Injecting pure N2 and CO2 to coal for enhanced coalbed methane: Experimental observations and numerical simulation

Enhanced coalbed methane (ECBM) core flooding experiments are a direct way to observe the gas displacement process, the competitive adsorption and the effect of coal swelling and shrinkage on coal permeability. This study reports two ECBM experiments. In the first experiment, pure N-2 is injected (N-2-ECBM) to a coal sample saturated with CH4 while, in the second experiment, pure CO2 is injected (CO2-ECBM) to the same coal sample cleaned and resaturated with CH4. We record the volumes and composition of the effluent gas with respect to time. Then the gas rate and gas composition are history matched using a commercial reservoir simulator. The results show that the breakthrough of N-2 occurs earlier than CO2 breakthrough (after approximately 0.1 day of injection compared to 0.43 day). The recovery factor of CH4 is 71% for the N-2-ECBM and 86% for the CO2-ECBM at a 10%-molar percentage of CH4 in the produced gas stream. The N-2 injection causes moderate increases in coal permeability whereas the injection of CO2 reduces coal permeability significantly. The maximum strain of CO2 injection is higher at the initial stage of CO2 injection but decreases after several days of injection. The extended Langmuir adsorption model predicts the compositional adsorption amounts of N-2 and CH4 better for the N-2-ECBM than for the CO2-ECBM. A co-optimisation concept is presented to analyse the coupling of ECBM with CO2 storage which shows that early times CO2 storage efficiency is higher than CH4 recovery efficiency. Later CO2 storage efficiency decreases due to CO2 production and CH4 recovery dominates the co-optimisation. (C) 2013 Elsevier B.V. All rights reserved.