Modeling the Significance of Mechanical Interfaces to Fracture Propagation and Morphology – Geomechanical Models Derived from Outcrop Analysis

Abstract

URTeC 1619873 Subsurface fault and fracture networks are important fluid-flow pathways in low-permeability rocks. We characterize the nature of discontinuities in well-exposed, fine-grained, low-permeability Jurassic Carmel Formation that exhibits evidence of subsurface fracture propagation and fluid migration. We identify mechanostratigraphic units and document the effects mechanical variations due to lithologic changes have on the distribution and nature of fractures. Interfaces mark the boundary between mechanical units where fractures deflect, propagate across, or arrest depending on contrasts in mechanical properties, such as Young's modulus. In this study we compare model results to outcrop fracture distribution to evaluate the effects that changes in mechanical unit thickness and associated variations in elastic moduli have on local strain and fracture distribution across locked mechanical interfaces. Studying the occurrence of, and changes in, outcrop fracture patterns and scaling these up for field-scale (km-scale) modeling is difficult due to the lack of direct correlation between outcrop observations and subsurface data. We bridge this correlation gap by first evaluating the meso-scale (mm-m) variability observed in outcrop and incorporate these observations into geomechanical models in order to identify the controlling properties in fracture propagation and morphology. In outcrop we observe variations in fracture densities from 1 to 18 fractures/meter and vein apertures that range from mm to cm in width. In outcrop fractures occur as calcite veins in the limestone rich facies of the Carmel Formation and in the shale facies are mineralized with calcite or gypsum and often have limonite alteration halos along the fracture margins. Fracture strike orientations parallel those in the underlying Navajo Sandstone and fault deformation bands. We incorporate this outcrop data with changes observed in dynamic Young's modulus (E) within the basal 9 meters of the heterolithic Carmel Formation and upper 3 meters of the Navajo Sandstone. Changes in elastic moduli, such as dynamic Young's modulus, which ranges from 17-34 GPa across interfaces within low permeability units, will ultimately influence fracture distributions and fluid behavior in the subsurface. Our characterization of rock strength variability is especially important for modeling the response of low permeability rocks to increased pore pressure, and is applicable to multiple geo-engineering scenarios such as exploitation of natural resources, waste disposal, and management of fluids in the subsurface. Our analyses provide analog fracture data for fine-grained mixed siliciclastic carbonate rocks and a dataset for better understanding the importance of heterogeneity in low permeability rocks.