Oil displacement efficiency

There are two principal mechanisms of enhanced oil recovery increasing volumetric sweep efficiency of the injected fluid and increasing oil displacement efficiency by the injected fluid. In both, chemicals are used to modify the properties of an injected fluid whether water, steam, a miscible gas such as CO2 or natural gas, or an immiscible gas, usually nitrogen. Poor reservoir volumetric sweep efficiency is the greatest obstacle to increasing oil recovery (9). 

Wettabihty is defined as the tendency of one fluid to spread on or adhere to a soHd surface (rock) in the presence of other immiscible fluids (5). As many as 50% of all sandstone reservoirs and 80% of all carbonate reservoirs are oil-wet (10). Strongly water-wet reservoirs are quite rare (11). Rock wettabihty can affect fluid injection rates, flow patterns of fluids within the reservoir, and oil displacement efficiency (11). Rock wettabihty can strongly affect its relative permeabihty to water and oil (5,12). When rock is water-wet, water occupies most of the small flow channels and is in contact with most of the rock surfaces as a film. Cmde oil does the same in oil-wet rock. Alteration of rock wettabihty by adsorption of polar materials, such as surfactants and corrosion inhibitors, or by the deposition of polar cmde oil components (13), can strongly alter the behavior of the rock (12). 

Microbial-enhanced oil recovery involves injection of carefully chosen microbes. Subsequent injection of a nutrient is sometimes employed to promote bacterial growth. Molasses is the nutrient of choice owing to its low (ca 100/t) cost. The main nutrient source for the microbes is often the cmde oil in the reservoir. A rapidly growing microbe population can reduce the permeabiHty of thief zones improving volumetric sweep efficiency. Microbes, particularly species of Clostridium and Bacillus, have also been used to produce surfactants, alcohols, solvents, and gases in situ (270). These chemicals improve waterflood oil displacement efficiency (see also Bioremediation (Supplement)). 

Combinations of hydrogen peroxide, sulfuric acid, and urea have been proposed [1]. The temperature influences the urea decomposition into ammonia and carbon dioxide that provokes pressure buildup in a formation model and a 19% increase of oil-displacement efficiency in comparison with water. 

The amount of oil recovery promoted by an injected fluid is related to its ability to displace the oil it contacts in the reservoir, termed the oil displacement efficiency (ODE), and to the relative amount of the reservoir invaded by the injected fluid, termed the volumetric sweep efficiency (VSE). Total oil recovery may be expressed as ... 

For example, consider a reservoir which has produced 40% of the oil originally in place. If an injection fluid contacts 70% of the reservoir and has an oil displacement efficiency of 70% of the remaining oil (42% of the oil originally in place) then the maximum enhanced oil recovery is 49% of the oil remaining in place or 29% of the oil originally present in the reservoir. (Trapping and other oil loss mechanisms are neglected in this simplified treatment.) Total oil recovery has increased to 69%. 

Intermixing of the polymer mobility control fluid with the surfactant slug can result in surfactant - polymer interactions which have a significant effect on oil recovery (476). Of course, oil - surfactant interactions have a major effect on interfacial behavior and oil displacement efficiency. The effect of petroleum composition on oil solubilization by surfactants has been the subject of extensive study (477). 

Oil displacement efficiency can be improved considerably when a composite slug is used in place... 

Oil displacement efficiency, 18 613, 614 in enhanced oil recovery, 18 628—629 Oil drilling, enzyme-based clean-up processes in, 10 306-307 Oil drilling platforms, with fiber-optic smart structures, 11 158-159... 

Surface wave, 17 422. See also S-wave Surfactant adsorption, 24 119, 133-144 at the air/liquid and liquid/liquid interfaces, 24 133-138 approaches for treating, 24 134 measurement of, 24 139 at the solid/liquid interface, 24 138-144 Surfactant blends, in oil displacement efficiency, 13 628-629 Surfactant-defoamers surface tension, <5 244t Surfactant-enhanced alkaline flooding,... 

Similarly, we used correlations developed by Fulcher et al. (1985) by fitting their experimental data to calculate the ratio of water to oil. The k, ratio of a high IFT system is compared with that of a lower IFT in Figure 7.40. The same observation can be made from this figure. We also checked other pubhshed data (not shown here to avoid tedious presentation), and they all show that the k, ratio is decreased when IFT is lower thus, the oil displacement efficiency is improved in the high aqueous phase saturation range as IFT is reduced. 

Pusch, G., Lotsch, T, Muller, T, 1987. Investigation of the oil displacing efficiency of suitable polymer products in porous media, aspects of recovery mechanisms during polymer flooding. DGMK—Report, German Society Petrol. Sci. Coal Chem., 295-296, Hamburg. 

Wagner, O.R., Leach, R.O., 1959. Improving oil displacement efficiency by wettability adjustment. Trans. AIME 216, 65-72. 

Wang, D.-M., Xia, H.-E, Liu, Z.-C., Yang, Q.-Y, 2001b. Study of the mechanism of polymer solution with viscoelastic behavior increasing microscopic oil displacement efficiency and the forming of steady Oil Thread flow channels. Paper SPE 68723 prepared at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Jakarta, 17-19 April. 

Pithapurwala, A.K., Sharma, R.C. and Shah, D.O. (1986) Effect of salinity and alcohol partitioning on phase-behavior and oil displacement efficiency in surfactant-polymer flooding. /. Am. Oil Chem. Soc., 63(6), 804-813. 

With the surfactant-cosurfactant system, it has been observed (6) that the best oil displacement efficiency is achieved when the surfactant system spontaneously emulsifies with the oil, followed by rapid coalescence of the emulsified oil droplets (2). 

