Sustainability of Fractures in EGS Systems – A Laboratory Investigation

Abstract

The aperture of an induced, open fracture in a geothermal reservoir can change over time, frequently resulting in loss of permeability which may require costly re-fracturing. Understanding how the fracture pore space geometry in various types of rock evolves under realistic reservoir conditions will lead to optimized reservoir selection for economic geothermal energy production. To this end, we have developed a specialized laboratory system and have begun a series of tests to examine changes in permeability of induced slip-shear fractures in geothermally relevant rock types over time, under conditions relevant to enhanced geothermal systems (EGS - effective fracture normal stress up to several tens of MPa, temperature up to 300°C). These tests will flow water at varying levels of equilibration with the rock through an induced fracture with a small initial shear offset. During experiments, which may take months, the normal (closure) displacement and the hydraulic permeability of the fracture (when possible), will be monitored and the effluent collected at regular intervals for chemical and isotopic analysis. Changes in the fracture surface morphology will be examined via pre-and-post-test optical surface profilometry and post-test SEM, X-ray CT, and/or optical thin section studies. We will examine our data for discrepancies between expected and observed results considering the heterogeneous stress and flow environment within a fracture, local pressure dissolution of contacting asperities, creation of fresh mineral surface during fracture compaction, fracture sealing caused by mineral precipitation, and heterogeneous flow distribution within a fracture. This paper provides a description of our method and equipment, as well as the results of initial characterization studies of the rock samples to be used in the first set of tests.