Finite-Element Simulation Studies for Consequences of Rock Layers and Weak Interfaces in Unconventional Reservoirs

Abstract

Unconventional reservoir systems are heterogeneous, thinly layered, and often exhibit strongly contrasting properties between layers. In addition, the interfaces between layers vary in strength (friction and cohesion) and, when weak, they provide preferential directions to rock failure and fluid flow. Traditional rock mechanics modeling for hydraulic fracturing, wellbore stability, stress prediction, and other petroleum-related applications assume homogeneous rocks and welded interfaces. This assumption is hard to reconcile with the strongly layered texture and varied layer composition observed in unconventional rocks. Using the finite element method (FEM), we investigated the consequences of the presence of rock layers and weak interfaces on three different subjects: 1) formation shear stress development, shear slip at interfaces, and wellbore stability; 2) hydraulic fracture height growth; and 3) casing shear impairment. For the first scenario in this work, three different layered rock models were simulated and compared: laterally-homogeneous, laterally-heterogeneous, and strongly laterally-heterogeneous. Results show that localized shear stresses develop along interfaces between layers with contrasting properties and along the wellbore walls. It was also seen that rock shear and slip, along interfaces between layers, may occur when the planes of weakness are pressurized (e.g., during hydraulic fracturing). In the second scenario, we used a range of tensile strength and fluid flow properties at the interfaces between layers, to investigate their impact on vertical propagation of hydraulic fracture. The results show a systematic decrease in fracture height and fracturing fluid efficiency with increasing interface hydraulic conductivity and/or decreasing interface strength. We also propose that fluid viscosity has a strong influence on fluid efficiency as well as fracture height growth. In the third scenario, finite-element simulations were also conducted in a casing-cement- formation system to evaluate casing curvature and plastic deformation caused by formation shear movement occurring with slippage along the weak interface between two distinct rock layers. The results indicate that the abrupt curvature change and the plastic deformation along the casing are generated near the slip surface. We also observe that casing shear at the peak temperature during a single thermal cycle of cyclic steam stimulation induces higher casing plastic deformations.