Abstract

Total worldwide CBM in-place reserves estimates are between 3500 Tcf and 9500 Tcf. Unminable coal beds have been recommended as good CO2 sequestration sites as the world prepares to sequester large amounts of greenhouse gases. In the U.S., these coal seams have the capacity to adsorb and sequester roughly 50 years of CO2 emissions from all the U.S. coal-fired power plants at today's output rates. The amount and type of gas adsorbed in coal has a strong impact on the permeability of the coal seam. An improved mixed gas adsorption isotherm model based on the extended-Langmuir theory is discussed and is applied to mixed gas sorption-induced strain based on pure gas strain data and a parameter accounting for gas-gas interactions that is independent of the coal substrate. Advantages and disadvantages of using freestanding versus constrained samples for sorption-induced strain measurements are also discussed. A permeability equation used to model laboratory was found to be very accurate when sorption-induced strain was small, but less accurate with higher strain gases.

Introduction

Coal bed reservoirs in the U.S. contain an estimated 141 Tcf of recoverable natural gas, which accounts for 10% of the total recoverable natural gas reserves in the U.S. (Nelson 1999). Coalbed methane (CBM) production also accounted for 10% of the total U.S. natural gas production in 2002 (Leach 2002). Total worldwide CBM in-place reserves estimates are between 3500 Tcf and 9500 Tcf (Olsen et al. 2003).

There is a strong relationship between CBM operations and CO2 sequestration. Methane production from coal beds can be enhanced by injection of other gases to displace or strip methane from the coal and accelerate its production at higher reservoir pressures in a process called enhanced coal bed methane (ECBM). The microporosity of coal can adsorb up to 10 times more CO2 than methane on a molecular basis, while it can adsorb roughly half as much nitrogen as methane (Reeves 2003). Because coal is such a strong adsorber of CO2, unminable coal beds have been recommended as good CO2 sequestration sites as the world prepares to sequester large amounts of greenhouse gases (mainly CO2) to limit their potential effects on climate change (Folger 2007). Coal seams in the U.S. that are either too deep or too thin to be economically mined have the capacity to adsorb and sequester roughly 50 years of CO2 emissions from all the U.S. coal-fired power plants at today's output rates of 90 Gt of CO2 per year (U.S. DOE 2007; Reeves 2003).

Unlike conventional gas reservoirs, methane in coal is not stored as free gas but rather as sorbed gas, at near-liquid densities on the internal surface area of the microporous coal (Puri and Yee 1990). As gas molecules are adsorbed onto adsorption sites within the coal matrix, the matrix blocks swell; and as gas is desorbed, the coal matrix shrinks. The more tightly the gas molecules are packed onto the adsorption sites, the larger the swelling. Chikatamarla et al. (2004) determined that at a given pressure, if a coal matrix block is saturated with a high-boiling-point gas, such as CO2, the volume of the coal block will be larger than when saturated with a low-boiling-point gas, such as helium.

The amount and type of gas adsorbed in coal has a strong impact on the permeability of the coal seam. Permeability of a coal bed is a function of cleat spacing and width (Robertson and Christiansen 2006). The swelling and shrinkage that occurs within the coal matrix blocks as different gases are injected into coal beds to displace methane or as gas reservoir pressure changes can cause a significant change in cleat width and a corresponding change in permeability. Being able to accurately predict permeability changes in coal beds as gases are produced or injected is important for designing surface facilities, predicting production and injection rates, and anticipating economic profitability of operations.

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