Experimental and Theoretical Studies of Foam Mechanisms in Enhanced Oil Recovery and Matrix Acidization Applications

by
Kenneth Richard Kibodeaux, Ph.D.

University of Texas at Austin, 1997
Supervisor: William R. Rossen

The presence of foam can greatly lower the mobility of gas in porous media, and foam is often used in enhanced oil recovery (EOR) and matrix acidization projects. Under some conditions foam may cause a large reduction in mobility (a "strong" foam), while under different conditions the foam may be "weak."

Since CO₂ foam is often found to be weak in laboratory corefloods, and N₂ strong, weak foam mechanisms were studied by comparing possible causes for CO₂ foam's weakness relative to N₂ foam. The two primary forces that destabilize a foam lamella, capillary pressure and the van der Waals component of the disjoining pressure, both appear smaller for CO₂ than N₂. Also, new experiments suggest CO₂ foam's low pH, like other possible causes studied, is not responsible for making CO₂ foam weak.

Previous modeling of foam diversion in matrix acidization required using parameter values extrapolated from scant data. New experiments provide relevant data while confirming the basic form of the previous model. An unexpected weakening of foam was discovered at later times, attributable to gas dissolution into the injected liquid. A model was formulated which estimates local phase mobilities and saturations during foam corefloods using a plausible relative-permeability function and sectional pressure gradient data, including effects of gas expansion.

New modeling efforts using fractional-flow theory reveal that a SAG (Surfactant-Alternating-Gas) injection strategy can combine high injectivity with low mobility at the front of the foam bank to help stabilize the displacement. Water is quickly displaced from the near wellbore region, weakening the foam there. Since near-wellbore behavior weighs heavily in ultimate injectivity, high injectivity is promoted. It is also demonstrated that in some cases a weak foam (as defined by conventional steady-state corefloods) can produce a larger, longer-lived increase in injection pressure in a SAG process than a stronger foam.

SAG effectiveness depends on foam behavior at low values of f_w, and new experiments are run in this range while measuring S_w and p_c. Results indicate a successful process, and reveal surprising foam behavior as the limiting capillary pressure is exceeded.

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