NUMERICAL MODELING OF WASTEWATER INJECTION AND INDUCED SEISMICITY IN GREELEY, COLORADO

On June 1, 2014, an M3.2 earthquake occurred in Greeley, Colorado, a previously aseismic area, spurring the temporary deployment of seismometers to study the possible induced seismicity. Detected earthquakes centered near an Underground Injection Control (UIC) Class II injection well (NGL-C4A) that disposes of wastewater from oil and gas activities in the Denver-Julesburg (DJ) Basin. NGL-C4A had been injecting wastewater, into the Lyons through Fountain Formations (disposal zone), at high rates of up to ~360,000 barrels per month for less than a year prior to the June 1 earthquake.

We conduct site-specific numerical modeling to explore possible pore-pressure influence on induced earthquakes. The model captures the geological asymmetry of the DJ-Basin and accounts for injection history in NGL-C4A from April 2013 through February 2016. To determine the proper range of hydrogeological parameters for the injection interval, we analyze step rate injection tests conducted at multiple wastewater disposal wells injecting into the same disposal zone. Step rate injection tests are conducted on injection wells during the permitting process to determine the fracture pressure of the injection formations. The injection interval is isolated and fluid is injected with injection rate increasing throughout the test in a step-wise fashion. In addition, we conduct constant-head permeameter tests on samples of the disposal zone from cores collected near the injection site and outcrop samples. Constant-head permeameter testing is used on samples by water flowing through the sample chamber of the permeameter at a constant rate. The hydraulic conductivity is calculated using a variation of Darcy’s Law. The range of hydraulic conductivity of the disposal zone is on the order of 10^-9 – 10^-6 m/s. Modeling results suggest that pore-pressure increase from the wastewater injection well was in the range of values that is sufficient to induce seismicity on pre-existing faults.