| Abstract |
Hydraulic stimulation (single and multi-stage) is the most commonly used method for enhancing deep geothermal reservoir permeability, but it lacks control over the fracture initiation direction around a borehole. Additionally, due to the stress shadowing effect in multi-stage stimulations, the fracture initiation direction can change at each stage. In this research, we introduce a borehole pre-conditioning method using a Shape Memory Alloy (SMA) based mechanical actuator to control the fracture initiation direction around a borehole, and to reactivate existing fractures. SMAs actuate through a solid phase transition from detwinned martensite to austenite phase when heated, and return to a twinned martensite phase when cooled, giving them a ‘shape memory’ effect. In this study, we used in-house developed NiTi and NiTiHf-based SMA actuators due to their superior corrosion resistance and high energy density. Constant stress temperature cycling was used to train SMA rods (10-15 mm length) for the shape memory effect. The trained NiTiHf and NiTi SMA exhibits 1.4% and 1.6% actuation strains, respectively. When actuated under constant strain, NiTiHf can generate 800MPa (at 240°C) stress while NiTi can generate 522MPa (at 140°C). Due to this, NiTi is suitable for low-temperature soft rock, while NiTiHf is more suitable for high-temperature hard rock pre-conditioning. Laboratory experiments and numerical simulations were performed on Rotondo granite specimens by inserting the actuator into a borehole and thermally actuating under in-situ stress conditions. The results indicate that the actuation and the generated stresses are sufficient to induce fractures and alter the stress state near the borehole. |