INDUCED SEISMICITY IN THE DENVER BASIN PROMPTS UPDATED BASEMENT FAULT CONFIGURATION MODEL

From June 2014 to January 2016 a swarm of earthquakes occurred in central Weld County Colorado in close proximity to two injection wells. The swarm occurred in an area with no known prior record of seismic activity. The earthquake hypocenters had a median depth of 5,000 m suggesting seismicity was occurring in the crystalline basement, but the well was injecting fluid into the overlying sedimentary section.

To test for basement communication, spinner surveys were collected from the deeper of the two wells. The spinner survey indicated that injected fluids were flowing the entire 3,297 meters of the well. The well was plugged back 143 meters and the spinner test was repeated indicating that the majority fluids were then entering units above 3,000 meters TVD. A reduction in the rate of earthquake events suggests that the plug-back reduced or removed basement communication.

Using gravity, aeromagnetics, and interpretations of overlying structure from well penetrations, basement fault configurations were mapped. Measurements of borehole breakouts and earthquake focal plane solutions were analyzed to understand the local stress regime and identify the orientations of faults that will be most likely to slip.

This study of the Denver Basin earthquake swarm in relation to nearby injection wells in conjunction with interpretations of the local stress regime and the basement fault configuration will enable regulators to locate injection wells safe distances from faults and better mitigate the risk and hazards from induced seismicity in the basin.