Abstract
The Enhanced Geothermal System (EGS) constructs an artificial thermal reservoir by hydraulic fracturing to extract heat economically from hot dry rock. As the core element of the EGS heat recovery process, mass and heat transfer of working fluid mainly occurs in fractures. Since the direction of the natural and induced fractures are generally perpendicular to the minimum principal stress in the formation, as an effective stimulation approach, horizontal well production could increase the contact area with the thermal reservoir significantly. In this paper, the thermal reservoir is developed by a dual horizontal well system and treated as a fractured porous medium composed of matrix rock and discrete fracture network. Using the local thermal non-equilibrium theory, a coupled THM mathematical model and an ideal 3D numerical model are established for the EGS heat extraction process. EGS heat extraction capacity is evaluated in the light of thermal recovery lifespan, average outlet temperature, heat production, electricity generation, energy efficiency and thermal recovery rate. The results show that with certain reservoir and production parameters, the heat production, electricity generation and thermal recovery lifespan can achieve the commercial goal of the dual horizontal well system, but the energy efficiency and overall thermal recovery rate are still at low levels. At last, this paper puts forward a series of optimizations to improve the heat extraction capacity, including production conditions and thermal reservoir construction design.
Highlights
The thermal reservoir is treated as a fractured porous medium composed of matrix rock and a discrete fracture network
With the increase of thermal recovery lifespan, the low-temperature zone in the reservoir will expand, and reach the production well, and the water temperature will gradually decrease. It can be clearly seen from the figure that the low-temperature zone near the fracture channel expands faster compared to the rock matrix region, because the penetrating fractures form the main water flow region with higher water velocity and more obvious convection effect
Using the local thermal non-equilibrium theory, a coupled and the ideal 3D numerical model are established for Enhanced Geothermal System (EGS) heat extraction mathematical model and the ideal numerical model are established for heat extraction capacity evaluation
Summary
Increasing the supply of local and renewable energy has become a central issue across society, which highlights the necessity to re-evaluate all alternative energy sources, those widely distributed throughout the country [1]. How to establish an artificial heat reservoir at commercial scale and develop the thermal energy cost-efficiently is still a significant measure to evaluate the promotion and application of EGS. Cao et al [11] simulated the heat extraction capacity of EGS using an ideal vertical well model based on the assumption of local non-thermal equilibrium, yet the existence of fractures is still not taken into consideration. Lei et al [16] and Rutqvist [17] respectively established a THM coupled model and solution method to realize the numerical simulation of the EGS production process.
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