macropores (>50 nm), mesopores (2-50 nm), and micropores (<2 nm) all increased following nitrogen injection, as did measured adsorption capacity. These observations are consistent with an increase in surface area and improved connection between macropores that were previously unconnected or poorly connected. Nitrogen was subsequently diffused into mesopores and micropores, and then adsorbed into the pore walls. Nitrogen adsorption decreases surface energy and develops internal stress by differential deformation of the different macerals, that destroys the connectivity of mesopores and micropores. The effect of nitrogen injection on specific pore volume, surface area, and adsorption capacity also differs for different material and reservoir conditions. Higher mineral content and even distribution of macerals may improve the effect of PSD and adsorption capacity change. The presence of water hinders both flow and adsorption of nitrogen, leaving the mesopores unaffected by nitrogen injection. Water has little effect on changes in micropore architecture as a result of nitrogen injection due to capillary exclusion.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology