Numerical Analysis of the Impacts of Multiscale Fractures on Geothermal Reservoir Capacity

A fracture network as the main channel of seepage and heat transfer has been a key focus in an enhanced geothermal system. The fractures in the geothermal reservoir usually have a strong multiscale nature in length, but the impacts of each scale fracture on geothermal reservoir productivity have not been studied so far. This paper investigates these impacts based on a discrete fracture matrix model. Firstly, the fractures are divided into four scales of micro-scale, small-scale, medium-scale, and large-scale in the dimension of a representative volume element (RVE). Secondly, governing equations are carefully formulated for matrix deformation, matrix percolation, fracture percolation, matrix heat transfer, and fracture heat transfer, thus a thermal-hydraulic-mechanical (THM) coupling model is established. Thirdly, the distribution of pore pressure and temperature, the fluid flow in fractures, and the deformation of multiscale fractured reservoirs are numerically simulated and those numerical results are comprehensively evaluated by three indicators. Finally, the impacts of each scale fracture on geothermal reservoir productivity are explored. The productivity of reservoirs with single-scale and full-scale fractures is compared. It is found that three pressure zones are formed along the mining direction in the fractured geothermal reservoir: high-pressure zone, medium-pressure zone, and low-pressure zone. Both pressure and temperature influence the reservoir deformation with a trend of first decreasing and then increasing. The impact of large-scale fractures on the steady power and cumulative heat recovery exceeds 90%. Excessive heat production from single medium-scale and large-scale fractured reservoirs induces a local thermal breakthrough prematurely. In contrast, the synergistic action of multiscale fractures can prolong thermal breakthrough time and maintain the reservoir at a higher steady power.