The effects of water-rock interactions on the permeability of the Precambrian Basement in Southern Saskatchewan
Creators: name: Smith, Taylor
Publisher: name: Graduate Studies
Year: 2023
Subjects: subject: Geothermal
subject: Hydrogeology
subject: Water-rock Interactions
subject: Saskatchewan
subject: Geochemistry
subject: FOS: Earth and related environmental sciences
subject: Geochemistry
subject: Geology
subject: Hydrology

description: Geothermal energy has gained significance as a potential low-carbon, renewable energy source. Approximately 70% of the world’s geothermal reservoirs are defined as low temperature (&lt;150°C), with the potential to power agricultural machinery, support direct heating of homes, buildings, and therapeutic facilities, and produce electricity using specialized facilities. The Precambrian basement underlying the Williston Basin in southern Saskatchewan, with a reservoir temperature of ~120°C, represents one such geothermal resource. Well tests and examination of recovered samples suggest that this, and other similar reservoirs globally, present suitable conditions for geothermal energy production. However, geochemically induced permeability alteration in these potentially highly reactive granitoid rocks presents a significant risk to sustained heat production from these low-temperature resources. To assess this risk, and potentially identify mitigation measures, we conducted a geochemical and mineralogical study of samples recovered from this reservoir. Mineral composition and thermal history of both altered and unaltered samples were quantified through a combination of petrographic, Raman Spectroscopy, and Electronic Microprobe analyses, and the resultant values were then utilized to parameterize geochemical simulations of water-rock interactions at relevant conditions. The simulations permitted prediction of mineral volume changes, and hence inferences about porosity and permeability changes, as a result of these interactions. The simulations suggest that mineral volume will increase as a result of water-rock interaction in both unaltered and altered zones of the basement rocks permeability, but that the geothermal alteration of the unaltered, and hence more reactive, rocks would yield mineral volume changes around 30% greater than the altered rock. Applying the porosity-permeability relationship equation developed by Verma and Pruess (1988), the calculated change in permeability indicates the unaltered Precambrian basement rock will experience a greater change in total permeability compared to the altered basement rocks. Additional calculations show that the Verma Pruess’s equation is sensitive to critical porosity and fitting parameter and predicts various scenarios resulting in a range of permeability change from 1x10-13 to 1x10-20 m2. Based on this result, we infer that altered, permeable zones of the Precambrian basement underlying the Williston basin are likely to provide conditions favorable for the sustained, multi-decade heat production that would be required to justify initial capital expenditures, and hence should be targeted over less altered zones. Geothermal heat extraction from permeable, altered zones of warm Precambrian granitoid rocks globally is an understudied but potentially fruitful resource to reliably produce low-carbon, renewable energy as we seek to decarbonize the global economy.