A Poromechanical Model for Coal Seams Injected with Carbon Dioxide: From an Isotherm of Adsorption to a Swelling of the Reservoir

S Nikoosokhan,P Dangla,M Vandamme

doi:10.2516/ogst/2012048

Abstract

Injecting carbon dioxide into deep unminable coal seams can enhance the amount of methane recovered from the seam. This process is known as CO2-Enhanced Coal Bed Methane production (CO2-ECBM). The seam is a porous medium whose porous system is made of cleats (small natural fractures) and of coal pores (whose radius can be as small as a few angström). During the injection process, the molecules of CO2 get adsorbed in the coal pores. Such an adsorption makes the coal swell, which, in the confined conditions that prevail underground, induces a closure of the cleat system of the coal bed reservoir and a loss of injectivity. In this work, we develop a poromechanical model which, starting from the knowledge of an adsorption isotherm and combined with reservoir simulations, enables to estimate the variations of injectivity of the coal bed reservoir over time during the process of injection.The model for the coal bed reservoir is based on poromechanical equations that explicitly take into account the effect of adsorption on the mechanical behavior of a microporous medium. We consider the coal bed reservoir as a dual porosity (cleats and coal porosity) medium, for which we derive a set of linear constitutive equations. The model requires as an input the adsorption isotherm on coal of the fluid considered. Reversely, the model provides a way to upscale an adsorption isotherm into a meaningful swelling of the coal bed reservoir at the macroscopic scale.The parameters of the model are calibrated on data on coal samples available in the literature. Reservoir simulations of an injection of carbon dioxide in a coal seam are performed with an in-house finite volume and element code. The variations of injection rate over time during the process of injection are obtained from the simulations. The effect of the compressibility of the coal matrix on those variations is discussed.

Highlights

Geological sequestration of carbon dioxide in coal seams is an environmentally appealing method to reduce the amount of greenhouse gas emissions
A Poromechanical Model for Coal Seams Injected with Carbon Dioxide: From an Isotherm of Adsorption to a Swelling of the Reservoir — Injecting carbon dioxide into deep unminable coal seams can enhance the amount of methane recovered from the seam
S Nikoosokhan et al / A Poromechanical Model for Coal Seams Injected with Carbon Dioxide: From an Isotherm of Adsorption 781 to a Swelling of the Reservoir where φc0 is the cleat porosity in the state of reference, pm(μm) is obtained from the state equation of the fluid considered, and the volumetric strain m of the coal matrix is obtained from the classical micro-macro relation [14]:

Summary

Introduction

Geological sequestration of carbon dioxide in coal seams is an environmentally appealing method to reduce the amount of greenhouse gas emissions. Producing methane from deep coal seams while sequestrating CO2 (a process known as CO2-ECBM (Enhanced Coal Bed Methane), in which CO2 is injected in one well and methane is withdrawn from another well) increases the economic viability of such a method. A decrease in the rate of injection is classically observed over time upon filling of a reservoir, in the case of the Allison pilot project this rate decreased faster than expected during the first months of injection of carbon dioxide (see Fig. 1). In order to mitigate this issue, one can inject carbon dioxide at a higher pressure or over longer periods of time but such alternatives

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Discussion

Conclusion