← Back to Sessions

Towards Thermo-hydro-mechanical Constitutive Models for Deep Geothermal Reservoirs: Experiments on Thermal Cracking Under Stress With Near-field Acoustic Sensing

Description: 

Deep geothermal heat mining at temperatures high enough for supercritical fluid extraction may become a significant source of carbon-free power generation. Thermal cracking may play an important role in the evolution of fracture networks that determine heat transfer and permeability in such reservoirs. During hydraulic stimulation, thermal cracking may couple strongly with hydraulic and background tectonic stresses to influence evolving fracture network characteristics.To better understand these couplings, we perform tri-axial laboratory experiments at 10 MPa, to produce thermal cracking in Westerly granite. We control stress buildup by ramping up temperature in stages under different mechanical boundary conditions. In some experiments, we lock the piston so that differential stress builds as the sample thermally expands, to reach failure that produces a rupture traversing the sample. We record continuous near-field acoustic data using novel high-temperature piezoelectric sensors stable up to about 500 C. Using the standard triggering method for detecting acoustic emissions (AEs), we find on the order of a hundred AEs per experiment. We then analyze continuous waveforms first using STA/LTA to build a catalog of template waveforms, and then perform matched filtering, which can detect tens of thousands of events, mostly below the noise level. Temporal clustering statistics identify multiple populations of events with burst-like and with random behavior. Finally, we perform unsupervised feature extraction (using SpecUFEx and hierarchical clustering) to discover temporal patterns in the spectral content of the signals, that evolve with increasing stress and temperature, providing potentially useful signatures of an evolving fracture network. As temperature increases towards and into brittle-ductile conditions, it is expected that the mean crack length scale decreases towards the grain scale, influenced by increasingly weak grain boundaries. Dense networks of grain-scale fractures may be essential for engineering geothermal reservoirs deep in the crust, potentially in conditions of brittle-ductile transitions.

Session: Seismology for the Energy Transition - I

Type: Oral

Date: 4/16/2025

Presentation Time: 05:00 PM (local time)

Presenting Author: Benjamin

Student Presenter: No

Invited Presentation: 

Poster Number:

Authors