Investigation on heat extraction characteristics in randomly fractured geothermal reservoirs considering thermo‐poroelastic effects

Abstract

A fully coupled thermal‐hydraulic‐mechanical (THM) model was developed to investigate the underlying response mechanisms during heat extraction in fractured geothermal reservoirs. The random fracture network in the stimulated zone was reproduced based on the fractal theory. The coupled model accounts for the dominant physical phenomena including (a) fluid flow, heat transport, and solid deformation in porous media and fractures; (b) local thermal nonequilibrium (LTNE) between rock matrix and flowing fluid; and (c) temperature‐dependent fluid thermodynamic properties and stress‐dependent pore and fracture permeability. The proposed model was validated by several analytic solutions. Sequentially, the evolution of pore pressure, equivalent temperature, effective stress, and reservoir permeability was analyzed. The sensitivity of the heat extraction performance to fracture network morphology was discussed. Results show that interconnected large‐scale fractures dominate mass and heat transport. Widely distributed small‐scale fractures contribute to the cooling of the heat rock mass along many flow paths in parallel. The change in effective stress associated with fully coupled thermo‐poroelastic effects may induce fracture shear dilation and pore expansion, resulting in an enhancement of the overall reservoir permeability. There is a significant temperature difference between the solid phase and the fluid phase in fractures, but not in porous media. It is more reasonable to use the LTNE theory to analyze the heat extraction of fractured geothermal reservoirs. The fracture network morphology has a profound effect on heat extraction efficiency and injectivity. Undeveloped fracture networks will result in a higher injection pressure, earlier thermal breakthrough, and shorter lifetime, but higher heat extraction ratio. Properly increasing the fracture density can delay the thermal breakthrough time, prolong the service life, and improve the injectivity. Fully connected fracture networks may result in thermal short‐circuiting and earlier thermal breakthrough. Generating a complex and scattered fracture network but without preferential channels is conducive to extracting more heat from geothermal reservoirs.

Combined Effects of Thermal Perturbation and In-situ Stress on Heat Transfer in Fractured Geothermal Reservoirs

“…These studies suggest that the thermal stress tends to have a dominant role in increasing fracture permeability, which is responsible for the enhanced flow channelization and faster thermal drawdown (Fu et al, 2016). The channelized flow and anisotropic heat transfer behavior are found to be controlled by fracture characteristics such as length and orientation (Han et al, 2019;Sun et al, 2017). These previous studies have mostly focused on analyzing the fracture aperture evolution either caused by thermal stresses, whilst the combined effects of thermal and in-situ stresses remain poorly elucidated, especially for complex discrete fracture networks.…”

“…A few recent studies have investigated the evolution of fracture transmissivity or aperture in fracture clusters (Vik et al, 2018;Zhao et al, 2015) or discrete fracture networks (Fu et al, 2016;Gan & Elsworth, 2016;Han et al, 2019;Koh et al, 2011;Sun et al, 2017;Yao et al, 2018). These studies suggest that the thermal stress tends to have a dominant role in increasing fracture permeability, which is responsible for the enhanced flow channelization and faster thermal drawdown (Fu et al, 2016).…”

Short‐term failure mechanism triggered by hydraulic fracturing

“…The principle of fluid load was similar to that of the needle tube, which could realize constant injection pressure or water injection rate by controlling the advancing pushing pressure. An acoustic emission monitoring system was used to monitor crack initiation and propagation, which could provide unique insights regarding the short‐term failure process under fluid injection . The data acquisition frequency could reach 40 MHz, ensuring reliable stability.…”

A Galerkin‐free/equation‐free model reduction method for single‐phase flow in fractured porous media

“…Applications, where fractures should be seriously studied for reservoir behavior, include development of tight sandstone oil and gas reservoirs, CO 2 geological sequestration, and geothermal energy extraction, among others. Fractures provide preferential flow paths and important heat transfer area in a low‐permeability reservoir.…”