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Basin-Scale Hydrogeological Modeling of the Fort Worth Basin Ellenburger Group for Pore Pressure Characterization


The Ellenburger Group in the Fort Worth Basin, north-central Texas, is the major target for disposal of produced water originating from the overlying Barnett Shale gas play. Ellenburger formations of Ordovician age consist of karstic platform carbonates with locally high injection potential, and commonly directly overly the Precambrian crystalline basement at 4500-15,000 ft depths. The thickness of the Ellenburger varies throughout the area of study from more than 4000 ft to less than 2000 ft decreasing toward the west and south. A few localities in the core area of the play have experienced seismic activity in the past decade. To better understand potentially induced seismicity and the relationship to oil and gas operations, a larger team have constructed, using the Petrel geocellular software, a 3D hydrogeological model of the Ellenburger Group in the Fort Worth Basin (30+ counties) with all available well log data, stratigraphic data (1200+ wells), petrophysical analyses (47 wells), faults from all possible sources including outcrops, controls on permeability anisotropy from outcrops and other data. The model is upscaled into 10+ layers and calibrated, using the CMG STARS software, with the help of injection pressure constraints while honoring injection volume history through 127 injection wells of the past decades. Major faults are implemented deterministically whereas fractures and minor faults, which considerably enhance the permeability of the carbonate system, are implemented stochastically and history-match the pressure data. Ellenburger formation water contributes to the water produced by the Barnett wells, which is then reinjected into the same formation through the injection wells. The recycled fraction is estimated at 10-30% of the total injected volume. This work in progress will ultimately provide basin-wide fluid budget analysis and pore pressure distribution in the Ellenburger formations. It will serve as a fundamental step to assess fault reactivation and basin-wide-seismogenic potential.