Abstract
Temperature variations often trigger coupled thermal, hydrological, mechanical, and chemical (THMC) processes that can significantly alter the permeability/impedance of fracture-dominated deep geological reservoirs. It is thus necessary to quantitatively explore the associated phenomena during fracture opening and closure as a result of temperature change. In this work, we report near-field experimental results of the effect of temperature on the hydraulic properties of natural fractures under stressed conditions (effective normal stresses of 5–25 MPa). Two specimens of naturally fractured granodiorite cores from the Grimsel Test Site in Switzerland were subjected to flow-through experiments with a temperature variation of 25–140 °C to characterize the evolution of fracture aperture/permeability. The fracture surfaces of the studied specimens were morphologically characterized using photogrammetry scanning. Periodic measurements of the efflux of dissolved minerals yield the net removal mass, which is correlated to the inferred rates of fracture closure. Changes measured in hydraulic aperture are significant, exhibiting reductions of 20–75% over the heating/cooling cycles. Under higher confining stresses, the effects in fracture permeability are irreversible and notably time-dependent. Thermally driven fracture aperture variation was more pronounced in the specimen with the largest mean aperture width and spatial correlation length. Gradual fracture compaction is likely controlled by thermal dilation, mechanical grinding, and pressure dissolution due to increased thermal stresses exerted over the contacting asperities, as confirmed by the analyses of hydraulic properties and efflux mass.
Highlights
IntroductionCoupled thermal–hydrological–mechanical–chemical (THMC) processes can significantly impact the long-term evolution of reservoir permeability, associated with geothermal energy extraction (Bažant and Ohtsubo 1977; Pandey and Vishal 2017; Kamali-Asl et al 2018), hydrocarbon production (Dobrynin 1962; Dusseault 2011; Liu et al 2016), nuclear waste disposal (Tsang 1987; Nguyen et al 1995), and geologic storage of carbon dioxide ( CO2 ) (Rutqvist and Tsang 2002; Kopp et al 2009; Luhmann et al 2012, 2014; Tutolo et al 2014, 2015a, b; Zhang et al 2015, 2016)
Two specimens of naturally frac‐ tured granodiorite cores from the Grimsel Test Site in Switzerland were subjected to flow-through experiments with a temperature variation of 25–140 °C to characterize the evolution of fracture aperture/permeability
Gradual fracture compaction is likely controlled by thermal dilation, mechanical grinding, and pressure dissolution due to increased thermal stresses exerted over the contacting asperities, as confirmed by the analyses of hydrau‐ lic properties and efflux mass
Summary
Coupled thermal–hydrological–mechanical–chemical (THMC) processes can significantly impact the long-term evolution of reservoir permeability, associated with geothermal energy extraction (Bažant and Ohtsubo 1977; Pandey and Vishal 2017; Kamali-Asl et al 2018), hydrocarbon production (Dobrynin 1962; Dusseault 2011; Liu et al 2016), nuclear waste disposal (Tsang 1987; Nguyen et al 1995), and geologic storage of carbon dioxide ( CO2 ) (Rutqvist and Tsang 2002; Kopp et al 2009; Luhmann et al 2012, 2014; Tutolo et al 2014, 2015a, b; Zhang et al 2015, 2016). Enhanced geothermal systems (EGS) rely on sustaining a deep bedrock fracture network of sufficiently high permeability to provide continuous heat exchange between the fractured rock and the circulation fluid In this scenario, coupled THMC effects might cause a decline in heat production and reduce or even prevent the feasibility of EGS operations under economically viable conditions. The hydraulic performance of fracture-dominated rocks is significantly more sensitive to THMC processes than that of intact rocks, and changes in the performance can span several orders of magnitude (Kranzz et al 1979; Stephansson et al 1996; Morrow et al 2001) It is, necessary to investigate the coupling between mechanical, hydrological, thermal, and chemical processes in fractures to develop well-calibrated models and predict the changes in hydraulic and transport properties of deep, fracture-dominated geological reservoirs
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