Transport of Tracers in Fractured Geothermal Reservoirs Including Fluid-Rock Interactions

To provide a sustainable heat extraction rate, an enhanced geothermal system (EGS) requires adequate circulation of the working fluid through a heat exchanger, which is comprised of a network of open fractures. The permeability of the fracture network constrains the fluid flux, and the surface area of the matrix rocks in contact with the fluid constrains the power or efficiency of the heat exchanger. Consequently, these parameters (surface area and permeability) are crucial for determining the capacity and longevity of EGS systems. One promising approach to estimate these properties is to analyze natural and/or artificial tracer data that are subject to fracture-matrix interactions including matrix diffusion and chemical reactions of the tracer with the solid phase of the rock matrix. Analytical solutions for tracer transport are commonly used to analyze tracer test data. However, fluid-rock interactions (e.g., precipitation and dissolution reactions) can impact the tracer behavior, and analytical solutions for tracer transport associated with precipitation-dissolution reactions are limited in the literature. This study develops semi-analytical solutions for tracer transport in both a single-fracture and a multiple-fracture system associated with precipitation-dissolution reactions under transient transport conditions. For verification, the semi-analytical solutions are compared with numerical simulation results. Several examples show that results are sensitive to fracture spacing, fracture surface area, and bulk reaction rate, indicating that the relevant flow and transport parameters can be inferred by analyzing tracer signals.