Coreflood Study of Surfactant-Alternating-Gas Foam Processes: Implications for Field Design

Abstract

The success of surfactant-alternating-gas (SAG) foam processes depends on foam behavior at very low liquid fractional flow fw - i.e., on whether and how foam breaks as it dries out during gas injection. It is difficult to conduct steady-state corefloods at extremely low fw, however. Therefore simulation studies of foam SAG processes rely on extrapolating steady-state data taken at much higher values of fw. Unfortunately, grossly different results are predicted depending on how these data are extrapolated. This Berea coreflood study of SAG foam processes uses a unique coreflood apparatus in which both water saturation Sw and capillary pressure Pc are measured throughout the flood. Steady-state data were obtained to as low a value of fw as feasible, followed by continuous gas injection. Because Sw and pressure drop were measured during this period, the fractional-flow function could 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 capillary pressure. Between fw = 0.008 and 0.002, foam weakened sharply, and, remarkably, reimbibition occurred: Sw rose and Pc fell as fw decreased. This suggests that the 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" state reported 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 are discussed. In particular, we illustrate how to extrapolate from a properly designed coreflood to field scale 1D displacement using fractional-flow methods. Introduction As applied to gas injection improved-oil-recovery (IOR) processes, foam has the 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/or reducing handling costs associated with large gas throughput (Schramm, 1994;Rossen, 1995). The foam may be administered by continuous coinjection of gas and surfactant solution, or by injecting a slug of surfactant solution followed by injection of a gas slug. The latter strategy is known as a surfactant-alternating-gas (SAG) process. SAG injection has certain advantages over continuous foam injection in foam IOR processes. SAG injection minimizes contact between gas and water in the injection facilities, which can help reduce corrosion. The alternating imbibition/drainage cycles in SAG injection can help to create foam in the reservoir (Rossen and Gauglitz, 1990; Chou, 1991). Finally, SAG injection can achieve the paradoxical benefits of high injectivity and low mobility at the displacement front (Rossen et al., 1995). High injectivity results as foam near the well dries out, weakens and collapses, while stronger, wetter foam further from the well maintains mobility control. Recent simulation results (Shi, 1996)show that SAG processes can overcome gravity override with less rise in injection-well pressure than is possible with continuous foam injection. The benefits of SAG foam injection are made clear by fractional-flow analysis. In fractional-flow methods (Buckley and Leverett, 1941; Helfferichand Klein, 1970; Lake, 1989; Rossen et al. 1994; Zhou and Rossen, 1995), one constructs wave diagrams for the displacement process from fractional- flow curves and initial and injection conditions. Saturation waves move with dimensionless velocities equal to the slope of the fractional-flow curve at the given saturations. P. 567^

Mechanistic Studies of Improved Foam Eor Processes

“…Red corresponds to high tracer concentration, dark blue to low concentration. Kibodeaux and Rossen (1997) including shock (incorrectly) suggested by them for SAG displacement. .…”

“…18 is that here there is a higher P c (S w ) function for strong foam than no-foam; this introduces an additional foam bank into the displacement. Kibodeaux and Rossen (1997) (cf. Fig.…”

“…. Revised construction of shock for gas injection based on fractional-flow data of Kibodeaux and Rossen (1997) (cf. Fig.…”

“…In particular, because in a strong foam some water is occupied in separating the gas bubbles, there is less water to occupy narrow pores; as a result, one would expect that at the same water saturation S w , P c (S w ) would be higher in a strong foam than a coarse foam. There is some evidence for this (Khatib et al, 1988;Kibodeaux and Rossen, 1997), but it is not clear how large an effect this would have in consolidated porous media.…”

Mechanistic Studies of Improved Foam Eor Processes

“…A novel apparatus, similar to that in Kibodeaux and Rossen (1997), monitors average water saturation in a core, moment-by-moment, by weighing the core. Our experiments find that water saturation increases more during liquid injection than previously conjectured -in other words, less gas is trapped by liquid injection than previously thought (cf.…”