Abstract
Hydraulic fracturing and premining gas drainage are important to safe mining and coalbed methane extraction. These technical processes cause the redistribution of in-situ stress and the regional variation of moisture contents within the affected zone. Therefore, we investigated the coupled effect of variable stresses (from 9 MPa to 27 MPa) and moisture contents (from 0.22% to 4.00%) on the permeability evolution of gas-saturated raw coal. The results show that (1) the relationship between the mean effective stress and the permeability can be described by a power function according to the permeability evolution model of the porous matrix established in this study. Besides, the influence mechanisms of moisture on fitting coefficients in the function were analyzed. (2) The permeability decreases with the increase of in-situ stress (e.g., confining pressure or volumetric stress) in a negative exponential manner. (3) The curves of permeability variations with moisture content are not always linear, and the permeability is more sensitive to the moisture content than the volumetric stress in the test range. Moreover, the sensitivity of permeability varies in different regions. These results would be beneficial for permeability prediction and surface well parameters design.
Highlights
Coalbed methane (CBM) is an abundant valuable resource in underground coal mines
Scholars have conducted extensive researches on the factors influencing permeability, including in-situ stress field [5], gas pressure [6], Klinkenberg effect [7], geothermal field [8], geoelectric field [9], acoustic field [10], and physical properties [11]. These findings have allowed the mechanics of permeability evolution to be deliberated and provided an adequate data volume and dimension for machine learning which has been proven to be a powerful tool for permeability prediction [12, 13]
A series of experiments were performed for grasping the permeability evolution of gas-saturated raw coal under the coupled influences of moisture contents and insitu stress
Summary
Coalbed methane (CBM) is an abundant valuable resource in underground coal mines. It is estimated that China bears approximately 36.8 trillion m3 of CBM in its reservoirs shallower than 2 km, ranking the third country in the world [1]. Scholars have conducted extensive researches on the factors influencing permeability, including in-situ stress field [5], gas pressure [6], Klinkenberg effect [7], geothermal field [8], geoelectric field [9], acoustic field [10], and physical properties [11]. These findings have allowed the mechanics of permeability evolution to be deliberated and provided an adequate data volume and dimension for machine learning which has been proven to be a powerful tool for permeability prediction [12, 13]. The large amount of fracturing fluids leads to Geofluids
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