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.