Abstract
Although coal swelling/shrinking during coal seam gas extraction has been studied for decades, its impacts on the evolution of permeability are still not well understood. This has long been recognized, but no satisfactory solutions have been found. In previous studies, it is normally assumed that the matrix swelling/shrinking strain can be split between the fracture and the bulk coal and that the splitting coefficient remains unchanged during gas sorption. In this study, we defined the fracture strain as a function of permeability change ratio and back-calculated the fracture strains at different states. In the equilibrium state, the gas pressure is steady within the coal; in the non-equilibrium state, the gas pressure changes with time. For equilibrium states, the back-calculated fracture strains are extremely large and may be physically impossible in some case. For non-equilibrium states, two experiments were conducted: one for a natural coal sample and the other for a reconstructed one. For the fractured coal, the evolution of permeability is primarily controlled by the transition of coal fracture strain or permeability from local matrix swelling effect to global effect. For the reconstituted coal, the evolution of pore strain or permeability is primarily controlled by the global effect.
Highlights
Coal permeability has been widely studied due to its vital importance for the effective extraction of coal seam gas
Understanding how to quantitatively describe this influence is crucial for the evaluation of both primary gas production from coal reservoirs and for CO2-enhanced coalbed methane recovery (ECBM) (Bergen et al 2009b)
Total fracture strain εf and total bulk strain εb can be decomposed into two parts (Connell et al 2010; Pan and Connell 2012); one that is caused by the mechanistic tractions and the other by the gas sorption, so Eq (2) can be written as f − f0 = Δ mf + Δ sf and b − b0 = Δ mb + Δ sb (3)
Summary
Coal permeability has been widely studied due to its vital importance for the effective extraction of coal seam gas. Seidle and Huitt (1995) assumed matrix swelling and shrinkage are proportional to the amount of gas adsorbed on the coal matrix, not the gas pressure, and that in situ coal deposits can be represented by a matchstick geometry Under this assumption, a permeability model as a function of sorption-induced volumetric strain was developed, in which considered that a change in the length of a matrix block (resulting from swelling or shrinkage) causes an equal, but opposite change in the fracture aperture. Most of the samples at the laboratory scale are under stress-controlled conditions rather than under constant volume or uniaxial strain conditions (Shi et al 2018) These assumptions may overestimate the effect of gas sorption on permeability under stress control conditions where the coal sample can expand outward (Robertson and Christiansen 2007; Zang et al 2015; Chen et al 2012; Liu and Rutqvist 2010). The back-calculated strains were analyzed both at the equilibrium and non-equilibrium states and discussed their implication on the validity of coal permeability models
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