Hydraulic Fractures in Reservoirs Bounded by Layers of Other Rocks

Abstract

This study investigates the characteristics of hydraulic fractures (HFs) formed in low permeability reservoirs that are bounded by salt layers. Three layered systems are modeled, where the thickness of the bounding salt layers differs with respect to the thickness of the shale layer (same thickness, thinner salt, and thicker salt). The width and total height of the models are the same. The interface properties match the properties of the weaker material, which is the salt. Both the shale and salt zones are modeled as homogeneous and impermeable materials, and water injection is modeled in the center of the middle shale layer. An additional model of hydraulic fracturing in the middle of a homogeneous and isotropic shale is included. All models are subjected to the maximum (major) principal stress in the vertical direction and the minimum (minor) principal stress in the horizontal direction with fixed boundary conditions. The hybrid finite-discrete element modeling technique is used for these analyses. Results show that the contrast between the mechanical properties and thickness of layers influence the state of stress in the layers. Specifically, the orientation of the major and minor principal stresses switch in the target shale layer. This leads to creation of inclined HFs in the bounded shale as opposed to vertical HFs that would form in a thick shale layer under normal anisotropic stress conditions. The thicker are the bounding salt layers, the more horizontally inclined the HFs are in the shale. These analyses inform us that the design of hydraulic stimulations is influenced by the properties and thickness contract between the reservoir and bounding layers.