Characterization of Fracture Plane Heterogeneity Using Variogram Models

Abstract

In geothermal reservoirs, where fractures are the main conduits for flow, the accurate modeling of flow through fractures is important in maximizing energy production. It is often assumed that the aperture is constant within the fracture. Though this is a good approximation, it does not take fracture roughness into account. Flow experiments in rough fractures have demonstrated that flow channeling can occur in fractures with heterogeneous aperture and permeability distributions. Flow channeling can impact geothermal energy production because it reduces the surface area available for heat conduction from the surrounding rock matrix. Characterization of the fracture aperture is important because aperture is directly related to permeability. In this study, the main goal was to present a structure for the characterization of fracture aperture distribution of different rock samples and pairing configurations. Variogram models and frequency distribution plots were used to characterize the roughness of fracture planes. Geostatistical techniques were used to analyze the spatial distribution of aperture values within the fracture plane. The best fit variogram models for fracture surfaces generated with a shear offset showed higher spatial continuity in the direction perpendicular to the direction of the shear offset. In contrast, the variogram models for mated fracture surfaces created artificially with a wedge exhibited no preferential orientation. Future work will include flow simulations to correlate characterization parameters to flow channeling effects.