Abstract

Although the influence of gas sorption-induced coal deformation on porosity and permeability has been widely studied, these studies are all under the invariant total stress condition. According to the principle of effective stress, the induced coal deformation is determined by the change in effective stress, which can be replaced by the change in pore pressure, under the assumption of null change in total stress. This is why terms representing effective stress or total stress are absent in all of these existing permeability models. In our previous work (Zhang et al, 2008), this assumption was relaxed and a new porosity and permeability model was derived. The FE model was also applied to quantify the net change in permeability, the gas flow, and the resultant deformation in a coal seam. In this work, the general porosity and permeability model was modified to represent both the primary medium (coal matrix) and the secondary medium (fractures), and implemented into a fully coupled coal deformation, CO2 flow and transport in the matrix system, and CO2 flow and transport in the fracture system model. The novel dual-poroelastic model was applied to quantify the mechanical responses of coal seam to the CO2 injection under in situ stress conditions. The simulation results illustrate how the CO2 injectivity is controlled both by the competition between the effective stress and the gas transport induced volume change within the matrix system and by the dynamic interaction between the matrix system and the fracture system.

1. INTRODUCTION

With the growing international concern over the issue of global warming, geological sequestration of CO2 is a significant contender in the mix of a greenhouse mitigation options. Recently, CO2 sequestration in deep coal seams has attracted more and more attentions as a method of reducing the output of greenhouse gases to the atmosphere [1]. Coal seam is the natural fractured reservoir for gas storage. The micro-pores and pores in coal matrix are the main storage space for gas, while the micro-fractures, fissures, fractures and faults build up the main passages for gas seepage and migration. The sorption-induced strain of coal matrix can change the porosity and the permeability, and the storativity of coal seam changes accordingly. In addition, in situ stresses also influence the CO2 sequestration. Therefore, the CO2 sequestration in q deep coal seam is a complex dynamic problem associated with gas flow and coal deformation.

The fractured coal seam comprises both permeable fractures and matrix blocks. Gas flow in such media may represent an intermediate characteristic between fracture flow and interstitial flow. Since Barenblatt et al. [2] conducted a theoretical study of the dual-porosity system in 1960s, different dual-porosity models have been proposed [3-8]. These models built up the relationship of matrix and fracture. However, all of these previous models were developed primarily for the flow of slightly compressible liquids without sorption and not applicable to the flow of compressible fluids such as CO2 where gas sorption is the dominant mechanism.

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