Dan Shan's thesis
by
Dan Shan, MSE
University of Texas at Austin, 2001
Supervisor: William R. Rossen
Foam can be placed in the reservoir by continuous injection or surfactant-
alternating-gas (SAG) injection to improve sweep efficiency in gas injection enhanced-
oil-recovery (EOR) processes. For continuous foam injection, the existence of a low-
quality regime does not appear to alter the ability of foam to overcome gravity override.
Foams in low-quality regime fit the same correlation for overcoming gravity override
previously derived for foams in the high-quality regime. Therefore, overcoming gravity
override for continuous foam injection in both regimes requires increasing injection
pressure, possibly risking fracturing the formation. In contrast, a constant-injection-
pressure SAG process is a better way to control gravity override with an acceptable rise
in injection pressure.
Using a numerical simulator with a variety of foam models, we test injection
strategies for SAG processes with finite slugs of liquid and gas. Applications of this
simulator to 1D SAG process match predictions of fractional-flow methods very well.
Under some conditions, constant-rate SAG processes can maintain constant injection
pressure in a 1D cylindrical reservoir. Applications of this simulator to 2D SAG
processes show that the optimal injection strategy for both liquid and gas is injection of
large slugs at constant pressure, just below the fracturing pressure of the formation. This
approach both minimizes the time of injection and prevents gravity override. Sufficient
surfactant solution and sufficient pressure gradient are two necessary conditions for foam
to overcome gravity override. Foam cannot overcome gravity override beyond the front
of surfactant bank. Sufficient pressure gradient helps prevent slumping of liquid and
override of gas. During liquid injection, gravity draws liquid downwards from the top of
reservoir, but, remarkably, during subsequent injection of gas, the resulting pressure
gradient tend to push liquid back up to the top of the reservoir and maintain foam there.
Significant numerical artifacts appear in simulation results with coarse grids.
Therefore, numerical issues are closely monitored in this study.
Back to theses index
