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Worldwide concerns about global warming and possible contributions to it from anthropogenic carbon dioxide have become important during the past several years. Coal seams may make excellent candidates for CO2 sequestration; coal-seam sequestration could enhance methane production and improve sequestration economics. Reservoir-simulation computations are an important component of any engineering design before carbon dioxide is injected underground. We have performed such simulations for a hypothetical pilot-scale project in representative coal seams. In these simulations we assume four horizontal production wells that form a square, that is, two wells drilled at right angles to each other forming two sides of a square, with another pair of horizontal wells similarly drilled to form the other two sides. Four shorter horizontal wells are drilled from a vertical well at the center of the square, forming two straight lines orthogonal to each other. By modifying coal properties, especially sorption rate, 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 CO2 sequestered and the rate of methane production. Values of total CO2 sequestered and methane produced are presented for multiple coal types and different operational designs.

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Carbon dioxide sequestration is a promising technology for reducinganthropogenic 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 morethan offset by the benefits of additional incremental hydrocarbon production.Primary production of coalbed methane is a well-developed technology, butsecondary production, through the injection of CO2 or N2has undergone relatively little study. Recent research suggests that carbondioxide preferentially sorbs to coal, displacing methane, makingCO2-enhanced coalbed methane production an ideal candidate forCO2 sequestration. We use PSU-COALCOMP, a dual-porosity coalbed methane simulator, to modelprimary and secondary production of methane from coal, for a variety of coalproperties and operational parameters. Our base well pattern consists of fourhorizontal production wells that form a square, with four smaller horizontalproducers/injectors at the square's center. Primary production of methane andwater is simulated until a specified reservoir pressure is reached, after whichCO2 is injected in the center wells to displace methane, extendingthe reservoir's production of methane. Production continues until theCO2 concentration in the produced gas is too high. By modifying coalproperties, such as permeability, porosity, degree of anisotropy, and sorptionrates, we have approximated different types of coals. By varying operationalparameters, such as injector length, injection well pressure, time toinjection, and production well pressure, we can evaluate different productionschemes to determine an optimum for each coal type. Any optimization requires considering a tradeoff between total methaneproduced (or CO2 sequestered) and the rate of methane production.Values of aggregate methane production and methane production rate arepresented for multiple coal types and different operational designs. Introduction Concern about the quantity of anthropogenic greenhouse gas emissions hasgrown steadily over the past few decades because of their potential forcontributing to global warming. One of the most promising new technologies forreducing carbon dioxide emissions while allowing for continued fossil fuel useis carbon sequestration. Coal seam sequestration is an option with greatpotential because CO2 becomes chemically bound or sorbed to the coalsurface, reducing its mobility and decreasing the chances that it will escapeinto the atmosphere, and because CO2 displaces methane in the coalseam, making the sequestration more economically viable.