Evaluation of Borehole Hydraulic Fracturing in Coal Seam Using the Microseismic Monitoring Method

Accurate evaluation of the influence range of borehole hydraulic fracturing (HF) in coal seam is crucial for optimizing the design scheme of HF. In this study, we adopted the microseismic (MS) monitoring technology to monitor and characterize the spatial shape of cracks caused by borehole HF in coal seam in an underground coal mine. And we also tested and analyzed the stress and moisture content changes of coal mass at different distances from the borehole after HF program. The number of MS waveforms and the energy of MS events show a good positive correlation with the water pressure curve. The response of the energy curve to the extension of the hydraulic cracks is ahead of the water pressure curve. Based on the short-time average/long-time average (STA/LTA), the interference signal recognition method (ISR) and the improved Akaike information criterion (AIC) method, we developed a comprehensive MS event detection and arrival time picking (CMDP) program that are suitable for the weak MS signals with low signal to noise ratio (SNR) induced by HF in coal seam. And then we were able to more accurately locate the MS events of the hydraulic cracks using the simplex source location method. We have conducted a comprehensive analysis of the relationship between the temporal and spatial distribution of MS events and hydraulic cracks propagation. The results show that there is an apparent correlation between the MS activities and HF operation. The expansion of hydraulic cracks generates MS events, and the larger the size of the cracks, the greater the energy of MS events. Based on the MS monitoring results, the HF produces a crack network of flat ellipsoid in the No. 6 coal seam, which indicates that there is obvious stimulated reservoir volume (SRV) fracturing effect during the borehole HF process. The influence radius of HF based on the moisture content is the smallest (about 20 m), followed by the stress monitoring (about 30 m), and the MS monitoring is the largest (about 40 m). The high-precision MS source location (location errors < 2.5 m) results combined with roadway roof watering phenomenon indicate that the influence radius of HF based on the moisture content and stress release may be underestimated. And the effective influence radius of the borehole HF is about 40 m for this HF program.