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Aquifer Controls on Coalbed Methane Development in the Powder River Basin, Wyoming
... Only when the coal seam pressure drops to the critical desorption pressure, can the methane be desorbed and diffuse from matrix pores to fracture network. In the Wyodak Anderson coalbed (Onsager and Cox, 2000;Cox and Onsager, 2002) of Fort Union Formation in the Powder River Basin, U.S., the 15# coalbed in Shouyang-Yangquan of the Qinshui Basin, China, and other coal measure strata (Meng 2010), water bodies exist in the upper or lower coalbed, in addition to the coalbed cleat system. The silty or muddy aquitard with a certain thickness of the coalbed and the aquifer has a certain permeability. ...
... Based on the data statistics of the Powder River Basin in the United States, the vertical permeability of such aquitard is 0.003-0.03 md, suggesting a certain impermeable capacity (Onsager and Cox, 2000). According to the research on the leakage process of the underground water by Hantush and Jacob, et al., the model is based on the following assumptions (Hantush and Jacob, 1955a; Hantush 1955b): 1). ...
To analyze the effects of the leakage recharge of the aquifer on the initial dewatering of coalbed methane wells, the mathematical seepage model of water in the coalbed considering the aquifer leakage was established by using the leakage coefficient according to the unsteady seepage theory. The model was solved after Laplace transform and the Stehfest numerical reverse inversion was used to obtain the solution in right space. Then, the log-log type curves of pressure and pressure derivative were created with new combinations of parameters. Based on the natural seepage mechanism, the influence of aquifer leakage on curve shape was judged. It is found that the radial flow ends earlier as the leakage coefficient increases. Moreover, it was proposed to obtain reservoir permeability, skin factor, and leakage coefficient by using type curve matching. The type curves are useful for quantitatively evaluating the level of leakage, thereby guiding the adjustment of the following production system for CBM wells.
To estimate methane recovery from an enhanced coal bed at the field scale, it is important to understand CO2 movement. Since coal beds are generally connected with aquifers, CO2 movement is affected by aquifer position and strength. Under conditions of no aquifer, CO2 initially has a tendency to move to the top layer of the coal seam due to the buoyancy effect. However, the permeability of the upper layer is decreased due to the swelling effect. The viscosity flow effectively improves CO2 movement due to high permeability in the bottom layer. Because of the offsetting effects of viscosity and gravitational flow, the vertical sweep efficiency of injected CO2 is very effective. Thus, methane recovery is highly calculated. Under conditions of an upper aquifer, CO2 flow in the bottom layer decreased as the injected CO2 leaked to the aquifer. As the hydraulic connection between the overlying aquifer and the coal seam is strong, the vertical sweep efficiency is weakened due to the high gas leakage rate. Therefore, methane recovery decreases. However, under conditions in which an aquifer is at the bottom, methane recovery should be high because the gas-leakage rate is almost negligible. Under conditions with an aquifer edge, injected CO2 in the coal bed has an asymmetric flow because pressure is only supported in the flow direction of the aquifer. As the pressure is greater at the aquifer, the asymmetric flow is gradually strengthened by changes in the equilibrium caused by viscosity and gravity. Consequently, enhanced coalbed methane recovery has irregular efficiency, in part due to the CO2 movement. However, by suitably adjusting the CO2 injection site and production well perforation, total methane recovery can be made more efficient, despite producing an asymmetric flow of injected CO2. These findings clearly indicate that production or injection plans for enhanced coalbed methane must be designed to consider CO2 movement.
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