Characteristics of Three-Dimensional Pore-Fracture Network Development and Enhanced Seepage Heat Transfer in Hot Dry Rock Stimulated by Temperature Shock Effects

Hot dry rock (HDR) geothermal is a sustainable and clean energy source. However, its development progress is hindered by creating seepage channels in deep reservoirs with low porosity and permeability. Traditional hydraulic fracturing techniques are ineffective for enhancing the permeability of these high-strength reservoirs. To address this, a cyclic nitrogen injection technique was proposed, which leverages the thermal gradients of the hot reservoir to stimulate a complex thermally induced fracture network. To study the three-dimensional pore-fracture structure and the flow characteristic of HDR under temperature shock effects, various high-temperature rock samples (200-500 °C) were treated with 5 cycles of liquid nitrogen cold shock. Using digital core technology, a visual pore-fracture network was reconstructed and the simulation of flow and heat exchange within this network was further performed. The main conclusions are as follows: Following the liquid nitrogen cold shock treatment with rock cores of 200-300 °C, only a few isolated micropores were formed, marked by low porosity and poor connectivity, yielding effective porosities of 0.79 and 1.52%, respectively. In contrast, the cold shock at 400-500 °C induced the formation of a reticulated pore-fracture network. This development was attributed to the combined effects of thermal stress and grain expansion, with an effective porosity reaching 12.58%. Further, a pore network model revealed a substantial increase in both the pore number and size, especially under the cold shock of 500 °C cores, where the largest pore radius reached 2133 μm. The permeability of the representative elementary volume increased significantly with the rising cold shock temperature difference, escalating from 13.79 μm² at 200 °C to 1101.39 μm² at 500 °C. This shift signifies a transition from localized to more extensive flow paths. Based on the actual pore-fracture network, a simulation of heat extraction from HDR was conducted, showing that the exchanged heat increased from 4.51 × 10^-8 to 8.34 × 10^-8 W with the rise in the temperature difference. Within the temperature range of 300-400 °C, a singular flow path was observed, characterized by minimal fluid transport but elevated exit temperatures. Meanwhile, at 500 °C, a superior heat exchange network was established, featuring improved fluid transport and heat exchange efficiency.