STATISTICAL ANALYSIS OF THE ARBUCKLE PRESSURE AND STATIC FLUID LEVEL

UIC (Underground Injection Control) Class I wells are used to dispose of industrial hazardous and non-hazardous waste into deep, confined saline aquifers. UIC Class II wells inject saltwater produced from oil-bearing reservoirs or unconventional developments. In Kansas and Oklahoma, both Class I and Class II wells dispose large volumes of wastewater, principally brines, into the Arbuckle Group aquifer, some at high injection rates. High injection rates have been linked with an increase in seismicity in the region. Annual fall-off tests are mandated for monitoring Class I wells in Kansas, requiring operators to report the well static fluid level (SFL) and pressure. The recorded pressures during fall-off tests can be extrapolated, using Horner analysis, to obtain a so called Horner false pressure, , linearly related to and often used instead of average reservoir pressure within the radius of investigation, the distance the pressure transient effect has traveled. 49 Class I wells in Kansas provide a unique dataset on reservoir pressures and SFLs. Here we combine these data with wastewater injection volumes from Class I and II wells to investigate temporal and spatial relationships between injection volumes and rates, pressures, and SFLs through bivariate (Pearson) correlation analysis. The results show that both Arbuckle reservoir pressures and SFLs are increasing. Several parameters are hypothesized to influence the pressure and SFL in the Arbuckle such as injection rate and volume of injection within a distance from the well. The aggregated results show that increases in SFL can be explained by the volume of fluid injected into the well, while the increase in well pressure is best explained by the cumulative volume of fluid injected in the vicinity of well (ca. 30 km distance). This study also offers models for the annual rate of increase in pressure and SFL under current injection rates. Though the thickness, lateral extent, and high permeability of the Arbuckle Group is expected to dampen any pressure change caused by injection rates; the observed increases in recorded pressure and SFL may explain the recent increase in seismicity across the midcontinent. Future efforts will focus on developing approaches and strategies that can be used by stakeholders for mitigation of seismicity and fluid-level increases.