An Attempt to Control Induced Seismicity and On-Fault Stress Pre-Conditioning in the Bedretto Underground Laboratory, Switzerland

Hydraulic stimulation has been used as a standard practice for rock mass treatment to fulfill the intended engineering goal of the stimulation procedure in various applications such as oil & gas, mining, and enhanced geothermal systems (EGS). High-pressure fluid injection during the hydraulic stimulation perturbs the local stress field around the injection borehole which has a direct influence on the hydro-mechanical behavior of the rock mass and the measured induced seismicity. Hydraulic stimulation is usually conducted using a predefined injection protocol, a sequence of pressure and/or flow rate injection cycles. Extensive attempts have been designed to develop a proper injection protocol in laboratory and field scales to reach the stimulation goals as well as reduce the risk of induced seismicity.

In the context of the ERC “Fault Activation and Earthquake Rupture (FEAR)” project, injection protocols are developed to further our understanding of earthquake rupture processes such as nucleation and premonitory slip. A series of stress-preconditioning injection strategies were tested in the Bedretto Underground Laboratory for Geoenergies and Geosciences, Switzerland (Ma et al., 2022, Solid Earth), using an existing stimulation and monitoring array (Plenkers et al., under rev. with Sensors). The main objective of these tests was to develop atypical injection strategies that could be implemented in different geological conditions that target aspects of induced seismicity associated with high-pressure fluid injection. We performed two injections and monitored the rock mass response using six monitoring boreholes each equipped with multi-component seismic monitoring systems, pressure, temperature, and distributed strain sensing arrays. Two different injection protocols have been applied in two packed-off injection intervals of a stimulation borehole, allowing us to characterize the hydro-mechanical response of the rock mass around these intervals. Each test was associated with pre- and post-characterization tests including hydraulic and hydro-mechanical tests such as HTPF tests.

The preliminary results of these tests show a direct dependency of the location and magnitude of seismicity with injection protocols. Moreover, the effect of the injection protocols on the characteristics of the rock mass such as transmissivity and normal stress could be quantified. Preconditioning of the fault appears to require low permeability, preferably undrained, fault structures and suffers if any fluids pathway are mitigated by the long range, slow pressurization of this region. Complex fluid pathways made re-stimulation and precondition of the target structures difficult. The outcomes of these tests play a key role in the design of upcoming FEAR stimulation experiments to better understand the earthquake physics on a field scale.

 

References

Ma, X., Hertrich, M., Amann, F., Bröker, K., Gholizadeh Doonechaly, N., et al., 2022. Multi-disciplinary characterizations of the BedrettoLab–a new underground geoscience research facility. Solid Earth, 13(2), pp.301-322.

Plenkers K., Reinicke A., Obermann A., Gholizadeh Doonechaly N., Krietsch H., et al., under rev. with Sensors, Multi-disciplinary monitoring networks for mesoscale underground experiments: Advances in the Bedretto Reservoir Project.