Abstract
When water is injected into a fracture-dominated reservoir that is cooler or hotter than the injected water, the reservoir permeability is expected to be altered by the injection-induced thermo-mechanical effects, resulting in the redistribution of fluid flow in the reservoir. These effects are important to be taken into account when evaluating the performance and lifetime particularly of Enhanced Geothermal Systems (EGS). In this paper, we compare the results from two dye tracer tests, conducted before (at ambient temperature of 13,^{circ } text {C}) and during the injection of 45,^{circ } text {C} hot water into a fractured crystalline rock at the Grimsel Test Site in Switzerland. Conducting a moment analysis on the recovered tracer residence time distribution (RTD) curves, we observe, after hot water injection, a significant decrease in the total tracer recovery. This recovery decrease strongly suggests that fluid flow was redistributed in the studied rock volume and that the majority of the injected water was lost to the far-field. Furthermore, using temperature measurements, obtained from the same locations as the tracer RTD curves, we conceptualize an approach to estimate the fracture surface area contributing to the heat exchange between the host rock and the circulating fluid. Our moment analysis and simplified estimation of fracture surface area provide insights into the hydraulic properties of the hydraulically active fracture system and the changes in fluid flow. Such insights are important to assess the heat exchange performance of a geothermal formation during fluid circulation and to estimate the lifetime of the geothermal formation, particularly in EGS.
Highlights
To produce energy from geothermal resources, fluid is injected and circulated through natural or artificially created reservoirs, the latter being so-called enhanced/engineered geothermal systems (EGS) (Tester et al 2006; Evans 2015)
We investigated the effects of hot water injection on the hydrodynamic properties of a hydraulically stimulated crystalline rock, using solute dye tracers
In addition to comparing the temporal moments and the associated tracer-swept volumes and flow geometries, we put constraints on fracture geometries by estimating the fracture surface areas contributing to heat exchange between the host rock and the fluid flowing in the fractures
Summary
To produce energy from geothermal resources, fluid is injected and circulated through natural or artificially created reservoirs, the latter being so-called enhanced/engineered geothermal systems (EGS) (Tester et al 2006; Evans 2015). Kumari et al (2018) conducted flow-through experiments in a granite core under different temperature conditions and demonstrated that increasing the temperature of the injected fluid from 20 to 300 ◦C , caused an 86% reduction in permeability before reaching 100 ◦C. Grimm Lima et al (2019) observed a 20–75% decrease in the hydraulic apertures of naturally fractured granodiorite cores from the Grimsel Test Site (GTS) in Switzerland (which is the field site in this paper), when increasing the temperature of the system from 22 to 140 ◦C . Kumari et al (2018) and Grimm Lima et al (2019) identified that the reductions in permeability and in fracture apertures as well as the closure of fractures are caused by the thermal expansion of the rock. Grimm Lima et al (2019) demonstrated pressure dissolution of contacting asperities
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