Effects of long‐term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of The Geysers geothermal field

Abstract

The long‐term temporal and spatial changes in statistical, source, and stress characteristics of one cluster of induced seismicity recorded at The Geysers geothermal field (U.S.) are analyzed in relation to the field operations, fluid migration, and constraints on the maximum likely magnitude. Two injection wells, Prati‐9 and Prati‐29, located in the northwestern part of the field and their associated seismicity composed of 1776 events recorded throughout a 7 year period were analyzed. The seismicity catalog was relocated, and the source characteristics including focal mechanisms and static source parameters were refined using first‐motion polarity, spectral fitting, and mesh spectral ratio analysis techniques. The source characteristics together with statistical parameters (b value) and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells. The observed temporal, spatial, and source characteristics were clearly attributed to fluid injection and fluid migration toward greater depths, involving increasing pore pressure in the reservoir. The seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud. A tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir.

Evolution of seismicity in relation to fluid injection in the North-Western part of The Geysers geothermal field

“…Equivalent moment magnitudes were calculated from original duration magnitudes using the formula M * W = 0.9M D + 0.47 (Edwards and Douglas, 2013). The equivalent moment magnitude of completeness as determined by Kwiatek et al (2015) is M * W C = 1.4. The selection of this particular data set was done for two reasons.…”

“…However, since the fluid volume injected in Prati-29 is proven to influence the total seismic activity (e.g. Kwiatek et al 2015), we considered the effect of the total injection rate and focused on the close vicinity of the open hole of Prati-9 injection well.…”

“…All events were recorded by the Lawrence-Berkeley National Laboratory surface network Berkeley-Geysers (BG) comprising 31 threecomponent short-period geophones. The events were relocated Evolution of seismicity in relation to fluid injection 1159 by Martínez-Garzón et al (2014) and Kwiatek et al (2015) from the original Northern California Earthquake Data Center (NCEDC) catalogue, using the double-difference relocation technique (Waldhauser & Ellsworth, 2000). The internal precision of relative hypocentre locations of the events was improved to 50 m due to the applied relocation.…”

“…In order to obtain significant results, high quality data sets should be analysed. For that reason, we selected our study area to be the northwestern part of The Geysers geothermal field, California and utilized a recent relocated seismicity catalog (Martínez-Garzón et al 2014;Kwiatek et al 2015). In the selected area, nearly immediate seismic response to the injection was noticed in several stimulation wells.…”

A Method for Calibration of the Local Magnitude Scale Based on Relative Spectral Amplitudes, and Application to the San Juan Bautista, California, Area

“…In the final inversion, there are 1600 earthquakes with 1 million spectral ratios from 200 stations. We use the observed ratios d ik − d jk to construct the system of equations (4), with unknown source amplitudes s i , as was done for corner-frequency estimation by, for example, Ide et al (2003Ide et al ( , 2004, Imanishi et al (2004), Oye et al (2005), Imanishi and Ellsworth (2006), Kwiatek et al (2011), Harrington et al (2015), and Kwiatek et al (2015).…”