Interpretation of microseismicity induced by time‐dependent injection pressure

Abstract

Fluid induced seismicity is often governed by linear pore pressure diffusion. Relating the perturbed pore pressure to induced seismicity, reservoir properties can be characterized. Analytical solutions of the diffusion equation are well known for the condition of constant injection source strength. But in some injection experiments, such as in Basel, Switzerland, the source strength is not constant over time. Here we present a solution of the diffusion equation which considers the special problem of linearly rising source strength. Using the solution, we accordingly modify already established methods for a seismicity based reservoir characterization (SBRC). These methods are based on a statistical approach and consider features of induced seismicity such as the spatial event density and the seismicity rate. We apply those methods to the Basel microseismic data which result in consistent estimates of hydraulic properties of the stimulated reservoir. We obtain a scalar permeability of 75 microDarcy by assuming an effective isotropic medium. Furthermore, we reconstruct the lower and upper limit of the criticality which statistically describes the strength of preexisting fractures. These critical pressures are distributed between a minimum criticality of about 5000 Pa and a maximum criticality of the order of 1 MPa.