Affiliations: [a] Hubei Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University, Wuhan, China | [b] College of Petroleum Engineering, Yangtze University, Wuhan, China
Abstract: Based on the energy release rate criterion, the behavior of vertical fracture penetration/deflection at interface between coal seam and mudstone layers are investigated by finite element method (FEM) in this study. Considering its interfacial effect that is simulated by linear spring model (LSM), the energy release rates (ERRs) of vertical fracture penetration and deflection at the interface are calculated using virtual crack close technique (VCCT). Combined with the critical energy release rates (CERRs) of coal seam layer, mudstone layer, and sandstone layer as well as the interface between them were tested in a laboratory. The layers were gathered from the southern Qinshui Basin. The judging methods of vertical fracture penetration/deflection at interface are presented and the maximum penetration length which extends into the mudstone layer is calculated. The result is consistent with the analyses from the field monitoring results. Furthermore, the effects of interfacial stiffness, coal seam thickness, Young’s modulus of sandstone cap and in-situ differential stress on the propagation behavior are also evaluated. The results show that the interfacial stiffness of interface between coal seam and mudstone layers vary between 109 Pa/m and 1012 Pa/m, which is about 1010 Pa/m. The state of interface is separation or perfect when the interfacial stiffness is smaller than 109 Pa/m or larger than 1012 Pa/m. With the increase of coal seam thickness, the maximum penetration length gradually and faintly decreases. In contrast, the maximum penetration length obviously decreases when Young’s modulus of sandstone cap raises or the in-situ differential stress Δ𝜎 declines. In other words, Young’s modulus of sandstone cap and the in-situ differential stress Δ𝜎 are the main factors which influence the behavior of vertical fracture penetration/deflection at the interface between coal seam and mudstone layers.
Keywords: Interfacial effect, energy release rate, interfacial stiffness, linear spring model, virtual crack closure technique
