--> Induced Seismicity in Oklahoma: An Update

AAPG ACE 2018

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Induced Seismicity in Oklahoma: An Update

Abstract

The frequency of earthquakes in Oklahoma peaked in 2015, with 903 M3.0+ earthquakes, declined to 623 in 2016, and is declining further in 2017, to a projected 300-350 M3.0+ earthquakes for 2017 (as of October). However, seismic moment peaked in 2016, with three earthquakes of M5.0+, unprecedented in Oklahoma’s recorded history. Seismic energy release is distinctly lower in 2017, a remarkable seismic lull in January and February has been followed by a return to levels similar to late 2016.

Rising seismicity has been attributed to greatly increased disposal of saline formation water from high water cut wells into underpressured and relatively permeable Arbuckle Group sedimentary rocks overlying Precambrian crystalline basement (for example, Walsh and Zoback, 2015). Pressure communication from the Arbuckle Group to faults in the basement is interpreted to have reduced stress on favorably aligned faults. More recently, poroelastic effects have been interpreted to have played a role as well (Barbour et al. 2017).

Reduction in frequency results from a decrease of >1,000,000 barrels per day of disposal in ~700 Arbuckle disposal wells in the seismic Area of Interest (AOI). Reductions in disposal occurred due to 1) reduced production driven by a significant oil price drop since 2014, and 2) shut in or reduced injection directed by the Oklahoma Corporation Commission. A recent model suggests that seismicity will decline toward background levels in a few years (Langenbruch and Zoback, 2017a), although these results have been debated (Goebel and others, 2017; Langenbruch and Zoback, 2017b). It remains uncertain whether the recent flattening/increase lies outside these projections.

At the same time, a second class of earthquakes has been identified outside the main Area of Interest that are associated in location and time with oil and gas well completion activities. These are generally smaller, less frequent, and readily mitigated by operator actions.

A recent issue of Seismological Research Letters highlights diverse new research results on the September 3, 2016 M5.8 Pawnee earthquake, the largest recorded in Oklahoma (Chen and Norikata, eds. 2017). This talk will discuss these and other recent research results on the nature of seismicity in Oklahoma. It will describe the evolution of Oklahoma seismicity and the regulatory actions taken to reduce it. It will also describe recent investigations of regional variation in earthquake frequency within the AOI.