Propagation Law of Hydraulic Fracture across the Coal–Rock Interface under the Co-Effect of Natural Fractures and Tectonic Stress

Abstract

Indirect fracturing from roof rock to coal using a horizontal well is a new and promising technology for coalbed methane surface exploitation in soft and low-permeability coal seams. In order to study the propagation law of hydraulic fractures across the coal–rock interface, a pore pressure cohesive element is used to establish a numerical model for indirect fracturing. Combined with practical engineering in a 3# coal seam in the Xinjing mine in China, the propagation behavior of hydraulic fractures across the coal–rock interface was researched, and the range of the horizontal well position for indirect fracturing was determined. The results show that: (1) the pore pressure cohesive element can be used to accurately simulate the interaction between hydraulic fractures and natural fractures, and the propagation of hydraulic fractures across the coal–rock interface. (2) As the vertical distance between the horizontal well and coal–rock interface decreases, the breakdown pressure of perforation decreases, while the injection pressure increases when the hydraulic fracture crosses the coal–rock interface. (3) For the indirect fracturing engineering in a 3# coal seam in the Xinjing mine, the vertical distance between the horizontal well and coal–rock interface should not be larger than 2.0 m to make the hydraulic fracture propagate into the coal seams.