Injection Induced Seismicity in the Raton Basin: Probabilistic Assessment of Fault Slip Potential with Application to Unmapped Faults
Advisor: Matt Weingarten
December 11, 2020
The Raton Basin of southern Colorado and northern New Mexico has been a source of coal-bed methane production since 1999. Produced water, coincident with gas production, is re-injected deep into the basin and since 2001 increased rates of seismic activity have been observed (Nakai et al., 2017). Previous studies in the Raton Basin (Meremonte et al., 2002; Zhang et al., 2013; Rubenstein et al., 2014; Nakai et al., 2017; Weingarten et al., 2020) indicate that the seismicity over the past two decades is likely induced as it correlates closely with wastewater injection rates. The lack of mapped faults in the area does not negate the existence of complex faulting in the basin, which is critical to understand the sensitivity of the basin’s faults to poroelastic stress changes from injection. Induced seismicity has been concentrated along three previously unmapped fault zones: Tercio, Trinidad and Vermejo Park. In this study, we quantitatively assess the probability of slip in each fault zone using probabilistic geomechanics in the Fault Slip Potential (FSP) framework. We first identify and characterize distinct linear fault trends within each fault zone by adapting the Fault ID algorithm (Skoumal et al., 2019) to the Raton Basin catalog of more than 2000 recorded seismic events. We then calculate both 3D deterministic and probabilistic geomechanical models of fault slip potential on each fault, varying several geomechanical parameters such as principal stress orientation, principal stress magnitude, as well as fault strike and dip. Our analysis identified six previously unmapped faults that have a fault slip potential greater than or equal to 1% in the preinjection state (0 psi change in stress). Poroelastic models of injection are incorporated with our FSP results to illustrate a percent increase in slip potential as poroelastic stress changes continue to rise into the year 2025 (Weingarten et al., 2020). The Trinidad sequence, host to the 2011 Mw5.3 earthquake, exhibits up to a 25% increase in FSP with a 28.5 psi stress increase from its preinjection state by the year 2025. Tercio shows up to an 11% increase with a 21.6 psi change in stress, and Vermejo Park shows up to a 22% increase in FSP with a 24.1 psi change in stress. Our probabilistic models also show that in FSP models with higher uncertainty in geomechanical input parameters, the identified faults are more likely to fail at smaller stress changes. These results give us a better understanding of the faulting hazards in the basin, provide a more integrated picture of induced seismicity in the basin both past and present, as well as allow us to assess the potential for seismicity under future injection operations.