A new simulator for foam-acid diversion is described. The simulator explicitly accounts for the first time for the effects of gas trapping on gas mobility in foam and in liquid injected after foam, and for the effects of pressure gradient on gas trapping. The foam model fits steady-state foam behavior in both high- and low-quality flow regimes and steady-state liquid mobility after foam. Previously, laboratory experiments suggested that a relatively slow transition between steady states during foam and acid injection may control the diversion process in the field. The simulator fits this transition period in laboratory corefloods qualitatively with no additional adjustable parameters. A procedure for fitting the simulator parameters to laboratory data is described.

The dynamics in the transition period are complex. For instance, simulations indicate that most of the core experiences a period of drier flow at the start of liquid injection, due to expansion of gas already in the core. Simulations and new laboratory results suggest that a dead volume present upstream of the core in previous studies strongly affects the transition period seen in those experiments. Simulations and data agree that the transition is faster at higher pressure (with lower gas compressibility) and that response to a shut-in period depends on how much gas escapes during the shut-in - i.e., on how long the shut-in lasts.

Extended to radial flow, the simulator suggests that the transition period may not be so crucial to field application as at first appeared from laboratory corefloods. In the cases examined, injection-well pressure approaches its steady-state value within about 15 minutes or less of the start of liquid after foam.

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