Leakage and Increasing Fluid Pressure Detected in Oklahoma's Wastewater Disposal Reservoir.

The Arbuckle Group is the principal reservoir used for wastewater disposal in Oklahoma. In Osage County—a seismically quiet part of the state—continuous measurements of fluid pressure reveal that pressure in the reservoir is increasing by at least 5 kPa annually and sometimes at a much higher rate. Tidal analysis reveals that fluid level changes lead the local strain tides, with no apparent influence from transient permeability changes; this indicates a response that is inconsistent with flow in a radially extensive, confined reservoir. We investigate whether this is due to vertical flow to the water table, vertical flow within the Arbuckle, or local distortions from fractures. While none of these alternative models can fully explain both the observed tidal phases and amplitude ratios, the observed response to teleseismic waves supports a mechanism related to leakage rather than fracture effects. At this location fluid influx associated with wastewater disposal is offset by migration into surrounding layers, which include the Precambrian basement below. Thus, our findings suggest the need to monitor for changes in the induced seismicity hazard, while pore pressures increase in a leaky disposal reservoir. Plain Language Summary: There is consensus among earthquake scientists that subsurface fluid pressure changes are the principal cause of the manmade (or "induced") earthquakes occurring in Oklahoma and around the globe. This is because greater fluid pressures reduce the clamping pressure that would otherwise prevent a fault from having an earthquake. The leading cause of pressure changes on faults in Oklahoma is the disposal of wastewater related to production of oil and natural gas. Disposing of wastewater is generally done by pumping fluid down boreholes into deep, highly permeable reservoirs, like the Arbuckle. One way for scientists to measure water pressure changes is by lowering specially designed instruments down unused wastewater disposal wells. The paper presents continuous measurements of fluid pressure at an Arbuckle well located in Osage County, Oklahoma. Our findings show an overall trend of fluid pressures increasing over time. The only conceivable source of this increase is due to the injection of wastewater. Furthermore, our findings show evidence that fluids are leaking out of the reservoir at a significant rate. Key Points: The tidal response of the Arbuckle reservoir in Oklahoma is inconsistent with confined, radial flow and shows evidence of leakageTime series show additional signals unrelated to tides and atmospheric pressure, including a robust teleseismic response and long‐term trendsFluid pressure in the Arbuckle is steadily increasing, presumably related to high volume wastewater disposal [ABSTRACT FROM AUTHOR]