Coal permeability model on the effect of gas extraction within effective influence zone

Abstract

In situ gas extraction is the main measure adopted by mines to achieve mining safety standards. Coal permeability is significantly influenced by coal shrinkage during gas release. Based on effective stress and additional damage that resulted from gas extraction, a unique model was proposed to describe the spatial and temporal distribution of coal permeability within the effective influence zone. Calculation models for the conditions of one and two extracting boreholes were developed. A parametric study on both two models was also conducted, in which the effect of extraction time, extraction pressure, interval between two boreholes, and extraction pressure combination were qualitatively analyzed. The results demonstrated that with the increase of extraction time, gas extraction can result in less permeability reduction in the beginning but can enhance permeability. Coal permeability obviously increases when extraction time increases but gradually stabilizes, presenting ''high middle and low around'' surrounding the extracting borehole. Meanwhile, a decreasing relative amplification implies that permeability ratio does not increase linearly with the increase in extraction pressure. Both intervals between two extracting boreholes and different combinations of extraction pressure significantly influence the distribution of coal permeability. A comparison with some previous permeability studies was conducted to validate the proposed model, and the predicted evolution results matched the permeability testing data. In conclusion, the proposed model is consistent with the phenomenon in real engineering, and the model results can provide preliminary guidance for the design of extracting borehole distribution.

Effect of Coal Rank on Various Fluid Saturations Creating Mechanical Property Alterations Using Australian Coals

“…Coal generally has a mixture of pore fluid, composed of water, N 2 , CO 2 and CH 4 and their composition can be changed with various applications such as CO 2 sequestration and N 2 injection for ECBM recovery. It is therefore necessary to have comprehensive knowledge of the relative effect created by each component.…”

“…The process of enhanced coal bed methane (ECBM) recovery is being implemented and tested as a viable option to store and reduce the amount of anthropogenic carbon dioxide (CO 2 ) in the Earth's atmosphere, as well as for the recovery of useful coal bed methane (CH 4 ) gas [1][2][3][4][5][6][7]. Overall, the ECBM process involves introducing CO 2 through injecting wells into deep coal seams and this CO 2 then acts as a displacing gas, which allows the already adsorbed CH 4 to be desorbed from the coal matrix.…”

“…Overall, the ECBM process involves introducing CO 2 through injecting wells into deep coal seams and this CO 2 then acts as a displacing gas, which allows the already adsorbed CH 4 to be desorbed from the coal matrix. Finally, CH 4 is obtained through a recovery well and used to produce energy in a cost-effective and environmentally friendly way.…”

“…Particularly in the geomechanical respect, the coal mass becomes weaker with the substitution of existing adsorbate CH 4 with the highly chemically potential CO 2 [13,14]. According to Perera et al [15], Ranathunga et al [16] and Vishal et al [17], the inherent brittleness of the coal mass becomes ductile with the plasticization effect of coal with the adsorption of CO 2 , and this phenomena is higher for super-critical CO 2 (beyond the critical temperature of CO 2 -31.8˝C and the critical pressure of CO 2 -7.38 MPa).…”

Mining induced strata movement and roof behavior in underground coal mine

“…Poor knowledge of the characteristics of strata movement due to longwall mining can create very serious ground hazards, potentially jeopardizing the safety and lives of mine personnel, as well as affecting the productivity and efficiency of a mining operation. For decades, extensive effort has been paid to study the strata movement and stress distribution law induced by mining excavation and obtain comprehensive understanding of the permeability evolution of coal seam (Chen et al 2014;Xie et al 2015;Yang et al 2011a, b) and a proper design of a coal pillar (Chugh et al 1990;Hammy and Fejes 1992;Holla 1997;Holla and Buizen 1991a;Islam et al 2009). Holla and Buizen (1991b) performed an empirical study of the sub-surface deformation caused by retreating longwall mining and monitored the strata movement over a longwall panel and indicated that the three-joint-arch structure formed by the broken key roof stratum is monotonically unstable structure and its limit value is determined by the breaking convergence value.…”