Nikolov et al. (1) were the first to find a direct correlation between pseudoemulsion film stability and crude oil displacement efficiency in... 

Figure 10. Effect of Oil Viscosity on Oil Displacement Efficiency by Foam Flooding.

The effect of oil viscosity on the displacement of oil is presented in Figure 10. In order to determine the oil displacement efficiency by foam flooding, the injection of gas phase was started at surfactant solution breakthrough. Both air and steam were employed to generate in-situ foams. The steam foam recovered more oil as compared to air foams. 

Fig. 1. The effect of capillary number, N, on the microscopic oil displacement efficiency in porous media of various size distribution (Ref. 1).

Fig, 11, Various phenomena occurring at the optimal salinity in oil/brine/surfactant/alcohol systems in relation to oil displacement efficiency. 

In summary, the formation of middle phase microemulsion at the optimal salinity is an important phenomenon with respect to ultralow interfacial tension, solubilization, rate of coalescence and oil displacement efficiency in porous media (10,11,25). Also, the optimal salinity can be shifted to a desired value by adjusting several variables. 

This is the type of information that is presented in a Salinity Requirement Diagram. Figure 6 is the Salinity Requirement Diagram for the system under discussion. The vertical bars show, as a function of overall surfactant concentration, the range of brine salinity over which the system is in a Type III phase environment (although not necessarily three phases). The position of the circle on the bar indicates midpoint salinity at that overall surfactant concentration. Optimal salinity for oil-displacement efficiency should be close to that level of salinity. The number within the circle is the volume fraction of surfactant in the invariant phase at midpoint salinity. Healy and Reed (12) found lower microemulsion /ex cess brine and microemulsion/excess oil interfacial tensions for systems in which the volume fraction of surfactant in... 

The effect of alcohol on surfactant mass transfer from bulk solution to the oil/dilute micellar solution interface was studied Various interfacial properties of the surfactant solutions and their ability for displacing oil were determined. For the surfactant-oil-brine systems studied, the interfacial tension (IFT) and surfactant partition coefficient did not change when isobutanol was added to the following systems 0.1% TRS 10-410 in 1.5% NaCl vs. n-dodecane and 0.05% TRS 10-80 in 1.0% NaCl vs. n-octane. On the other hand, the interfacial viscosity, oil drop flattening time (i.e. the time required for an oil droplet to flatten out after being deposited on the underside of a polished quartz plate submerged in the micellar solution) and oil displacement efficiency were influenced markedly by the addition of alcohol. 

In order to delineate the effect of surfactant mass transfer on in situ behavior of oil ganglia, we carried out several oil displacement experiments using equilibrated and nonequilibrated oil/ micellar solution systems. For equilibrated systems, the oil displacement efficiency showed an excellent correlation with IFT and capillary number. However, for unequilibrated systems, the oil displacement efficiency depended on salinity. Below optimal salinity, the oil displacement efficiency almost remained the same for both equilibrated and nonequilibrated systems, whereas at and above optimal salinity the oil displacement efficiency was higher for nonequilibrated systems as compared to equilibrated systems. This was attributed to mass transfer rate effects in these systems. 

Laboratory studies on oil displacement efficiency by surfactant-polymer flooding process have been reported by a number of investigators (1-10). In general, the process is such that after being conditioned by field brine or preflush, a sandstone core or a sandpack is oil-saturated to the irreducible water content. It is then waterflooded to the residual oil level. Finally, a slug of surfactant solution followed by a mobility buffer is injected. 

In general, the surfactant formulations used for enhanced oil recovery contain a short chain alcohol. The addition of alcohol can influence the viscosity, IFT and birefringent structures of micellar solutions as well as coalescence rate of oil ganglia. The present paper reports the effect of addition of isobutanol to a dilute petroleum sulfonate (< 0.1% cone) solution on IFT, surface shear viscosity, surfactant partitioning, the rate of change of IFT (or flattening time) of oil drops in surfactant solutions and oil displacement efficiency. The two surfactant systems chosen for this study indeed exhibited ultralow IFT under appropriate conditions of salinity, surfactant concentration and oil chain length (11,15,19). 

Oil displacement in porous media Horizontally mounted sand-packs and Berea cores encased in an air-circulating constant temperature box were used for oil displacement efficiency tests. 

Table 1 shows the effect of the addition of isobutanol on various properties of oil/brine/surfactant systems for TRS 10-410 and TRS 10-80. Because the same IFT values were obtained for the systems with and without IBA (Table 1), the observed differences in oil recovery cannot be explained in terms of any change in IFT. The presence of alcohol did not significantly influence the partition coefficient of surfactant in n-dodecane or n-octane. It is important to emphasize that the partition coefficient changes sharply near the ultralow IFT region (19). Thus, the partition coefficient does not appear to correlate with the oil displacement efficiency. However, the presence of isobutanol decreases the interfacial viscosity and markedly influences the flattening time of the oil droplets. It has been suggested (18) that a rigid potassium oleate film at the oil/water interface can be liquefied by the penetration of the hexanol molecules in order to produce spherical microemulsion droplets. It has been shown (14) also that for a commercial petroleum sulfonate-crude oil system, the oil droplets with the alcohol coalesce much faster than the ones without alcohol. For the systems studied here, IBA is believed to have penetrated the petroleum sulfonate film as seen by the decrease in IFV. The decrease in interfacial viscosity would presumably promote the coalescence in porous media. 

Table 1. The Effect of IBA on Flattening Time, IFT, IFV, Partition Coefficient, and Oil Displacement Efficiency... 

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