Carbon dioxide sequestration is a promising technology for reducing anthropogenic greenhouse gas emissions while fossil fuels are still being used. The costs associated with CO2 sequestration are often high; however, in certain circumstances (e.g., enhanced oil recovery) these costs can be more than offset by the benefits of additional incremental hydrocarbon production. Primary production of coalbed methane is a well-developed technology, but secondary production, through the injection of CO2 or N2 has undergone relatively little study. Recent research suggests that carbon dioxide preferentially sorbs to coal, displacing methane, making CO2-enhanced coalbed methane production an ideal candidate for CO2 sequestration.

We use PSU-COALCOMP, a dual-porosity coalbed methane simulator, to model primary and secondary production of methane from coal, for a variety of coal properties and operational parameters. Our base well pattern consists of four horizontal production wells that form a square, with four smaller horizontal producers/injectors at the square's center. Primary production of methane and water is simulated until a specified reservoir pressure is reached, after which CO2 is injected in the center wells to displace methane, extending the reservoir's production of methane. Production continues until the CO2 concentration in the produced gas is too high. By modifying coal properties, such as permeability, porosity, degree of anisotropy, and sorption rates, we have approximated different types of coals. By varying operational parameters, such as injector length, injection well pressure, time to injection, and production well pressure, we can evaluate different production schemes to determine an optimum for each coal type.

Any optimization requires considering a tradeoff between total methane produced (or CO2 sequestered) and the rate of methane production. Values of aggregate methane production and methane production rate are presented for multiple coal types and different operational designs.

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