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The Influence of Wastewater Injection Wells on Induced Seismicity in the Denver Basin Combined Disposal Zone, Weld County, Northeast Colorado


Colorado is a large producer of oil and natural gas, and uses UIC Class II wastewater injection wells to handle produced water throughout northeast Colorado, specifically in Weld County. In June 2014, an M3.2 earthquake occurred in Greeley, CO, a previously aseismic area in Weld County. Following the earthquake, University of Colorado Boulder installed a seismic array in the area near disposal well NGL-C4A, the wastewater injection well closest to the June 2014 event, to record the seismicity; the array was extended during summer 2016. The seismic monitoring indicates the area has experienced small earthquakes intermittently from 2013-2016. NGL-C4A and an additional 18 wastewater injection wells within a 30 km radius of NGL-C4A inject into a part or the entire Denver Basin combined disposal zone. The combined disposal zone includes the Lyons Formation, a sandstone unit, and extends to the Fountain Formation, an arkose unit just above the crystalline basement. In this study, we conduct site-specific numerical modeling to explore the influence of pore pressure changes on seismicity. We include the asymmetry of the basin in the model and use hydraulic conductivity values (10-10 to 10-6 m/s) for the disposal zone, determined from constant head permeameter tests on disposal zone rocks and re-analysis of step rate tests conducted at disposal wells in the area. Constant-head permeameter analysis tests samples by flowing water through the sample chamber of the permeameter at a constant rate. The hydraulic conductivity is then calculated using a variation of Darcy's Law. Incorporating these results, we model both the influence of injection at NGL-C4A alone and the influence of injection from all wastewater injection wells within the 30 km radius on pore pressure. We use the model to understand the pore pressure changes through time, the influence of pore pressure changes on seismicity locations, and the interactions between wells. This case study can be used to refine well spacing and injection rate constraints in relation to the occurrence of induced seismicity.