Natural Hydraulic Fracturing, Fluid Pressure, and Saltwater Disposal in the Arbuckle: Implications for Induced Seismicity

Although petroleum has been produced in the midcontinent for more than a century, water management by the industry has only recently been in the foreground. After separating water from oil and gas at the wellhead, producers are left with co-produced brine that is typically disposed of via saltwater disposal (SWD) wells. The Cambrian to Ordovician age Arbuckle Group is the primary zone for SWD in the midcontinent. Recent research has suggested a link between fluid injection and seismicity in the midcontinent; however, mechanisms for transferring pore pressure from zones of fluid injection to critically-stressed faults haven’t been well documented. The objectives of this research were to explore geologic processes that resulted in natural hydraulic fracturing of the Arbuckle, and to describe recent spatial and temporal variability of fluid pressures in the Arbuckle.

It is believed that accumulation and loading from rapid sedimentation caused abnormally high fluid pressures (i.e., fluid pressures above hydrostatic) in the Arbuckle when the midcontinent was part of an inland seaway. Other overpressuring processes such as hydrocarbon generation, diagenic dehydration, and aquathermal pressuring followed and caused fluid pressures within the sealed Arbuckle to exceed lithostatic pressures and result in natural hydraulic fracturing. Subsequently, abnormally low fluid pressures (i.e., fluid pressures below hydrostatic) resulted from uplift, erosion, and unloading of the same geologic materials that caused natural hydraulic fracturing. Present-day pressures in the Arbuckle, as a function of depth, in comparison to a pressure gradient of 0.456 psi/ft indicate that the Arbuckle is widely underpressured. Preliminary data also shows that pressure distribution varies with time and local zones of increasing and overpressuring are beginning to emerge. Follow-up research will examine the process of pore pressure diffusion from SWD wells and be integrated with seismological data to develop SWD best management practices for reduced risk for seismicity.