Locating Microseismic Events Using Fat-Ray Double-Difference Tomography for Monitoring CO2 Injection at the Aneth EOR Field

Abstract:

During CO₂ injection, the increase in pore pressure and volume may change stress distribution in the field, and induce microseismic events as brittle failure on small faults or fractures. An accurate location of these induced microseismic events can help understand the migration of CO₂ and stress evolution in the reservoir. A geophone string spanning 800-1700 m in depth was cemented into a monitoring well at the Aneth oil field in Utah in 2007 for monitoring CO₂ injection for enhanced oil recovery (EOR). The monitoring continued till 2010. A total of 24 geophone levels recorded induced microseismic events, including 18 levels of three-component geophones and six vertical-component levels spaced 106.7 m (350 ft) apart to take full advantage of the entire array aperture. We apply a fat-ray double-difference tomography method to microseismic data acquired at the Aneth EOR field. We obtain high-precision locations of microseismic events and improve the velocity structure simultaneously. We demonstrate the improvements by comparing our results with those obtained using the conventional double-difference tomography.