Monitoring Geochemical Alteration of Fracture Surfaces in Granite with Electrical Conductivity Measurements
Authors: Christoforidou E. , Bobet A. , Pyrak-Nolte L. J. 
Published: August 2024

ABSTRACT:
Understanding the behavior of fluid flow in fractures over time is crucial for ensuring the long-term performance and sustainability of Enhanced Geothermal Systems (EGS). Geophysical observation techniques, including electrical methods, are often used to monitor subsurface alterations, during geothermal operations, from coupled thermo-hydro-mechanical and chemical processes. Here, we describe an experimental system and procedures for high-temperature laboratory fluid flow tests on rock fracture surfaces developed to measure, concurrently, fluid electrical conductivity and temperature. Laboratory fluid flow experiments were conducted on individual rock fracture surfaces for constant flow rate, solution chemistry, and temperature. Potential geochemical alteration of fracture surfaces was investigated by conducting surface roughness measurements before and after the flow tests using laser profilometry and 3D X-ray microscopy. A maximum change of 160% in fluid conductivity at 25 °C was observed during room temperature flow tests on halite specimens, accompanied by measurable alterations in surface geometry. Tests performed on individual rock fracture surfaces in Sierra White granite, created through axial splitting, found that the changes in electrical conductivity were less than 5 %. This result was supported by fracture surface roughness measurements that determined that the alterations were within the calculated measurement error.

1. INTRODUCTION
Geothermal systems typically consist of fracture networks within hot, low-permeability rocks. Enhanced Geothermal Systems (EGS) utilize engineering stimulation techniques, such as hydraulic fracturing, to create or enhance the permeability of hot rock formations. Hydraulic stimulation is a key process that involves injecting cold fluids into hot rock formations to enhance the permeability of the fracture network. Since fractures play a crucial role in controlling fluid and heat transport in geothermal systems, understanding the behavior of fluid flow in fractures and its temporal changes is crucial for ensuring the long-term performance and sustainability of EGS.
Geophysical observation techniques such as seismic and electrical methods, have been widely used to monitor subsurface changes during geothermal operations (Okamoto et al., 2021; Taira et al., 2018; Didana et al., 2017). Electrical methods involve measuring the electrical resistivity (ρ) or conductivity (σ) of subsurface materials to gain insights into the fluid distribution and rock porosity. Changes in electrical resistivity have been observed in certain EGS during hydraulic stimulation (Peacock et al., 2012). Given that rocks in EGS are rather impermeable, these changes can be related to alterations in fluid ionic composition and/or fracture aperture caused by coupled thermo-hydro-mechanical and chemical processes. Chemically mediated changes during fluid-rock interactions, such as pressure solution at contacting asperities and free-face dissolution at fracture walls, can gradually affect the hydraulic and transport properties of fractured rocks (Polak et al., 2004). Several researchers have conducted laboratory flow-through experiments on fractured rock specimens at high temperatures and pressures. They observed a reduction in fracture permeability caused by thermo-chemical dissolution phenomena (Polak et al., 2003; Morrow et al., 2001; Moore et al., 1983). Hence, monitoring fracture surface alterations to assess changes from coupled thermo-hydro-chemical dissolution phenomena is essential for the maintenance of geothermal resources.