REFEREED PUBLICATIONS IN JOURNALS
Gischig, V.S., D. Giardini, F. Amann, "et al.", Keith F. Evans, "et al.", A. Kittilä, X. Ma, "et al.", M.O. Saar, and "et al.", Hydraulic stimulation and fluid circulation experiments in underground laboratories: Stepping up the scale towards engineered geothermal systems, Geomechanics for Energy and the Environment, 100175, 2020. [Download PDF] [View Abstract]The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS.
Ma, X., M.O. Saar, and L.-S. Fan, Coulomb Criterion - Bounding Crustal Stress Limit and Intact Rock Failure: Perspectives, Powder Technology, 374, pp. 106-110, 2020. [Download PDF] [View Abstract]In this perspective article, we illustrate the importance and versatility of the Coulomb criterion that serves as a bridge between the fields of powder technology and rock mechanics/geomechanics. We first describe the essence of the Coulomb criterion and its physical meaning, revealing surprising similarities regarding its applica- tions between both fields. We then discuss the rock mechanics applications and limitations at two extreme scales, the Earth's crust (tens of kilometers) and intact rocks (meters). We finally offer thoughts on bridging these scales. The context of the article is essential not only to the rock mechanics/geomechanics community but also to a broader powder technology community.