Oil bank formation

Oil Bank Formation. If a surfactant or surfactant-forming material is injected into a reservoir and mobilizes residual oil, then oil recovery is more efficient if the mobilized oil droplets can coalesce to form an oil bank. 

In oil recovery processes, the formation of an oil bank is very important for an efficient oil displacement process in porous media. This was established from studies on the injection of an artificial oil bank followed by the surfactant formulation which can produce ultralow interfacial tension with the injected oil. We observed that the oil recovery increased considerably and the residual oil saturation decreased with the injection of an oil bank as compared to the same studies carried out in the absence of an injected oil bank (54). Figure 17 schematically represents the oil bank formation and its propagation in porous media, which is analogous to the snowball effect. If an early oil bank is formed then it moves through the porous medium accumulating additional oil ganglia resulting in an excellent oil recovery, whereas a late oil bank formation will result in a poor oil recovery. 

N. Shah, S. M. Shah, R. Chan, M. Nevrekar, P. Pasquarelli, C. Srivatsa, S. Vehkataraman, K. "The Mechanism of Oil Bank Formation, Coalescence in Porous Media and Emulsion Stability", 4th Annual DOE Symposium, Tulsa, OK, Aug. 1978. 

The present study utilizes a microwave attenuation tech nique to study oil bank formation and propagation during linear core tests. This technique, first developed by Parsons (12), was employed to monitor the dynamic in-situ water concentration during the alkaline core flooding experiments. 

The effect of a dynamic interfacial tension will be to increase the probability of a moving oil blob being trapped. The overall kinetics of blob entrapment and mobilization which depend on the dynamics of interfacial tension variation will determine whether or not the blobs will aggregate this will be a key factor in formation and stabilization of an oil bank. In addition, any interfacial rheological resistance will reduce the probability of mobilization, and of drop coalescence during oil bank formation. The quantitative assessment of interfacial dynamic properties is therefore of major importance in the development and optimization of chemical FOR systems. 

A complete screening of an EOR surfactant must include determination of the kinetics of interfacial tension changes in addition to their equilibrium values. Considerable work remains to be done to characterize dynamic processes such as oil droplet mobilization, entrapment and oil bank formation. 

Such studies provide information pertaining to the behavior of individual ganglia and form the basis for the analysis of the collective behavior of large populations of interacting ganglia (4). The latter problem, in turn, is central to the understanding of oil bank formation and/or attrition. 

Multifunctional multipolymeric surfactant mixtures which can be produced from the FRRPP process are capable of efficiently recovering trapped oil from subterranean sources. A typical oil reservoir, shown in Figure 5.1.1, is an anticline (inverted-dome) rock formation wherein the oil is trapped within the open-pore rock formation from sandstone or carbonate material bound by impermeable rock on top and brine in porous rock at the bottom. Immediately above the oil bank could be a layer of natural gas at high pressure, since this formation is originally underground where the pressures could at least be that of the ground overbearing material. Other oil bank formations cited in the literature include salt formations, reefs, etc. 

Oil bank formation (oil rate increase and water rate decrease)... 

Oil bank formation Production tests oil to water breakthrough time ratio... 

Since Thin Film Spreading Agents do not produce ultralow interfacial tensions, capillary forces can trap oil in pore bodies even though the oil has been displaced from the surface of the porous medium. Therefore, recovery of incremental oil is dependent on the formation of an oil bank. Muggee, F. D. U.S. Patent 3 396 792, 1968. 

No sign of an oil bank was detected at the observation well. Before the test began, the average oil saturation in this well had been 19% in the lower 20 ft of the formation and just 4% in the upper 10 ft. 

The formation and displacement of the oil bank depends upon the nature of the phases formed in the porous medium and their relative permeabilities, which may also change as a result of changes in wettability. Detailed discussion of these factors is beyond the scope of this chapter Chapter 6 and references 37 and 38 address this topic. 

Figure 2. Schematic presentation of the role of coalescence of oil ganglia in the formation of the oil bank.

Transient Processes. There are several transient processes such as formation and coalescence of oil drops as well as their flow through porous media, that are likely to occur during the flooding process. Figure 12 shows the coalescence or phase separation time for hand-shaken and sonicated macroemulsions as a function of salinity. It is evident ithat a minimum in phase separation time or the fastest coalescence rate occurs at the optimal salinity (53). The rapid coalescence could contribute significantly to the formation of an oil bank from the mobilized oil ganglia. This also suggests that at the optimal salinity of the system, the interfacial viscosity must be very low to promote the rapid coalescence. 

