Abstract
Initial cracking pressure and locations are important parameters in conducting cross-measure hydraulic fracturing to enhance coal seam permeability in underground coalmines, which are significantly influenced by in-situ stress and occurrence of coal seam. In this study, stress state around cross-measure fracturing boreholes was analyzed using in-situ stress coordinate transformation, then a mathematical model was developed to evaluate initial cracking parameters of borehole assuming the maximum tensile stress criterion. Subsequently, the influences of in-situ stress and occurrence of coal seams on initial cracking pressure and locations in underground coalmines were analyzed using the proposed model. Finally, the proposed model was verified with field test data. The results suggest that the initial cracking pressure increases with the depth cover and coal seam dip angle. However, it decreases with the increase in azimuth of major principle stress. The results also indicate that the initial cracking locations concentrated in the second and fourth quadrant in polar coordinate, and shifted direction to the strike of coal seam as coal seam dip angle and azimuth of maximum principle stress increase. Field investigation revealed consistent rule with the developed model that the initial cracking pressure increases with the coal seam dip angle. Therefore, the proposed mathematical model provides theoretical insight to analyze the initial cracking parameters during cross-measure hydraulic fracturing for underground coalmines.
Highlights
High efficient extraction of coal bed methane is of great importance to the safety of coalmine production and the development of state energy strategy in China [1]
The purpose of this study is to investigate the influences of the in-situ stress and coal seam dip angle on the initial cracking pressure and locations in hydraulic fracturing of underground coalmines
The in-situ stress analysis is assumed to obey the stress distribution rule that is suitable for Mainland China [28,29]
Summary
High efficient extraction of coal bed methane is of great importance to the safety of coalmine production and the development of state energy strategy in China [1]. The gas pre-extraction technique by boreholes has been widely used to recover the underground coal bed methane and prevent coalmine disasters [2,3]. Research efforts in recent year have proved that the hydraulic fracturing technology can enhance the coal seam permeability significantly, and has been used during the stages of coal roadway development and rock cross-cut coal uncovering, where a special technique of cross-measure hydraulic fracture is usually used [7]. In the implementation of cross-measure hydraulic fracturing, boreholes are drilled through seam floor into the targeted coal seam in a special gas roadway or main gate road. The typical spatial relationship between the borehole and coal seams is shown in
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