Investigation of the stress evolution under the effect of hydraulic fracturing in the application of coalbed methane recovery

摘要

The study of stress transfer can delimitate the hydraulic fracturing (HF) range to evaluate the extraction efficiency of coalbed methane (CBM). However, less research has been done to explore the stress change of underground coal seam due to the field test limitations. In this paper, a total of 9 stress monitoring boreholes (SMBs) are designed to investigate the stress evolution of coal seam during HF through field experiments. Although the middle-low pressure fracturing may cause some stress concentrations, the stress could completely release in the period of subsequent mining, which will not cause potential safety hazards. The stress increase direction is more likely to extend to the original fracture area, especially the direction of loose ring in front of coal seam. The range of stress increase is inversely proportional to the distance to the hydraulic fracturing borehole (HFB) in the same direction, except for the geological structure development areas. The plastic ring will expand with an increasing radius R1 along with the stress increase of coal seam until the radial stress increment Delta sigma rp of outer wall of plastic ring decreases to zero. The effective stress affecting range is about 140 m according to the analysis of stress transfer at different time during HF. The stress no longer increases and gradually decreases to stability after fracturing. The stress transfer method is one of ways to represent the effective region of field HF and enrich the evaluation system for HF affecting range.