Figure 10.24 shows that the oil bank breaks through earlier in the model with the Initial 2 data. The initial formation water pH in Initial 2 is about 8.1 compared to 7.4 in Initial 1. Soap is probably generated earlier in Initial 2 than in Initial 1. Thus, the oil bank is formed earlier and breaks through earlier in Initial 2. 

Mobilisation of NAPL generally leads to the formation of an oil bank (see Chapter 10.3) in front of the surfactant solution. If the solubilisation capacity of the surfactant solution is too low, large amounts of emulsions will be formed, which can clog the pore space. As the flow in columns is forced, these experiments may not correctly reflect the behaviour of the multiphase system under free flowing conditions in a three-dimensional pore space. 

We have recently reported (6, 7) that those surfactant formulations which yield good oil recovery exhibit both low interfacial tensions and low interfacial viscosities. Our experiments have shown that surfactant formulations which ensure low interfacial viscosity will promote the coalescence of oil droplets and thereby decrease the emulsion stability, thus enhancing the formation of a continuous oil bank. It has been demonstrated that the requirements for emulsion stability are the presence of an interfacial film of high viscosity and a film of considerable thickness. We have observed that the surfactant concentration which minimizes the interfacial tension may not simultaneously minimize the interfacial viscosity. Hence, our results indicate both interfacial tension and interfacial rheology must be considered in selecting surfactant formulations for tertiary oil recovery. 

Results of our experimentation QJ suggests that the occurrence of permeability reductions during enhanced oil recovery may be avoided and the formation of a continuous oil bank may be initiated and maintained by using a slug of an extracted resinous fraction. These results support the work of Lichaa and Herrera (10,11), where they found that severe permeability reductions due to asphaltene deposition, could be avoided by the injection of a mixture of highly resinous Boscon Crude (29% wt. resin) with a Boscon refined oil. Cooke ( recommended a similar process where a bank of highly acidic crude oil would be injected prior to the injection of the alkaline water for cases where the crude oil acid concentration is low. 

Figure 5. Formation of an Oil Bank in Relation to Petroleum Recovery.

Chiang, M., Role of Surfactant Mass Transfer and the Formation of an Oil Bank in Displacement of Oil Through Porous Media., Ph.D. Dissertation, University of Florida, Gainesville, Fla. (1979). 

In addition to the factors mentioned earlier (e.g., ultralow interfacial tension and coalescence of oil ganglia), mobility control of the oil bank and surfactant slug is an important requirement for a successful oil recovery process. As shown in Figure 8, pol3rmer solutions are used as drive fluids for proper mobility control of the oil recovery process. The dispersion of surfactant, polymer, oil and brine during the flow could lead to emulsion formation and/or phase-separation due to surfactant-polymer incompatibility (7). Efforts should be made to minimize the formation of... 

The flow of oil bank, surfactant slug and the polymer solution through porous media results in the formation of emulsions and mixed surfactant plus polymer region. The formation of stable emulsions and high viscosity surfactant plus polymer structures should be minimized for efficient oil recovery. 

In the tertiary mode floods. Figures 8 and 9, an oil bank is formed which is preceded by a sharp rise in both the pressure drop and the pH. The sharp pressure peaks in Figures 8 and 9 demons-strate that the oil phase was mobilized by locally high gradients in pressure whereas the diffuse pressure profile of the tertiary high pH/high salinity alkaline flood appears to be caused by the formation of swollen ganglia which restrict aqueous flow and which increase the macroscopic displacement efficiencies. 

The plot of dimensionless pressure drop versus pore volumes injected for the secondary mode alkaline flood (Figure 15) indicates that breakthrough occurs at the same amount of pore volumes during the waterflood (i.e., T = 0.58 PV approximately) and during the calcium hydroxide flood. However, in the secondary mode alkaline flood, the pressure drop continues to increase because of the in situ formation of a water-in-oil emulsion phase after breakthrough. As in the tertiary mode alkaline flood, the emulsified phase restricts the flow of the displacing phase and increases the pressure drop. The increased AP indicates the formation of an oil bank... 

The pressure history of the secondary alkaline flood reflects the formation of a secondary oil bank behind the immiscible phase oil bank. This secondary oil bank results in an overall recovery which is above that obtained by secondary waterflooding or by secondary caustic flooding with an univalent ion of high electrolyte concentration. The concentration history of the fractional water production during the secondary calcium hydroxide flood represents the total consumption of the hydroxyl ion. This consumption curve is made up of consumption due to adsorption of the silica surfaces and consumption due to the in situ chemical reaction which forms the more oil-soluble, surface-active salt, calcium oleate. 

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