DERIVING HYDRAULIC PROPERTIES FOR A CONFINED RESERVOIR FROM RESPONSES TO EARTH-TIDAL STRESS

Since the mid-20th century, water level fluctuations as a response to earth tides have been used to calculate aquifer properties. Because earth tides act as a cyclic stress they cause a strain or water level fluctuation that is a function of the time of year, tidal cycle, latitude of the well, properties of the fluid, and properties of the rock. The same “earth tide analysis” approach can be used to calculate rock properties in deep reservoirs where it is not practical to conduct a pumping or a slug test. Numerous pressure transducers were deployed, beginning in August 2016, in a deep reservoir in central- and north- central Oklahoma for a project titled “Monitoring and Analysis of Arbuckle Group Pressures”. The program was designed to measure downhole pressure in the area of interest for studying seismicity that is believed to be largely related to high rates of SWD into wells completed in the Arbuckle Group. Because pressure or fluid-level elevation is recorded every 30 seconds, time differential stress of the tides are expressed in the monitoring wells as a strain or fluid level fluctuation.

Several properties can be derived from the time series including specific storage, storage coefficient, transmissivity, porosity, matrix compressibility, hydraulic conductivity, and hydraulic diffusivity. Preliminary results indicate that the macroscale or completion interval scale property values derived from the observed earth tides differ substantially from values used in several previous studies. Because the previous studies focused on modeling with highly uncertain input parameters, these property values derived from our work will allow for more realistic predictions of subsurface fluid behavior, pore pressure diffusion, and other geomechanical and seismological processes.