Hydraulic Stimulations with Hydro-Mechanical Pre-Conditioning at the BedrettoLab

The heterogenous nature of geological formations is further complicated by natural and induced discontinuities such as fractures and faults. These geological features introduce a complex network of pathways and barriers that alter the local in-situ stresses as well as fluid flow dynamics. A comprehensive understanding of hydro-mechanical (HM) interactions during fluid injection experiments may provide insights into the effective manipulation of underground for energy extraction (e.g., hydraulic stimulation) as well as prediction and mitigation of induced seismicity in response to various stimulation techniques. Significant effort has been devoted to understand the rock-fluid interactions in energy contexts (e.g., shale gas, enhanced geothermal system) and earthquake seismology through various methodologies including laboratory studies, in-situ experiments and numerical simulations over the last decades. However, systematic studies of such interactions under in-situ conditions with natural heterogeneity require systematic manipulation of injection parameters in a well-characterized reservoir or underground research laboratory.

The BedrettoLab (Bedretto Underground Laboratory for Geosciences and Geoenergies) with a local overburden of over 1000 m, located in the Swiss Alps, is a suitable site for conducting such studies, as the rock volume has been well instrumented and characterized [Plenkers et al., 2023]. In a close collaboration between two running projects in BedrettoLab, i.e., VALTER (Validating of Technologies for Reservoir Engineering) and FEAR (Fault Activation and Earthquake Rupture), a series of controlled hydraulic stimulations were conducted where various HM pre-conditioning steps were included (or left out) in the injection protocol. The main objective of the HM pre-conditioning was to understand and control microseismicity by pre-determining the pressurized patch on the fault/fracture volume via the injection protocol prior to the main injection. The installed monitoring system captured ongoing HM processes during each hydraulic stimulation, enabling systematic testing of these pre-conditioning hypothesis across various fractures. In the next step, similar protocols were applied to the first stimulation experiment as part of the project which provided a comprehensive insight of the HM and seismogenic characteristics of the stimulated fault, in response to pre-conditioning.