Stress-strain-sorption behaviour of coal matrix material exposed to CO2

Abstract

Coal swells when it adsorbs carbon dioxide (CO2). The stress-strain behaviour associated with adsorption is of key importance in determining the feasibility of extracting methane (CH4) from coal via Enhanced Coalbed Methane production. ECBM involves injection of preferentially sorbing CO2 into the target coal seam, providing a potential means of geologically storing CO2, coupled with recovery of desorbing CH4. However, almost all field experiments performed to date, show major CO2 injectivity reductions, suggesting that a strong coupling exists between CO2 sorption, changes in the mechanical state of the coal matrix and changes in the transport properties of the system. Unfortunately, the fundamental physical processes controlling this coupling are very poorly understood. The present thesis aims at understanding the stress-strain-sorption behaviour of coal matrix material, with the aim to a) better understand ECBM field performance, and b) help advancing existing ECBM reservoir “simulators” towards predictive reservoir models. The approach adopted involves both theoretical developments and mechanical tests (conditions: T=40 ºC, P=0-100 MPa, σe=0-35 MPa). Several important observations were made. First, the adsorption of CO2 in the coal matrix gives rise to swelling. The total volumetric strain occurring under unconfined conditions can be realistically modelled as the sum of an adsorption-related expansion term and an elastic compression term. Second, effective in situ stresses in the range 25-35 MPa will directly reduce the sorption capacity, and associated swelling, of the coal matrix significantly. A general thermodynamic model for the effect of a 3-D stress state on adsorbed CO2 concentration was developed. “Self-stressing”, as a result of CO2 adsorption occurring under conditions of restricted or zero strain (i.e. fully constrained conditions), will more than double the expected in situ stresses under typical (E)CBM reservoir conditions. A constitutive equation was developed to describe the full coupling between stress state, total strain (i.e. combined strain of adsorption processes and poroelasticity) and sorption. Third, it was observed that microfractures form in coal due to exposure to CO2 under unconfined conditions. However, in situ stresses will likely prevent opening of such fractures, and will limit equilibration of the coal matrix with CO2. The findings of this study all lead to the conclusion that CO2 access to, and CO2 uptake by the coal matrix remains a major problem for ECBM operations, especially in the case of stiff, highly swelling coals situated at (unmineable) depth. Major improvements in access to the coal matrix by CO2 can probably only be achieved by creating space to accommodate coal swelling. The required removal of coal mass, or creation of void volume, may be achievable either by hydrofracturing and injection of solvents to remove either organic or mineral components, by performing active mining of the coal and/or the over- or underlying strata, or possibly by pore pressure depletion of porous sediments enclosing the target coal seams.