Summary

In the past two years, Kansas has seen a two-orders-ofmagnitude increase in the rate of seismic activity in the southcentral part of the state relative to nearly forty years of instrumented monitoring. The elevated seismicity follows a recent rise in oil and gas production and associated disposal of produced formation water. The strong spatial and temporal correlation between disposed fluid volumes, injection well locations and disposal interval, and earthquake epicenters suggests that the recent seismic activity may be induced. Key to informed regulatory decisions are locations of faults capable of generating earthquakes. We propose a zoned model for fluid injection to mitigate seismic hazard in areas with multiple injection wells. Earthquake clusters were identified using the Seismic Action Score, a formula intended to establish a quantitative means to potentially discriminate induced earthquakes from natural earthquakes. Statistical declustering was performed to focus related epicenters. Linear patterns were identified that were consistent with other geologic and geophysical lineaments that have been interpreted as possibly related to faults or fractures. The state regulatory body for oil and gas operations enforced a systematic decrease in injection volumes within a relatively small footprint surrounding the proposed faults to address the increased seismic risk. This geologically-based model allows operators and regulators to establish a zoned approach to mitigate seismic risk from multiple injection wells injecting into seismically sensitive areas and avoid failure of a nearby fault.

Introduction

The state of Kansas has a history of relatively low seismic activity, with only one earthquake of magnitude (M) 3 or greater occurring approximately every two to three years. In 2014, over 40 such earthquakes were detected in a twocounty area in south-central Kansas with very little historic seismic activity. The increase in seismicity occurred shortly after a significant increase in the volume of brine injected in saltwater disposal wells in the affected counties. For decades, it has been recognized that fluid disposal can induce seismic activity (Nicholson and Wesson, 1990). The first widely accepted case of induced seismicity occurred in the 1960s at Rocky Mountain Arsenal near Denver, Colorado. Wastewater disposal in a deep well caused over 1500 earthquakes in six years, the largest of which—originally estimated as a magnitude 5.0 to 5.5—occurred over a year after injection was terminated (Healy et al., 1968). Fluid injection raises pore pressure within the injection interval and at any fault or fracture in hydraulic communication with the injection interval. Increased pore pressure reduces the effective stress (i.e. the frictional resistance) on the fault (Hubbert and Rubey, 1959) increasing the potential for an earthquake to occur (Nicholson and Wesson, 1990). Many confirmed cases of induced seismicity directly link seismic activity to fluid injection in a single nearby well (e.g. Seeber et al., 2004; Ake et al., 2005; Frohlich et al., 2011).

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