Abstract
Global efforts to tame CO2 emissions include the use of renewable energy sources, such as geo-energy harnessing. However, injecting pressurised fluids into the deep underground can induce earthquakes, hence converting CO2-related risk into seismic risk. Induced seismicity hazard is characterised by the overall seismic activity afb that is normalised by the injected fluid volume V and the parameter b of the Gutenberg–Richter law. The (afb,b) set has so far been estimated for a dozen of reservoir stimulations, while at least 53 geothermal fluid stimulations are known to exist, based on our survey. Here, we mined the induced seismicity literature and were able to increase the number of estimates to 39 after calculating afb from related published parameters and by imputing b with its expectation where this parameter was missing (0.65 ≤ b ≤ 2.9, with mean 1.16). Our approach was a two-step procedure: we first reviewed the entire literature to identify seismic hazard information gaps and then did a meta-analysis to fill those gaps. We find that the mean and median afb estimates slightly decrease from afb ≈ −2.2 to afb = −2.9 and −2.4, respectively, and that the range of observations expands from −4.2 ≤ afb ≤ 0.4 to −8.9 ≤ afb ≤ 0.4, based on a comprehensive review unbiased towards high-seismicity experiments. Correcting for potential ambiguities in published parameters could further expand the range of possibilities but keep the mean and the median relatively close to original estimates, with afb ≈ −2.3 and −2.4, respectively. In terms of the number of earthquakes induced (function of 10afb), our meta-analysis suggests that it is about half the number that could previously be inferred from published afb estimates (i.e., half the seismic hazard). These results are hampered by high uncertainties, demonstrating the need to re-analyse past earthquake catalogues to remove any ambiguity and to systematically compute afb in future geothermal projects to reduce uncertainty in induced seismicity hazard assessment. Such uncertainties are so far detrimental to the further development of the technology.
Highlights
IntroductionThe main challenge facing the enhanced geothermal system (EGS) industry today is the risk of induced seismicity [3,4,5,6]
It has been stated that underground fluid stimulation data are “too hetIt has beenerogeneous stated thatfew underground stimulation data are “too heteroand too in numberfluid to allow firm conclusions to be drawn on the basis geneous and too few in number to allow firm conclusions to be drawn on the basis of single-parameter correlation with seismic response” [23] and thatofoperational pasingle-parameter correlation with seismic response”
By formalising hazard in in terms two main parameters a f b and induced b and byseismicity systematically assessing them by terms of the two main parameters a and b and by systematically assessing them by mining all available data from the literature (Table 1; Table A1), we made the first attempt mining all available data from the literature
Summary
The main challenge facing the EGS industry today is the risk of induced seismicity [3,4,5,6]. The same problem is faced by any geothermal project requiring underground stimulation, an EGS. Injection-induced seismicity is due to complex thermo-hydro-mechanical-chemical (THMC) processes involving fault activation, pore pressure diffusion and other alterations of the rock material [7,8]. Despite the apparent complexity of the physical processes involved and the heterogeneities of the underground, induced seismicity follows (in most of the cases) surprisingly simple empirical laws at a first level of analysis; in particular, a linear relationship between injected flow rate and induced seismicity rate, as well as a parabolic growth of the seismicity cloud over time during the injection. The laws have been verified at many locations and have been explained by both nonlinear poro-elasticity and geometric operations on an overpressure field [9,10,11,12,13,14]
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