Accelerated Fill‐Up of the Arbuckle Group Aquifer and Links to U.S. Midcontinent Seismicity

Abstract

The Arbuckle Group aquifer is the principal disposal zone for oil and gas field brines and hazardous/nonhazardous wastewater across the U.S. midcontinent and is traditionally viewed as an infinite capacity aquifer. Thousands of wells annually dispose hundreds of millions of barrels of wastewater into the aquifer across Kansas and Oklahoma, but direct links between injection and recent increases in seismicity have been hindered by a lack of pressure data for the Arbuckle Group. Here we present a newly compiled data set for 49 wells across Kansas that provides a unique perspective on the aquifer's performance over two decades. Statistical analysis of falloff test pressures, static fluid levels, and injection volumes shows that Arbuckle pressures and fluid levels are rising, recently at faster rates, likely associated with increased wastewater injection. The new data also suggest that the pressure diffusion, the primary driver of induced seismicity, can reach distances up to 25 km from an injection point and is connected to static fluid level rises. The compiled dataset explains the recent surge in midcontinent seismicity. The data set also suggests that the Arbuckle has finite storage capacity and that wastewater disposal across parts of the midcontinent may soon require alternatives.

Comment on “Accelerated Fill‐Up of the Arbuckle Group Aquifer and Links to U.S. Midcontinent Seismicity” by Ansari et al. (2019)

“…distance can be interpreted as evidence that pore pressure diffusion is limited to within 25 km of the injection point, as suggested by Ansari et al (2019).…”

“…We are grateful to Brownie Wilson, Mike Killion, and Mary Brohammer for their assistance in preparing maps and illustrations and to Julie Tollefson for editing the manuscript. P* and disposal volumes for Kansas wells were reported by Ansari et al (2019).…”

“…Furthermore, the results are most likely influenced by a screening effect— the tendency for nearby observations to reduce the influence of more distant observations (Chilès & Delfiner, 1999; Journel & Huijbregts, 1978). Consequently, we question whether the leveling off of correlation with increased distance can be interpreted as evidence that pore pressure diffusion is limited to within 25 km of the injection point, as suggested by Ansari et al (2019).…”

Reply to Comment by Peterie Et Al. on “Accelerated Fill‐Up of the Arbuckle Group Aquifer and Links to U.S. Midcontinent Seismicity”

“…The consistency of the pressure increases suggests that changes are likely governed by the cumulative injection into the wells or in close proximity to them. Other wells in central Kanas show similar patterns (e.g., Wells KS-01-097-001 and KS-01-113-001; see supplementary materials in Ansari et al, 2019), calling into question far-field injection as the cause of observed pressure increases in these wells.…”

“…The noted gauge-depth changes typically occur in only one or two of the reported years, and for most wells, the documented changes are small (<30 m) relative to the depth of the Arbuckle Group in these wells. Analysis of the gauge-depth data indicates that a maximum of 15 fall-off tests out of 715 tests used in Ansari et al (2019) may have had influential depth changes (see supporting information), and these 15 data points are scattered on 18 years' worth of data (see Figure 6 of Ansari et al, 2019).…”