Analysis of Seismicity Resulting from Time-dependent Fluid Injection Source Pressures

Fluid induced seismicity is often governed by linear pressure diffusion. We relate the perturbed pore pressure to the induced seismicity to study the fluid - rock interaction, to examine physical processes and to characterize the reservoir. Solutions of the diffusion equation exist for the condition of constant source strength. But in some injection experiments, such as in Basel (Switzerland), the source strength is not constant over time. Here we present the solution of the diffusion equation which considers the special problem of linear 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. Our analysis result in consistent estimates of hydraulic properties of the stimulated reservoir. We obtain a scalar permeability of around 75 microDarcy by assuming an effective isotropic medium. Furthermore, we study the criticality field which describes the strength of preexisting fractures in the reservoir. We observe that it is bounded by a minimum criticality of approximately 2000 Pa and a maximum criticality of about 0.75 MPa.