Long-time diversion in surfactant-alternating-gas foam enhanced oil recovery from a field test

The ability of foam to divert gas flow during a long period of gas injection in a surfactant-alternating-gas (SAG) foam process is important for the economics of foam-diversion processes for enhanced oil recovery (EOR). Here, we interpret field data from the foam test in the Cusiana field in Colombia (Ocampo et al. 2013). In this test, surfactant was injected into a single layer that had been taking approximately half the injected gas before the test; then, gas injection resumed into all layers. On the basis of the size of the surfactant slug injected and estimates of adsorption and of water saturation in the foam in situ, we estimate that the treated region extended approximately 5.3 m from the injection well; fortunately, the results to follow are not sensitive to this estimate. On the basis of the change in injection logs before the test and at Day 5 of the test, when approximately 30 pore volumes (PVs) of gas (relative to the volume of the treated zone) had been injected, foam still reduced gas mobility in the treated layer to approximately 11% of its pretrial value. We base this estimate on the decrease of injection into the treated layer and the increase of injection into the other layers; the results are consistent among the layers. After 35 and 152 days of injection (220 and 1,250 treatment PV of gas injected), foam reduced gas mobility in the treated zone to approximately 26 and 50% of its value before the test, respectively. This result indicates that foam continued to reduce mobility by a modest amount even after long injection of gas. On the other hand, foam did weaken progressively as it dried out. Foam models in which foam remains strong at irreducible water saturation would greatly overestimate foam effectiveness at long times in this test. In this test, the large volume of gas had quickly penetrated far beyond the edge of the surfactant bank. Mobility in the foam-treated region in this test, after passage of many treatment PVs of gas injection, mimics that very near the injection well in a process with a larger slug of surfactant.