Geothermal Induced Seismicity – Risks and Rewards

Abstract

A review has been undertaken of international experiences in geothermal induced seismicity, from both conventional operating projects and stimulation (EGS) projects. From a number of case studies, the review reached conclusions that, we hope, will assist developers and regulators manage this issue in a pragmatic manner, while enhancing fracture permeability and thereby energy recovery. Injection or extraction of fluid induces changes in reservoir pressure and temperature which perturbs in-situ stress conditions. This may be sufficient to trigger seismicity through a range of mechanisms. Understanding of the mechanisms involved has now advanced to the stage that mathematical models have been developed to simulate the processes. The phenomenon of post shut-in seismicity, for example, has been simulated. In many cases, the maximum magnitude of the induced events appears limited by the geometry of the volume of the stimulated reservoir. This leads to the conclusion that monitoring the spatial growth of seismicity in real time can help constrain the risk of inducing large damaging earthquakes on nearby faults. For site risk assessment, the level of background knowledge regarding natural rates of seismicity, in-situ stress state and rock strength parameters is important. Ongoing collaborative research, through IEA-GIA Annex XI and IPGT working groups is addressing outstanding issues with the objective of revising existing protocols, and reducing the risks. Key outstanding issues include: discriminating natural from induced events, thermo-elastic effects from long-term production-induced cooling, the effective extent of stress-field perturbations, the choice of stimulation methods to minimize ground shaking, and ways to reduce uncertainties in forecasts of induced seismicity effects.