Shear slip and opening of existing faults during fluid injection: insights from tilt measurements

Fluid injection in the subsurface for the purpose of CO2 sequestration, geothermal heat extraction or energy storage has frequently caused faults activation and seismicity, raised the communities’ concerns and ultimately resulted in project shutdown. In order to understand earthquakes, a set of unique experiments are being conducted in Bedretto Underground Laboratory for Geosciences and Geoenergy located at 1,000 m depth under the Swiss Alps. In these suites of experiments, small-scale non-damaging earthquakes are induced via water injection into a well-known and well-characterised fault.

A set of three borehole tiltmeters were deployed in the vicinity of the injection borehole and its data were used for analysing the fault’s behaviour during and after injection. A 3D finite element model (CSMP-HF) was utilised to predict the tilt vectors at specified stations from a set of prescribed input data (geometry, loading, stiffness, etc.), and a residual (cost) function was defined based on Bayesian framework to evaluate the closeness of the model predictions to the field measurements. Finally, a Differential Evolution optimisation technique was used to locate the global minima of the residual (cost) function, corresponding to the best set of input data. The inversion model results confirmed that both shear slip and opening (dilation) deformations occurred not only on the target fault, but also on another transverse fault. The inversion model was capable of accurately finding the location of “unknown” secondary fault which was consistent with log data gathered from another observation wellbore. The shear slippage consisted of both dip-slip (vertical) and strike-slip (horizontal) deformation, consistent with measured in-situ stresses.