Containment and what might constitute a barrier to the created fractures is an important technical issue. The question is how to maximize the stimulated volume without encroaching on neighboring strata. Key to effective fracturing and its containment is the understanding of the underlying mechanisms in shale fracturing and its interaction with the bounding layers. The ability of the adjacent strata to act as a barrier depends on the stress state, ductility, fracture toughness, elastic properties, and permeability as well as the interface or discontinuity properties. Rock heterogeneity influences the rock mass stiffness and fracture propagation rates and thus needs to be considered. In this paper we focus on the role of overburden and underburden mechanical properties and toughness contrast on growth of a hydraulic fracture. In view of the 3D problem complexity, we use a fully coupled 3D numerical model to analyze height growth from the payzone into bounding layers. The DD method is very efficient for this type of a problem using different approaches. A more efficient approach for a reservoir bounded by contrasting layers above and below is a 3D DD for multiple bonded planes. The 3D DD method proposed in this study is coupled with a finite difference fluid flow model to simulate the propagation of hydraulic fractures under constant injection rate or injection pressure in layered rocks.
Our results show that fracture geometry distorts from its radial shape to a more elliptical shape as stress barriers restrict height propagation. The fracture width profile also starts to deviate from its symmetric shape as the fracture approaches the barrier which has implications in the proppant placement. The simulations indicate that the degree of containment varies depending upon the magnitude of stress barriers, fluid viscosity, pumping rate, and leak-off. The contrast between the elastic modulus of the adjacent layers also influences height containment but its effect was found to be less than that of stress contrast.
Although fracture containment and the influence of stress barriers and material heterogeneity is studied in the past, the widely-used P3D models are restricted due to their underlying of the vertical plane-strain condition which dictates a PKN type of fracture. The current model, however, does not prescribe any type of propagation geometry and fracture geometry is determined based on the in-situ conditions. Moreover, the rigorous hydro-mechanical coupling helps to further our understanding of proppant placement in layered rock.
