Coreflood Study of Surfactant-Alternating-Gas Foam Processes: Implications for Field Design
- K.R. Kibodeaux (The University of Texas at Austin) | W.R. Rossen (The University of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Western Regional Meeting, 25-27 June, Long Beach, California
- Publication Date
- Document Type
- Conference Paper
- 1997. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 4.3.4 Scale, 5.3.2 Multiphase Flow, 5.4.2 Gas Injection Methods, 2.5.2 Fracturing Materials (Fluids, Proppant)
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The success of surfactant-alternating-gas (SAG) foam processes depends onfoam behavior at very low liquid fractional flow fw - i.e., on whether and howfoam breaks as it dries out during gas injection. It is difficult to conductsteady-state corefloods at extremely low fw, however. Therefore simulationstudies of foam SAG processes rely on extrapolating steady-state data taken atmuch higher values of fw. Unfortunately, grossly different results arepredicted depending on how these data are extrapolated.
This Berea coreflood study of SAG foam processes uses a unique corefloodapparatus in which both water saturation Sw and capillary pressure Pc aremeasured throughout the flood. Steady-state data were obtained to as low avalue of fw as feasible, followed by continuous gas injection. Because Sw andpressure drop were measured during this period, the fractional-flow functioncould be extrapolated below the range in fw of the steady-state data.
During steady-state foam injection, for fw >0.01, foam weakened and Swfell as fw decreased. Foam was stable to a remarkably high value of capillarypressure. Between fw = 0.008 and 0.002, foam weakened sharply, and, remarkably,reimbibition occurred: Sw rose and Pc fell as fw decreased. This suggests thatthe fractional-flow curve for foam is not single-valued. Stated differently,there are at least two steady states, a "strong foam" and "weak foam" state,over a range of water saturations, and the behavior observed depends oninjection history. This sudden foam collapse and reversion to the "weak foam"behavior under dry conditions mirrors the sudden jump to "strong foam" statereported in studies of foam generation. For lower fw and during gas injection,Sw again declined as fw declined.
Implications of these results for field design of foam SAG processes arediscussed. In particular, we illustrate how to extrapolate from a properlydesigned coreflood to field scale 1D displacement using fractional-flowmethods.
As applied to gas injection improved-oil-recovery (IOR) processes, foam hasthe potential to relieve several common problems through better areal sweep,better vertical sweep (less gravity override), less viscous fingering,diversion of gas from higher-permeability (or previously-swept) layers, and/orreducing handling costs associated with large gas throughput (Schramm, 1994;Rossen, 1995). The foam may be administered by continuous coinjection of gasand surfactant solution, or by injecting a slug of surfactant solution followedby injection of a gas slug. The latter strategy is known as asurfactant-alternating-gas (SAG) process.
SAG injection has certain advantages over continuous foam injection in foamIOR processes. SAG injection minimizes contact between gas and water in theinjection facilities, which can help reduce corrosion. The alternatingimbibition/drainage cycles in SAG injection can help to create foam in thereservoir (Rossen and Gauglitz, 1990; Chou, 1991). Finally, SAG injection canachieve the paradoxical benefits of high injectivity and low mobility at thedisplacement front (Rossen et al., 1995). High injectivity results as foam nearthe well dries out, weakens and collapses, while stronger, wetter foam furtherfrom the well maintains mobility control. Recent simulation results (Shi, 1996)show that SAG processes can overcome gravity override with less rise ininjection-well pressure than is possible with continuous foam injection.
The benefits of SAG foam injection are made clear by fractional-flowanalysis. In fractional-flow methods (Buckley and Leverett, 1941; Helfferichand Klein, 1970; Lake, 1989; Rossen et al. 1994; Zhou and Rossen, 1995), oneconstructs wave diagrams for the displacement process from fractional- flowcurves and initial and injection conditions. Saturation waves move withdimensionless velocities equal to the slope of the fractional-flow curve at thegiven saturations. P. 567^
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