eprintid: 2124 rev_number: 18 eprint_status: archive userid: 2 dir: disk0/00/00/21/24 datestamp: 2018-09-07 10:10:57 lastmod: 2021-12-15 06:08:49 status_changed: 2018-09-07 10:10:57 type: article metadata_visibility: show creators_name: Garcia-Aristizabal, Alexander corp_creators: Istituto Nazionale di Geofisica e Vulcanologia title: Modelling fluid-induced seismicity rates associated with fluid injections: examples related to fracture stimulations in geothermal areas subjects: MP4 subjects: RA1 subjects: RU1_1 subjects: SG divisions: IP13 divisions: SHEER-6 divisions: SHEER-5 full_text_status: none keywords: Probabilistic forecasting; Induced seismicity; Statistical Seismology abstract: In this paper, we present a model for describing relationships between fluid-induced seismic- ity and operational parameters of fluid injections. Considering seismic sequences occurring during sustained fluid injections, we present a novel covariate approach in which a probability distribution is defined as a basic template function for modelling the interevent times (i.e. the time intervals between consecutive events), and the possible dependencies on operational parameters are modelled writing the parameters of the probabilistic model in terms of deter- ministic functions of explanatory covariates that are selected from the operational data. The implemented model is tested using data from two cases of reservoir stimulations in geother- mal systems (The Geysers, US and Cooper Basin, Australia). We have found that a template exponential distribution of interevent times, with a linear function relating the logarithm of the distribution’s μ parameter and the logarithm of the injection rate, is the model that better describes the observations in all the analysed cases. This result suggests that the μ parameter and the injection rate have a power-law relationship. The value of the power-law exponent, α 1 , is an indicator of the relative change in the seismicity rate associated with a relative change in the injection rate, and it results particularly important for understanding the behaviour of seismicity at high injection rates. α 1 =− 1 indicates the special case of a linear relationship between seismicity rate and injection rate (which usually is the model assumed in the litera- ture); conversely, α 1 > − 1( α 1 < − 1) indicates that the relative change in the seismicity rate associated with a relative change in the injection rate is lower (higher) than the relative change expected by assuming a linear relationship between these two parameters. Regarding the cases analysed in this paper, the exponent of the power law varies between − 1.04 and − 0.78 in The Geysers, and between − 1.22 and − 0.73 in Cooper Basin. While no clear temporal trends are observable in the α 1 values obtained for The Geysers, in Cooper Basin α 1 increased as the stimulation tests proceeded (from an initial value of α 1 =− 1.22 calculated for the first fracture initiation test to α 1 =− 0.73 associated with the third fracture initiation test). This behaviour can be a consequence of the so-called Kaiser effect indicating that, to trigger events, the pore pressure during new injections must exceed the values already reached in previous injection operations. Finally, we also studied the gradual decline of seismicity rates in the post-injection phases in Cooper Basin using the modified Omori law. We explored this data set for looking, in particular, if there exists a pattern between the parameters controlling the post-injection seismicity decay rate and the characteristics of the precedent fluid injection. date: 2018 date_type: published publication: Geophysical Journal International volume: 215 number: 1 publisher: Oxford University Press pagerange: 471-493 id_number: doi:10.1093/gji/ggy284 issn: 0956-540X official_url: http://doi.org/10.1093/gji/ggy284 access_IS-EPOS: unlimited software_references: MIDSTREAM owner: Publisher acknowledgments2: IS-EPOS_platform acknowledgments2: SHEER_project citation: Garcia-Aristizabal, Alexander (2018) Modelling fluid-induced seismicity rates associated with fluid injections: examples related to fracture stimulations in geothermal areas. Geophysical Journal International, 215 (1). pp. 471-493. DOI: https://doi.org/10.1093/gji/ggy284