Probability of earthquake occurrence and magnitude estimation in the post shut-in phase of geothermal projects

Abstract

Induced seismicity in geothermal projects is observed to continue after shut-in of the fluid injection. Recent experiments show that the largest events tend to occur after the termination of injection. We use a probabilistic approach based on Omori's law and the Gutenberg-Richter magnitude frequency distribution to demonstrate that the probability of exceeding a certain maximum magnitude still increases after shut-in. This increase is governed by the exponent of Omori's law q and the Gutenberg-Richter b-value. For a reduced b-value in the post-injection phase the probability of occurrence directly after shut-in can be even higher than the corresponding probability for an on-going injection.For the reference case of q = 2 and a 10% probability at shut-in time t S to exceed a given maximum magnitude we obtain an increase to 14.6% for t = 2t S at a constant Gutenberg-Richter b-value after shut-in. A reduction of the b-value by one quarter leads to a probability of 20.5%. If we consider a constant probability level of occurrence for an event larger than a given magnitude at shut-in time, this maximum magnitude increases by 0.12 units for t = 2t S (0.26 units for a reduced b-value).For the Soultz-sous-Forêts (France) injection experiment in 2000 recent studies reveal q = 9.5 and a b-value reduction by 14%. A magnitude 2.3 event 9 h after shutin falls in the phase with a probability higher than for the continued injection. The probability of exceeding the magnitude of this post-injection event is determined to 97.1%.

Validating induced seismicity forecast models—Induced Seismicity Test Bench

“…Presumably, this problem is not specific to the data we considered here because in several other projects the biggest event occurred after shut‐in [ Baisch et al , ; Asanuma et al , ]. Focusing on shut‐in and the events that follow, Barth et al [] showed theoretically and also confirmed with the analysis of the data from Soultz‐sous‐Forêts 2000 that probability of exceeding a certain magnitude can be higher after shut‐in than it would have been for ongoing injection. Segall and Lu [] proposed a descriptive model that includes complete poroelastic coupling—changes in pore pressure induce stresses, and changes in mean normal stress induce changes in pore pressure—and concluded that an abrupt shut‐in can produce sharp increase in the seismicity rate.…”

Induced seismicity in geothermal reservoirs: A review of forecasting approaches

“…Further experience is required to assess the capabilities of an initially estimated "seismogenic index" to forecast induced seismicity in geothermal reservoirs and to use it as a key parameter. The addition of the modified Omori's type law to the model for the period following the injection (see section 0) allows for the prediction of the seismicity during shutin [179]. Hence, this additional feature, together with the a priori Gutenberg-Richter distribution, combines a pressure diffusion model with a statistical seismicity approach.…”

Project permitting, finance, and economics for geothermal power generation

“…Majur and Baria (2007) point out that there is a growing public perception of seismic hazards that may be associated with geothermal operations. Barth et al (2013) point to the biased "decision threshold due to some ambiguity on hazard and risk estimates" that drive this issue, and Evans et al (2009) point out the obvious, namely that the uncertainty introduced by this issue poses a severe challenge for developers and financiers, especially given the paucity of real field data to validate or refute the condition. Barth et al (2013) point to the biased "decision threshold due to some ambiguity on hazard and risk estimates" that drive this issue, and Evans et al (2009) point out the obvious, namely that the uncertainty introduced by this issue poses a severe challenge for developers and financiers, especially given the paucity of real field data to validate or refute the condition.…”