Induced Seismicity in the Denver Basin Prompts Updated Basement Stress and Fault Configuration Model

Abstract

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 study the recent seismicity in the Denver Basin, the basement stress and fault configuration was revisited to identify zones in the basement where critically stressed faults are likely to occur. Conclusions of this study were derived from existing basement maps updated with interpretations of publically available geophysical surveys and well logs. A software application was developed to scrape the public dataset for 6-arm caliper logs and interpret orientations of borehole breakouts to determine the direction of maximum horizontal stress. The basement units and faults were updated using quantitative lineament and predictive unit filters; informed by basement well penetrations, to process the overlying structure, gravity and aeromagnetic data. By comparing the orientations of the mapped faults to the orientation of a projected maximum horizontal stress field, determined from borehole breakouts, we identified areas that are more likely to contain critically stressed basement faults and generate seismic events. This study identified a previously unmapped basement structure oriented parallel to the direction of the maximum horizontal stress in close proximity to the seismicity associated with deep water injection. The results of the study can be validated by comparing the faults identified to be parallel to maximum horizontal stress with the record of seismicity in the Denver Basin. All of the recorded seismicity within the Denver Basin has occurred within 10km of the identified fault zones. Results of this study will enable operators and regulators to site deep waste water disposal wells in a way that will reduce the risk of induced seismicity. Reduced induced seismicity can remove public pressure and avoid the possible 520% midrange cost increase of disposal by treatment. The economic limit of water cut can be a deciding factor in which petroleum plays are profitable. In addition, the tool developed to interpret the direction of maximum horizontal stress could have applications to improving drilling and completions efficiency in oil and gas operations.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017