Can Foams Show Better Placement Properties than Gelants When Used as Blocking Agents?
(Yes, under limited circumstances.)
Placements
A considerable volume of theoretical, laboratory, and field work has been performed to evaluate the use of foams as mobility-control agents during steam and high-pressure gas floods (see Refs. 2-69 in Ref. 13). Much less work has been done to evaluate the use of foams as blocking agents. The distinction between a blocking agent and a mobility-control agent is an important concept to understand. A mobility-control agent should penetrate as much as possible into the less-permeable zones so that oil can be displaced from poorly swept zones. In contrast, we wish to minimize penetration of blocking agents into the less-permeable, oil-productive zones. Any blocking agent that enters the less-permeable zones can hinder subsequent injected fluids (e.g., water, CO2, steam) from entering and displacing oil from those zones.
Many field results demonstrate that foams usually act more effectively as mobility-control agents than as blocking agents. For example, in cases where vertical injection profiles were measured before, during, and after foam injection, the profiles were consistently improved during foam injection—demonstrating the ability of the low-mobility foams to shift flow from high-permeability zones into less-permeable zones.76-80 Also, when gas or water injection was resumed after foam injection, the profiles quickly reverted to profiles that were the same or worse than those observed before foam injection.76-81 This behavior is consistent with expectations for injection of a high-mobility fluid following a bank of low-mobility fluid in a heterogeneous system.44 This behavior is opposite to that desired for a blocking agent.
Nevertheless, in concept, several phenomena could allow foams to be superior to gels as blocking agents, in some circumstances. At present, these circumstances are hypothetical; very few conditions have been verified experimentally or in field applications. Details of our analyses of these circumstances are presented in Refs. 13, 28, and 82. In what follows, we summarize the findings of these analyses.
Two phenomena, the limiting capillary pressure82-85 (see Fig. 24) and the minimum pressure gradient for foam generation,76 could allow low-mobility foams to form in high-permeability zones but not in low-permeability zones. Exploiting these phenomena during foam placement requires that (1) under given reservoir conditions, a gas/liquid composition must be identified that will foam in high-permeability zones but not in low-permeability zones, (2) the foam must not easily collapse or wash out from the high-permeability zones, and (3) the aqueous phase must not contain a gelant or other reactive blocking agent.
An ideal placement could be realized if foam forms in the high-permeability zone but not in the low-permeability zone. In contrast, an extremely unfavorable placement results if foam forms in both zones. The foam can penetrate almost as far in the low-permeability zone as in he high-permeability zone. This behavior is expected for viscous fluids when free crossflow can occur.44,46 However, if crossflow cannot occur, the relative distance of penetration into the low-permeability zone is significantly greater than expected for simple viscous fluids.13 If gelant is included with the foam, a very undesirable placement results regardless of whether foam forms in the less-permeable zone.13
In cyclic steam projects, foam placement could be aided by gravity effects combined with very large mobility contrasts between the foam and the displaced oil. For cyclic steam injection projects where the foam was intended to act as a blocking agent, a common observation for successful field applications was that steam and oil flow after the foam treatment was diverted away from upper zones in favor of the middle or lower zones.86 These results suggest that gravity effects aided foam placement in the upper zones.
A circumstance where the presence of a preformed gel could aid placement of a foam can be inferred from the work of Craighead et al.87 During hydraulic fracturing, foamed gels show significantly lower leakoff rates than foams or foamed polymers.87 Logically, preformed foamed gels may propagate substantial distances along fractures with minimum leakoff. This argument parallels that given for injecting preformed gels into fractured systems.16,17 However, a potential advantage over ordinary gels is that the foamed gels may be more likely to extrude through fractures without developing excessive pressure gradients. This concept needs to be tested experimentally.
Permeability Reduction
Problems with foam propagation and stability present challenges for foam applications both as mobility-control agents and as blocking agents.13 In many cases, foam stability is significantly reduced by the presence of oil.88,89 Hypothetically, this phenomenon could be exploited to optimize the use of a foam blocking agent in oil production wells. When oil wells are returned to production after foam injection, foams could collapse more rapidly in oil zones than in water zones. This behavior is most likely to be exploitable if the water zones contain no residual oil. Foam washout from the water zones could be reduced by incorporating a polymer or gel into the foam. If a gelant is used, the foam must be produced from the oil zones before gelation occurs; otherwise, the oil zones could be damaged.13
Another potential advantage of foamed gels is that they may allow more control in achieving low or intermediate residual resistance factors.90 To explain, strong gels (without foam) can provide predictable and reproducible residual resistance factors because gelation in the porous medium is fairly complete.8 Because these gels fill most of the aqueous pore space,8 residual resistance factors are usually very high (103-106). However, we sometimes desire lower residual resistance factors (e.g., 1-100), that are associated with weak gels. Unfortunately, for the reasons mentioned earlier, weak gels provide low to intermediate residual resistance factors that are often unpredictable.8 If a foamed gel is used that incorporates a strong gel in the aqueous phase, the thin gel films that separate the gas bubbles should be formed reproducibly, and they may allow intermediate residual resistance factors to be attained more reliably. This concept also needs to be tested experimentally.
For foams, gas residual resistance factors can increase with increasing permeability.88 This behavior could be exploited when using foam as a gas blocking agent. A similar phenomenon has not been observed for water residual resistance factors in the presence of foam.88 Gels and foams are known to show different permeability reductions for different phases.27,88,89 Experimental work is needed to establish the permeability reduction properties of foamed polymers and foamed gels.91