Digital Quantitative Study on Fracture Gas Storage Space for a Three-Layer Composite Residual Mining Area

Abstract

Quantifying the fracture gas storage space is the key to improving the coalbed methane (CBM) extraction efficiency in residual mining areas (RMAs). In this paper, a new digital quantitative description method of fractures is proposed when using a digital image correlation (DIC) system to monitor strata displacement, which improves the accuracy of fracture statistical data. The results show that with the evolution of RMA from single to three layers, the rock strata area with displacement greater than 3.5 mm increases radially and the maximum fracture rate of the uppermost RMA increases by 64.26%. The fracture rate increases exponentially from top to bottom in a long-distance composite RMA and distributes parabolically in the horizontal partition. The area with the highest average fracture rate (12.65%) in the close-distance composite RMA is defined as the concentrated growth area. The longitudinal fracture rate of a cross-layer fracture area in the three-layer RMA exceeds 60%. The cross-layer fracture area connecting the composite RMA at the open-cut side is a favorable extraction location for surface drilling. The research results will provide theoretical support for the safe and sustainable exploitation of CBM and residual coal in composite RMA.