Supporting Information for "Pressure monitoring of disposal reservoirs in North-Central Oklahoma: implications for seismicity and geostorage"
Underground wastewater injection into deep reservoirs confronts complex hydrogeologic conditions potentially leading to induced seismicity. We focus on the geologic units of the Arbuckle Group carbonates in Oklahoma that are frequently used for disposing of produced water associated with oil and gas production. Subsurface pressures and fluid flow figure prominently in most explanations for induced seismicity and are important in evaluations of future storage potential of the Arbuckle. To understand subsurface pressure conditions within the Arbuckle we monitored the water levels in 15 inactive wells. The wells were monitored at 30-second intervals, with eight wells monitored since September 2016, and an additional seven from July 2017. All of the wells were monitored until early March 2020. Since 2016, 13 of the 15 wells showed a net decrease in well level (a.k.a. hydraulic head), proportional to near-borehole fluid pressure. The pressure patterns observed in each well vary from one another, and some wells display a gradual decrease in pressure over time, some have a rapid decrease, and others show irregular changes. The well pressures respond to Earth tides and injections into nearby wells as well as response to distal and proximal seismic waves, though responses vary considerably between wells. Moreover, there appears to be a threshold injection rate above which pressure changes level off. The data illustrate that Arbuckle hydrogeology is a multi-scale, temporally dynamic system, with regional heterogeneity of porosity and permeability. These dynamics exert important controls on seismic hazard and storage capacity estimates.