Analytical model for simulating and analyzing the influence of interfacial slip on fracture height propagation in shale gas layers

摘要

During hydraulic fracturing, fractures with interfacial slip are distorted into complex geometries with horizontal components that prevent simulations by the conventional fracture height propagation models. Interfacial slip is a general phenomenon caused by structural weak interfaces in shale gas, such as natural fractures with different angles, thin interlayers and structure disconformity planes. This paper aims to study the influence of interfacial slip on fracture height propagation and investigate measures to prevent fracture height overgrowth during shale gas fracturing simulations. To know the effect of interfacial slip on hydraulic fracture height propagation clearly, this paper introduces the K criterion and COD criterion to establish a new analytical model based on the equivalent planar fracture assumption to simulate fracture height propagation with interfacial slip. The computed results show the difference between the K criterion fracture height propagation model and the COD criterion fracture height propagation model is tiny due to the two criterions having the same function in fracture mechanics. The results also show that the pressure drop caused by vertical distorting structures could be higher than the stress difference between the upper layer and the pay layer, so that the interfacial slip has a significant controlling effect on hydraulic fracture height propagation; and the interfacial slip is one of the reasons why the observed fracture height was always lower than the one predicted by conventional models. However, in shale gas fracturing, the extremely high net pressure caused by high treating pressure and pumping speed in very special treatments still has the possibility to drive the fracture height to propagate and decrease stimulated reservoir volume. Therefore, the influence of interfacial slip on fracture height propagation must be taken into consideration in shale gas fracturing simulations, and in some cases, the net pressure should also be restrained to avoid fracture height overgrowth.