Influences of hydraulic fracturing on microfractures of high-rank coal under different in-situ stress conditions

The influence of hydraulic fracturing on coal microfractures (10-1000 mu m) is an important part of the stimulation mechanism of coalbed methane (CBM) hydraulic fracturing, which is of great significance to improve the productivity of CBM. In this work, high-rank coal samples from Chengzhuang and Sihe Mine were selected for hydraulic fracturing simulation experiments, using stereoscopic microfracture scanning and binarization image processing methods, the mean aperture, surface density, connectivity, mean length and fracture porosity of microfractures were compared and analyzed under different in-situ stress conditions before and after hydraulic fracturing, and the permeability changes were calculated. It is found that hydraulic fracturing could increase the permeability of microfractures in coal seam significantly, and the permeability of microfractures after hydraulic fracturing was 0.43-14.82 times that before fracturing. Hydraulic fracturing did not induce new microfractures, but only the expansion of the original microfractures, which caused the mean aperture, mean length and fracture porosity of microfractures to increase by 39.85%, 47.70% and 115.59%, respectively. Stronger heterogeneity and larger horizontal in-situ stress difference will inhibit the expansion of microfractures. Under the same hydraulic fracturing conditions, there are larger interlayer differences for coal samples with stronger heterogeneity, and hydraulic fracturing will prioritize microfractures in the direction of parallel bedding planes. In addition, the smaller the horizontal in-situ stress difference, the microfractures expand uniformly during hydraulic fracturing, which is more conducive to the connection of microfractures.

Automated summary

Hydraulic fracturing significantly increases the permeability of coal microfractures by expanding existing fractures rather than inducing new ones. The impact of hydraulic fracturing on microfractures is influenced by the heterogeneity of coal samples and differences in in-situ stress.