Interpreting Reservoir Microseismicity Detected During CO2 Injection at the Aneth Oil Field

Microseismic monitoring is expected to be a useful tool in CO2 sequestration projects for mapping pressure fronts and detecting fault activation and potential leakage paths. Downhole microseismic monitoring and several other techniques are being tested for their efficacy in tracking movement and containment of CO2 injected at the Aneth oil field located in San Juan County, Utah. The Southwest Regional Partnership on CO2 Sequestration is conducting the monitoring activities in collaboration with Resolute Natural Resources Company, under the support of the U.S. Department of Energy’s National Energy Technology Laboratory. The CO2 injection at Aneth is associated with a field-wide enhanced oil recovery operation following decades of pressure maintenance and oil recovery by water-flood injection. A 60-level geophone string was cemented into a monitoring well equipped with both 3-component and vertical component geophones spanning from 800 to 1700 m depth. The top of the oil reservoir in the study area is at approximately 1730 m depth. Over the first year of monitoring, approximately 3800 microearthquakes have been detected within about 3 km of the geophone string. The Aneth reservoir events are relatively large with magnitudes ranging from approximately -1 to 1. For comparison, reservoir seismicity induced during hydraulic fracturing treatments typically result in events with magnitudes <-1, unless pre-existing faults are pressurized by the treatments. The Aneth events delineate two NW-SE oriented fracture zones located on opposite flanks of the reservoir. Injection activity is fairly uniform over the entire field area, and the microseismicity does not correlate either temporally or spatially with any anomalous changes in injection or production activities near the source locations. Because the activity is fairly isolated and relatively energetic, I speculate that the seismicity may be due to critically stressed structures driven by longer-term production- and/or injection-induced stress changes. Ongoing analysis includes extracting precise arrival time to improve relative source locations and looking for correlations of event occurrence and moment release with field-wide rates of injection and production.