Numerical Models for Upscaling of Caprock Properties from Small Sample Laboratory Testing

Abstract

Abstract This work had as objective to provide simple numerical models capable of bridging between small sample or cuttings mechanical tests in the laboratory and reservoir-scale models. Numerical models are developed for tests such as the Brazilian indirect tensile test and the direct shear punch test. Intermediately upscaled shale models can be developed to model shale caprock behavior under subsurface storage operations such as CO2 or H2 storage. Of importance to creating a useful material model is to take into account the anisotropic nature of shale caprock, by introducing parallel weak planes in the models. Simulating laboratory tests allows one to tune spacing and properties of such planes with no need for microscopic and detailed accuracy. The effect of weak plane spacing and orientation on the elastic deformation of a layered material is investigated using a simple finite difference scheme. In uniaxial deformation, weakening or hardening is included through a stress-dependent stiffness modulus. Strength dependence on weak plane orientation is modelled using the finite element code DIANA. Tuning of weak plane frequency and stiffness contrast to rock bulk results in stress strain plots where the upscaled stiffness corresponds to simple analytical models. This makes it possible to specify anisotropy parameter inputs for large-scale models. Further tuning to laboratory experiments is possible through the use of the stress-dependent stiffness, making more accurate predictive upscaled models. Similarly, results from tensile strength and shear strength numerical testing highlight the fractures and their interaction with weak planes.