Unlocking the potential of shape memory alloys for near-borehole fracture stimulation: Numerical and experimental insights for underground energy extraction

This paper investigates the possibility of utilizing Shape Memory Alloy (SMA) rock splitters to stimulate nearborehole fractures for subsurface energy storage and extraction applications such as Enhanced Geothermal Systems. We trained binary NiTi50 SMA elements under isobaric conditions and produced a laboratory-scale rock splitter to perform fracturing experiments. The mechanical response of the SMA and its specific work density were calculated during training. Based on the experimental observations, a specific work-based numerical model was developed and implemented in 3DEC to simulate the fracturing potential of SMA splitters on two calibrated grain-scale rock assemblies. The numerical simulations show that rock type, confining stress, and in-situ stress anisotropy affect SMA splitter fracture performance, with stiffer rocks yielding greater fracture area than softer rocks. Simulations also indicate SMA splitters effectively induce fractures under in-situ stress conditions, with fractures aligning predominantly normal to the SMA actuation direction. Following the simulations, SMA splitter fracturing experiments were performed on sandstone and granite cores representing soft and hard rocks. The experimental results verified the fracture patterns produced by numerical simulations. In addition, the numerical simulations provided insights into stresses developed at the SMA interfaces and the energy utilization. SMA actuation under anisotropic stresses indicates that the directional actuation of SMA may be used to equalize or amplify the local stress field anisotropy around the borehole, presenting a novel mechanism to overcome nearborehole stress-shadowing effects observed in conventional stimulation methods. These findings highlight the promise of SMA rock splitters for near-borehole fracture stimulation and mechanical pre-conditioning to supplement hydraulic stimulations in subsurface reservoir engineering.