Earthquake Science Center Seminars

Towards Strongly coupled Thermo-Hydro-Mechanical Constitutive Models: Experiments on Thermal Cracking Under Stress with Near-field Acoustic Data

Date & Time

Wednesday, May 7, 2025 at 10:30 AM Pacific

Location

Online-only seminar via Microsoft Teams

Summary

Thermal cracking may play important roles in fracture network (FN) evolution and permeability in geothermal reservoirs. During hydraulic stimulation in geothermal systems, boreholes and reservoirs can undergo varying degrees of thermal shock and thermal stress that couple to hydraulic pressure changes. Thermal cracking, in such situations, may couple strongly with hydraulic and background tectonic stresses to influence seismicity rates and fracture network evolution. Thermal stress and cracking may play an important role in creating dense networks of grain-scale fractures (e.g. "cloud fracture") at high temperatures. 
We perform tri-axial laboratory experiments at 10 MPa, to produce thermal cracking in Westerly granite while varying stress paths for the same thermal paths. Some samples reach failure levels that are clear in the mechanical data and in the microstructures as a rupture that traverses the entire sample. We record continuous near-field acoustic data using novel high-temperature piezoelectric sensors stable up to about 500 C, and apply a high resolution acoustic emission (AE) detection method that produces far larger catalogs than standard threshold methods. We then analyze the temporal clustering statistics and identify multiple populations of events with different degrees of burst-like behavior. Finally, we perform unsupervised feature extraction (using SpecUFEx and hierarchical clustering) to detect/discover temporal patterns in the spectral content of the signals, which evolve as temperature and stress increase. We infer that the samples are generating isolated or minimally-connected fractures during early stages of thermal cracking, at the grain scale, followed by rapid weakening as these fractures connect up. Whether we can identify this change in fracture connectivity in patterns in the acoustic data (with both AE temporal clustering statistics and spectral patterns) is currently work in progress. As temperature increases towards and into brittle-ductile conditions, it is expected that the mean crack length scale decreases, dominated by increasingly weak grain boundaries. Understanding the influence of the stress state on thermal crack propagation and fracture network characteristics at high temperature may be a valuable tool in engineering deep, supercritical reservoirs in brittle-ductile conditions.