CO2 Foam With Surfactants Used Below Their Critical Micelle Concentrations
- M.I. Kuhlman (Shell Development Co.) | A.M. Falls (Shell Development Co.) | S.K. Hara (Shell Development Co.) | T.G. Monger-McClure (Shell Development Co.) | J.K. Borchardt (Shell Development Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1992
- Document Type
- Journal Paper
- 445 - 452
- 1992. Society of Petroleum Engineers
- 2.5.2 Fracturing Materials (Fluids, Proppant), 5.4.3 Gas Cycling, 5.4.1 Waterflooding, 4.1.2 Separation and Treating, 5.4.2 Gas Injection Methods, 2.4.3 Sand/Solids Control, 4.1.5 Processing Equipment, 5.6.5 Tracers, 1.6.9 Coring, Fishing, 5.2.1 Phase Behavior and PVT Measurements, 1.2.3 Rock properties, 3.3 Well & Reservoir Surveillance and Monitoring, 5.4 Enhanced Recovery, 5.3.2 Multiphase Flow, 5.4.6 Thermal Methods
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Using surfactants below their critical micelle concentrations, C CMC, toreduce gas mobility throughout a reservoir can be advantageous. When used inreservoir brines below their C CMC inexpensive anionic surfactants propagatethrough rock with little delay owing to adsorption. Yet gas mobility is reducedbelow that of a waterflood in either surfactant-water-alternating-gas (SWAG) orcoinjection experiments. Moreover, oil recovery is higher than in processeswithout surfactant. In contrast, at concentrations above the C CMC, surfactantadsorption is much higher and the injected surfactant concentration can begreatly delayed. In addition, CO2 mobility can be reduced so far below that ofa waterflood that injectivity suffers. Moreover, oil recovery can be reducedbecause oil is emulsified and its mobility is lowered. This type of foam shouldbe considered only for near-wellbore fluid diversions.
The viscosity and density of miscible gases such as CO2 are lower than thoseof the fluids they displace. This can cause fingering or channeling throughhigh-permeability streaks or gravity override if no vertical permeabilitybarriers exist. Foam has been suggested to correct these shortcomings. Threeproblems have limited the application of foam to near-wellbore fluid diversion.The first, surfactant adsorption limits foam propagation. The second is thelarge pressure gradient observed in some foams. Laboratory pressure gradientscan be 10 to 100 psi/ft. However, gradients larger than 1 to 2 psi/ft are notpractical in reservoir-wide applications because injectivity can practical inreservoir-wide applications because injectivity can suffer. The third problemis that oil recovery decreases when large mobility reductions are achieved.This problem occurs because CO2 foam is initially an ensemble of oil dropletsspreading around CO2 bubbles and is durable only when a CO2/oil/water emulsionis viscous. These three problems could be corrected if a surfactant thatexhibited low adsorption was a weak foamer and emulsifier yet reduced gasmobility sufficiently.
Surfactant Propagation Theory. A material balance on surfactant indimensionless form is
where xD and tD are dimensionless length and PV throughput, respectively. Inthis analysis, water is the only fluid present; Cf, the normalizedconcentration of surfactant in the aqueous phase, is Cs,w/C CMC; C is the totalamount of surfactant in the phase, is Cs,w/C CMC; C is the total amount ofsurfactant in the rock, [Cf + (1 - )/ /C CMC; and is the adsorption. The C CMCis the concentration at which surfactant micelles begin to form. A micelle is asmall group of surfactant molecules (Fig. 1). From coherence theory, for aconstant C, dCf = 0. Because no oil is present, Eq. 1 reduces to
Eq. 3 shows that the velocity of a flowing concentration of surfactantdecreases when the slope of the surfactant adsorption isotherm ( / Cf)increases.
Surfactant Adsorption Data. Consider the case of a limited amount ofadsorption data, which usually are fit with the Langmuir isotherm [ = ACf/(1 +BCf)]. The slope of this isotherm decreases with increasing concentration.Consequently, from Eq. 3, the injected concentration propagates faster than anyother; thus, only the injected concentration propagates. For many surfactants,only high concentrations should propagate quickly enough to be useful. A foammade with a high concentration of surfactant, however, can be viscous andimpair injectivity. Moreover, surfactant cost becomes prohibitive. Thus, ifadsorption isotherms were Langmuirian, CO2 prohibitive. Thus, if adsorptionisotherms were Langmuirian, CO2 foam probably could not be designed forreservoir-wide application. From previously published data, there appears to bea solution to these problems. First, detailed surfactant adsorption isothermsdeviate significantly from the Langmuir isotherm below the C CMC. Moreover,surfactants can propagate with little delay at low concentrations, and very lowconcentrations of surfactant can reduce gas mobility. The representativeadsorption data of Scamehorn et al. and Somasundaran et al. are compared inFig. 1 with the Langimuir fit through the first few data points below the CCMC. Adsorption below the C CMC is much less than, and has a larger slope than,the Langmuir fit. Above the C CMC, a micelle is in equilibrium with a bilayerof surfactant on the surface of the rock. Below the C CMC, surfactant exists asisolated monomers. These are in equilibrium with isolated patches of surfactantadsorbed on the rock and at fluid/fluid interfaces. and foam stability areconsequently lower.
Implications of Non-Langmuirian Adsorption Data. Fig. 2 is an example of thesurfactant effluent concentration calculated from Eq. 3 and the adsorptionisotherm fit with cubic splines to Somasundaran et al.'s data in Fig. 1. Theproduction history is calculated for a surfactant injected at 20 times its CCMC of 0.01 meq/L (3.4 ppm). Adsorption at the C CMC is 0.17 mg/g and porosityis 30%. A low concentration, Cfl, propagates quickly. porosity is 30%. A lowconcentration, Cfl, propagates quickly. The injected concentration, Cfo, isdelayed over 4 PV. Thus, when isotherms are more detailed than that usually fitwith the Langmuir isotherm, a dilute concentration can propagate with highervelocity than the injected concentration. These clues suggest that researchersusing the Langmuir isotherm did not expect the surfactant to propagate veryfar. Yet low concentrations could have propagated and reduced gas mobility.
Mobility Control With Dilute Surfactants. This analysis suggests that asurfactant with low adsorption should have a high C CMC and low adsorption atits C CMC and be a weak foamer. If this hypothesis is true, then reservoir-widefoam mobility control may be possible. Fig. 3 is an example of the surfactanteffluent concentration of a hypothetical surfactant with C CMC = 2 meq/L (600ppm) and adsorption at the C CMC = 0.06 mg/g. Cfo is one-half the surfactant'sC CMC. Surfactant production begins after a 0.02-PV delay. Cfo is reached inanother PV. Gas mobility reduction with dilute, weakly foaming surfactantsolutions has been reported. Long ago, Raza used 10 ppm of surfactant. Sanchezet al. reported that 2.6 and 10 ppm of Suntech IV(TM) (C CMC = 1,540 ppm)reduced gas mobility by factors of 40 and 100. Recently, Yang and Reed reportedusing the weak foamer Dowfax XDS 8390 (TM) at 1,000 ppm (C CMC = 1,780ppm).
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