Seismic Response to Injection Well Stimulation in a High‐Temperature, High‐Permeability Reservoir

Abstract

Fluid injection into the Earth's crust can induce seismic events that cause damage to local infrastructure but also offer valuable insight into seismogenesis. The factors that influence the magnitude, location, and number of induced events remain poorly understood but include injection flow rate and pressure as well as reservoir temperature and permeability. The relationship between injection parameters and injection‐induced seismicity in high‐temperature, high‐permeability reservoirs has not been extensively studied. Here we focus on the Ngatamariki geothermal field in the central Taupō Volcanic Zone, New Zealand, where three stimulation/injection tests have occurred since 2012. We present a catalog of seismicity from 2012 to 2015 created using a matched‐filter detection technique. We analyze the stress state in the reservoir during the injection tests from first motion‐derived focal mechanisms, yielding an average direction of maximum horizontal compressive stress (SHmax) consistent with the regional NE‐SW trend. However, there is significant variation in the direction of maximum compressive stress (σ1), which may reflect geological differences between wells. We use the ratio of injection flow rate to overpressure, referred to as injectivity index, as a proxy for near‐well permeability and compare changes in injectivity index to spatiotemporal characteristics of seismicity accompanying each test. Observed increases in injectivity index are generally poorly correlated with seismicity, suggesting that the locations of microearthquakes are not coincident with the zone of stimulation (i.e., increased permeability). Our findings augment a growing body of work suggesting that aseismic opening or slip, rather than seismic shear, is the active process driving well stimulation in many environments.

Potential influence of overpressurized gas on the induced seismicity in the St. Gallen deep geothermal project (Switzerland)

“…The temperature linearly increases with depth by 35.5 • C km −1 and is held constant during the injection. Although injection of low temperature fluid can cool the rock and influence both the stress and pressure conditions (e.g., Ghassemi and Tao, 2016;Hopp et al, 2019; Rinaldi et al, 2015), we do not expect a significant cooling effect over the relatively short-term and small volume injection that occurred in St. Gallen. We choose the boundaries to be open for fluid flow everywhere except at the boundary Y=0 km (symmetry boundary), where we apply no flow conditions (Fig.…”

Seismic response to evolving injection at the Rotokawa geothermal field, New Zealand

“…At reservoir depths, the drilling operation sustained full fluid losses. Similar drilling losses induced a large number of seismic events during drilling in southern Ngatamariki (Hopp et al, 2019) and may have had a similar effect at Rotokawa. Finally, swarm-like behavior (defined as any day on which more than 15 events occurred) has been observed at Rotokawa in the past and is thought to be related to pressure perturbations induced by power plant shutdown and startup during regular maintenance operations (Sewell et al, 2015b).…”

“…During this period, little new development was undertaken at Rotokawa as the resource was adjusting to the significant increase in production associated with the commissioning of NAP in 2010. Changes in injection and production are therefore more subtle than those analyzed at the nearby Ngatamariki field for the same period, where more substantial changes were associated with well stimulation and power plant commissioning (Clearwater et al, 2015;Hopp et al, 2019). Table 1 contains periods identified by the operator, Mercury, from 2012-2015 during which the character of seismicity may help address various outstanding questions about the nature of the reservoir.…”

“…As described in detail by Hopp et al (2019), the Mercury seismic network covers an area of approximately 450 km 2 surrounding the Rotokawa and Ngatamariki geothermal areas. While most of the instruments are 4.5 Hz Geospace GS-11D short-period geophones owned by Mercury, we have also incorporated a number of stations operated by Contact Energy at their nearby Wairakei geothermal field, as well as three broadband instruments and one short-period instrument operated by the GeoNet national network ( Figure S1).…”

“…We used the matched-filter correlation detection approach described by Hopp et al (2019) to increase the number of events included in the GNS Science earthquake catalog mentioned above. This approach increases the number of events detected without unduly increasing the rate of false detection and is well suited to detection at geothermal fields with many noise sources and dense clusters of small seismic events.…”