Repurposing Oil/Gas Wells for Geothermal Applications: an Experimental Study on Geochemical Impacts of the Cycling System

Abstract

Repurposing late-stage, depleted, and/or inactive hydrocarbon wells for geothermal systems can be environmentally sustainable and economically profitable. Studies of such transition systems tend to focus on geothermal properties, geomechanics, and economic benefits using simulation analysis. Laboratory research designed to assess the geochemical impact of wells not originally drilled for geothermal purposes needs to be conducted. Reservoir fluids may contain various dissolved salts and aqueous silica depending on the geologic formation type. Oil and gas reservoirs’ fluids may also contain residual hydraulic fracturing chemicals and high total dissolved solids. The fluid-rock and fluid-material interactions during the geothermal cycles may enhance pipe corrosion and scale mineral formation (silica, salts, etc.). It is crucial to study reactions of the complicated fluids with site-specific reservoir geochemistry in the geothermal system cycles to understand impacts to long-term performance. Solid-fluid chemical interactions may introduce new scale species like iron (hydro)oxides to the downhole and reservoir, resulting in changes to the fluid chemistry and causing subsequent reactions as the fluid cycled through the entire geothermal system over time. In this study, we designed an experimental system mimicking the fluid paths cycling through the fractured or porous reservoir at elevated temperature and pressure, and through the geothermal infrastructure materials (e.g., steel) at reduced temperature and pressure relevant to the operation conditions. We focused on answering fundamental geochemical questions associated with site-specific geothermal systems, including traditional sandstone/limestone aquifers and repurposed oil/gas reservoirs. The field water was collected from the Tuttle Geothermal project, which targeted retrofitting retired oil wells in Tuttle, Oklahoma, to produce geothermal energy for direct heating. The site-specific conditions were used for the experimental setup. We identified the major steel-fluid and reservoir-fluid interactions that may cause potential efficiency degradation of geothermal systems in operation. By understanding the potential geochemical impacts on geothermal systems, this study provides insights on the geothermal transition from oil/gas fields.