Continuous Pressure and Temperature Monitoring in Lower Arbuckle Saline Aquifer in Wellington Field, Sumner County, Kansas - Response to the M5.8 Pawnee Earthquake

Since April 2016 a downhole P/T gauge has been continuously recording high-resolution data (1 /sec, 0.7 kP and 0.01 °C accuracy) in the lower Arbuckle saline aquifer at a depth of 1524 m in the Berexco Wellington KGS #1-28 well. The cased well was perforated and tested in August 2011 and has remained idle since. The wellsite located in south-central Kansas has experienced the introduction of numerous, low magnitude earthquakes since early 2015. The downhole pressure measured at the start of continuous monitoring in April 2016 was 216 kP above pressures measured five years before in August 2011. Pressure has risen at a rate of up to 4.1 kP /mo until 9-1-16, two-days prior to the Pawnee earthquake, after which time, long-term pressure has remained essentially level (mid October 2017). In addition, short-term, multiple, hours-long, asymmetric pressure spikes of up to 345 kP accompanied by temperature falls of 0.3 °C have been observed. The M5.8 Pawnee earthquake was detected by pressure oscillations (1.4 to 5.5 kP and 0.1 sec freq) coinciding closely with the larger amplitude portion of the earthquake's CODA. Oscillations are attributed local aquifer and wellbore effects from the wave train of the passing earthquake. The validation of a regional pressure field forecasted by simulation of brine disposal by Bidgoli et al. (2016) suggests hydrologic continuity of the regional Arbuckle aquifer and a possible explanation for the observed expansion of the seismicity linked to the pressure field. Short term spikes of pressure increases and temperature falls suggest cooler brine entering casing perforations that accompany the pressure increase. These short term anomalies are attributed to changes in disposal rates and casing pressure of wells in the local vicinity. The leveling off of pressure that occurred 2-days before the Pawnee earthquake may be just a coincidence, but could potentially have occurred either from moving brine from the Arbuckle into another formation or basement, or from a reduction in fluid disposal. Further work remains to compare pressure changes with 1) coincident seismic activity including swarming, 2) details of well disposal histories, and 3) other pressure monitoring in strategic locations to better understand this hydrogeologic system and the perturbations cause by large volume brine disposal.