Modeling a Miscible Flood in the Prudhoe Bay Field
WITHERS, ROBERT J
A miscible gas injection pilot project was undertaken in the Prudhoe reservoir to evaluate the efficiency of water-after-gas (WAG) injection of miscible gas (MI). It was proposed to collect a repeat seismic survey to map the movement of the gas front in a plane between three wells.
Fluid saturations between the DS13-06 injector, DS13-96 monitor and DS13-05 producing wells were calculated at different times from a reservoir simulator using the production history. Reservoir velocities were generated using Gassmann's equation based upon the openhole wireline logs and knowledge of the reservoir fluid properties. The elastic properties of the rock matrix were determined from wireline logs and initial reservoir conditions. In the later stages of production, a composite fluid based on the saturations calculated by simulator was combined with the skeletal matrix for a time varying velocity computation.
The reservoir was modeled at three production stages. After initial production, the pressure was reduced and a light gas phase developed. Later cyclic injection of the heavier miscible gas and water increased gas saturation as the reservoir was swept. Velocities were found to increase locally by up to 164 f/s (50m/s) and decrease by 300 f/s (91 m/s). An irregular pattern of velocity and density changes resulted from differential mobility of the gas and water and to stratigraphic variability. It was shown that extremes in gas saturations between 30 percent and 60 percent were indistinguishable by surface or crosswell seismic methods. From the model study, it was concluded that using a repeat seismic survey to map the gas front would be futile and could not be economically justified.