Abstract
To investigate the mechanism of hydraulic fracture propagation in coal seams with discontinuous natural fractures, an innovative finite element meshing scheme for modeling hydraulic fracturing was proposed. Hydraulic fracture propagation and interaction with discontinuous natural fracture networks in coal seams were modeled based on the cohesive element method. The hydraulic fracture network characteristics, the growth process of the secondary hydraulic fractures, the pore pressure distribution and the variation of bottomhole pressure were analyzed. The improved cohesive element method, which considers the leak-off and seepage behaviors of fracturing liquid, is capable of modeling hydraulic fracturing in naturally fractured formations. The results indicate that under high stress difference conditions, the hydraulic fracture network is spindle-shaped, and shows a multi-level branch structure. The ratio of secondary fracture total length to main fracture total length was 2.11~3.62, suggesting that the secondary fractures are an important part of the hydraulic fracture network in coal seams. In deep coal seams, the break pressure of discontinuous natural fractures mainly depends on the in-situ stress field and the direction of natural fractures. The mechanism of hydraulic fracture propagation in deep coal seams is significantly different from that in hard and tight rock layers.
Highlights
Coal seam gas that mainly contains methane is an economical and promising solution for the world’s energy crisis [1]
Micro-seismic monitoring in the field confirms that hydraulic fractures in coal seams usually form a network of fracture branches using the horizontal well technique [99]
The results indicate that: (1) hydraulic fracturing can create a large number of secondary fractures in coal seams; (2)
Summary
Coal seam gas that mainly contains methane is an economical and promising solution for the world’s energy crisis [1]. China has the world’s third largest CBM reserve, totaling approximately 37 Tm3 [3,4]; as they are influenced by coalification, geological structure and depth, most Chinese CBM reservoirs have low permeability [5,6,7], which makes gas extraction difficult, and even leads to coal and gas outbursts [8]. Hydraulic fracturing has been applied to destress coal seams to prevent coal and gas outbursts. Other significant applications of hydraulic fracturing include the measurement of geo-stress [15,16], Processes 2018, 6, 113; doi:10.3390/pr6080113 www.mdpi.com/journal/processes
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