Impact of Various Parameters on the Production of Coalbed Methane

Abstract

Coalbed-methane (CBM) reservoirs are naturally fractured formations, comprising both permeable fractures and matrix blocks. The interaction between fractures and matrix presents a great challenge for the forecast of CBM reservoir performance. In this work, a dual-permeability model was applied to study the parameter sensitivity on the CBM production, because the dual-permeability model incorporates not only the influence from matrix and fractures but also that between adjacent matrix blocks. The mass exchange between two systems is defined as a function of desorption time constant at the standard condition, coal matrix porosity, and the difference of gas pressure between two systems. Correspondingly, gas diffusivity in matrix is considered as a variable and represented by a function of shape factor, gas desorption time, and reservoir pressure. These relations are integrated into a fully coupled numerical model of coal geomechanical deformation and gas desorption/gas flow in both systems. This numerical approach demonstrates the important nonlinear effects of the complex interaction between matrix and fractures on CBM production behaviors that cannot be recovered without rigorously incorporating geomechanical influences. This model was then used to investigate the sensitivity of CBM extraction behavior to different controlling factors, including gas desorption time constant, initial fracture permeability, fracture spacing, swelling capacity, desorption capacity, production pressure, and fracture and matrix porosities. Modeling results show that the peak magnitudes of gasproduction rate increase with initial fracture permeability, sorption and swelling capacities, and matrix porosity, and decrease with gas desorption time constant and production pressure. These results also show dramatic increase in gas-production efficiency with decreasing magnitudes of fracture spacing. The comparison of the transient contributions of the desorbed gas and the free phase gas from the matrix system to gas production shows that the free phase gas plays the dominant role at the early stage, but diminishes when the adsorption phase gas takes over the dominant role, indicating the necessity of incorporating free phase gas impact in simulation models. The numerical model was also applied to match the history data from a gas-production well. A better matching result than that for the single-permeability model demonstrates the potential capability of the dual-permeability model for the forecast of CBM production. IntroductionCoal seams are naturally fractured reservoirs, comprising both permeable fractures and matrix blocks. The micropores within coal matrix are the main gas-storage site, whereas the microfractures, fissures, fractures, and faults comprise the main conduits for gas seepage and migration. Coal permeability changes with CBM reservoir-pressure depletion during production, whereas the desorption of gas from the coal matrix tends to increase fracture openings and increase coal permeability. These two processes are competitive ...

Swelling of clay minerals and its effect on coal permeability and gas production: A case study of southern Qinshui Basin, China

“…). Moreover, during drilling and gas production, fluid velocity sensitivity caused by particle migration and pore throat blocking is also a major cause for coal reservoir permeability decrease . Besides stress sensitivity and velocity sensitivity, water sensitivity, alkali sensitivity, and acid sensitivity can also cause damages to coal reservoirs, especially water sensitivity for high‐rank coal reservoirs .…”

Modifying subterranean fluid flow patterns using barriers to improve pre-drainage in coal mining

“…Over time, models have evolved from steady-state, to unsteady state, then transient flow solutions to attempt to better describe the pressure behaviour of gas desorbing from the coal into the low-porosity, Darcy flow network representing the cleat or fracture network (Chase, 1977;King, 1985;Mavor and Cinco-Ley, 1979;Wicks et al, 1986;Zuber et al, 1987). Further, many commercial and research reservoir simulators can now effectively model compositional changes in gas based on adsorption of mixed gases, stress dependent changes in permeability and porosity, and physical changes in coal based on coal shrinkage (Chen et al, 2013;Law et al, 2002 …”

The impact of time-dependent deformation on coal permeability and gas drainage efficiency​​​​​​​

“…The increase or decrease in permeability depends on the prevailing phenomenon. The impacts of the two mechanisms on coal permeability have been extensively studied (Shi and Durucan, 2004a;Gray, 1987;Harpalani and Chen, 1995;Shi and Durucan, 2005;Connell, Lu and Pan, 2010;Zhang, Liu and Elsworth, 2008;Wu et al, 2010a;Wang, Massarotto and Rudolph, 2009;Chen et al, 2013).…”

“…The cleats are naturally occurring fractures in coal that consist of face cleats parallel to the bedding and butt cleats perpendicular to the face cleats. The gas flow in coal cleats is laminar and governed by Darcy's law (Taheri et al, 2016;Flores, 2013;Chen et al, 2013;Clarkson and Bustin, 1999;Connell, Lu and Pan, 2010). Adsorption is the main storage mechanism for gas that primarily occurs in the micropores of the coal matrix.…”

“…They deduced that permeability change with gas production is dependent on factors such as coal seam orientation, the stress field in reservoir, and the cleat structure. Decrease and increase in permeability due to cleat closure and matrix shrinkage has been widely acknowledged in the literature (Gorucu et al, 2005;Chen et al, 2013;Cui and Bustin, 2005;Connell, Lu and Pan, 2010;Harpalani and Chen, 1997;Wu et al, 2010a). It is believed that the challenge for accurate simulation of gas flow in reservoir exists in gas desorption-induced coal matrix shrinkage in response to decrease in pore pressure during gas production (Gray, 1987).…”