EVOLUTION OF THE HYDRAULIC PROPERTIES OF FRACTURES DUE TO HYDRO-CHEMICAL AND THERMO-HYDRO-MECHANICAL PROCESSES

Abstract

Fractures often act as fluid conduits in subsurface formations, and understanding the evolution of their hydraulic properties is essential to energy extraction, resources exploitation, and environment management. This thesis research addressed fracture evolution in the context of two engineered systems.

The first part of the thesis investigated permeability evolution due to hydro-chemical processes for a leakage pathway in a caprock in geologic carbon sequestration (GCS). A model, named Permeability Evolution due to Calcite reactions (PEC), was developed and applied to systems for a synthetic caprock, the Eau Claire caprock, and the Callovian caprock. The effects of a number of important geologic and injection-related parameters were investigated. The results show that pathway permeability can increase by up to three orders of magnitude in 50 years, and we identified several conditions under which pathway permeability tends to significantly increase. A novel semi-analytical model, PEC Interface Progress (PECIP), was developed too. PECIP is two to five orders of magnitude faster than PEC, but it tends to over-estimate permeability increase. The results show that the pathway permeability from PECIP is close to that from PEC in more than half of the cases investigated. This study provides in-depth understanding of geochemical alteration of pathway permeability and guidance for future GCS assessments.

The second effort examined flow channeling in a single fracture due to thermo-hydro-mechanical processes in engineered/enhanced geothermal systems (EGS). It focused on the effects of autocorrelation length and standard deviation of aperture field on flow channeling and heat production. A new model was developed on the platform of GEOS at Lawrence Livermore National Laboratory. The results show that an EGS system tends to have enduring heat production if the initial aperture field enables tortuous preferential flow paths. Longer correlation length in fracture aperture field generally leads to worse and more variable reservoir performance. Aperture standard deviation tends to exacerbate flow channeling and reduce the amount of useful heat only when correlation length is long. This study provides important insights into fracture behavior with spatially heterogeneous aperture field during EGS heat production, as well as practically useful guidelines for developing sustainable EGS.