SRIMA: A fast tool to assess seismicity and seal integrity related to fluid injection.

The safe and effective deployment of geothermal energy and storage of carbon dioxide requires an assessment of potential induced seismicity and fracturing through the seal above the reservoir. To aid such assessment we have built the SRIMA tool (Seal and Reservoir Integrity through Mechanical Analysis) and we have made it available online. The tool can be used in the Standard extended Seismic Hazard Analysis, which is part of the seismic hazard and risk assessment for geothermal projects in the Netherlands. SRIMA is a fast semi-analytical tool that provides a scenario-based analysis of pressure and temperature changes around an injection well, the resulting stress changes on nearby faults, reactivated fault area, the maximum credible earthquake magnitude, the resulting PGV distribution and an estimate of damage. SRIMA also computes the potential for development of tensile fracture in the seal and base. SRIMA has been designed to give first-order estimates of these results. The speed of the calculations facilitate them to be performed in a stochastic framework, which allows the assessment of failure probabilities.

 

SRIMA is based on semi-analytical expressions for the fast calculation of temperatures, pressures, and induced poro-elastic and thermo-elastic stresses due to the injection of cold fluid. The expressions for flow and induced stresses have been developed for a homogeneous, isotropic layer cake model under radial symmetry. In the injection layer the flow is assumed to be fully developed and temperature transfer is in an advective way. In the bounding seal and base layers, the pressure and temperature dynamics are assumed diffusive. The derived expressions capture the first-order characteristics of the pressure, temperature and stress changes. Validation of the expressions has been achieved through comparison with finite-difference and finite-element codes for temperature, pressure, and stress changes around an injection well. A fault without offset cutting through the seal, reservoir, and base can be specified within the model space. Poro-elastic and thermo-elastic stress changes are transformed to fault stresses and fault criticality. The fault area over which stresses are critical (i.e. fault reactivation occurs) is used to estimate the magnitude of the largest credible earthquake for each model scenario, assuming that the entire reactivated fault area participates in a single event, slip cannot propagate beyond the reactivated area, and all assumed slip over the fault area is seismic slip. An ensemble of magnitudes is converted to exceedance curves of peak ground velocities (PGV), using nationwide developed Ground Motion Prediction Equations. The resulting PGV distribution as a function of epicentral distance serves as input for calculating the probability of exceeding Damage State 1 using empirical fragility functions for unreinforced masonry buildings. This contribution will summarize details and assumptions behind each step.