Association Between Injection and Microseismicity in Geothermal Fields With Multiple Wells: Data‐Driven Modeling of Rotokawa, New Zealand, and Húsmúli, Iceland

• Published in: Journal of geophysical research. Solid earth Vol. 128; no. 4
• Main Authors: Yu, Pengliang, Dempsey, David, Rinaldi, Antonio Pio, Calibugan, Aimee, Ritz, Vanille A., Archer, Rosalind
• Format: Journal Article
• Language: English
• Published: 01.04.2023
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB025952

Understanding injection‐induced microseismicity in geothermal systems can provide insight into reservoir connectedness. However, fault and reservoir complexity are difficult to represent in simple analytical models, which makes it difficult to discern clear relationships from incidental associations. Here, we have used data‐driven models to study how fluid injection and microseismicity are related in the Rotokawa (New Zealand) and Hellisheiði (Iceland) geothermal fields. We tested two classes of model: (a) lagged linear regression of seismicity rate as a function of well injection rates; and (b) systematic extraction of injection time series features that are then evaluated for associations with the seismicity. These models allowed us to determine which wells had the greatest correlation with microseismicity and to explain this association in a reservoir context. Finally, exploring different data types and transformations, we were unable to establish a link between rapid changes in injection rate and seismicity spikes, as suggested by some theoretical models. Plain Language Summary To better understand the small earthquakes triggered in geothermal fields, we used two kinds of data‐driven models: time series feature engineering (a machine learning method) and the non‐negative linear regression method. We applied these techniques to microearthquakes recorded in the Rotokawa geothermal field, New Zealand, and the Húsmúli reinjection area in the Hellisheiði field, Iceland. We searched for associations and patterns hidden within the noisy injection and seismicity data. These patterns are further complicated by the complex injection schedules that involve several wells that switch off and on over time. We found a stronger association between injection and long‐term microseismicity rate as opposed to short‐term fluctuations driven by noise. We showed that particular injection wells are strongly associated with the microseismicity, which we interpret in terms of reservoir connectedness and stress changes in the rock. We found that injection rate and volume are the most useful data for understanding microseismicity, whereas there was no evidence that rapid injection rate changes triggered seismicity at these two locations. Key Points Relative associations of geothermal wells with seismicity rate are evaluated using regression analysis and time series feature engineering Reservoir operational changes such as injection transfers between wells can affect the association between wells and microseismicity There is a stronger correlation between injection and long‐term seismicity rate than short‐term seismicity rate at Rotokawa and Hellisheiði site