Residual oil saturation

On a microscopic scale (the inset represents about 1 - 2mm ), even in parts of the reservoir which have been swept by water, some oil remains as residual oil. The surface tension at the oil-water interface is so high that as the water attempts to displace the oil out of the pore space through the small capillaries, the continuous phase of oil breaks up, leaving small droplets of oil (snapped off, or capillary trapped oil) in the pore space. Typical residual oil saturation (S ) is in the range 10-40 % of the pore space, and is higher in tighter sands, where the capillaries are smaller. 

Field analogues should be based on reservoir rock type (e.g. tight sandstone, fractured carbonate), fluid type, and environment of deposition. This technique should not be overlooked, especially where little information is available, such as at the exploration stage. Summary charts such as the one shown in Figure 8.19 may be used in conjunction with estimates of macroscopic sweep efficiency (which will depend upon well density and positioning, reservoir homogeneity, offtake rate and fluid type) and microscopic displacement efficiency (which may be estimated if core measurements of residual oil saturation are available). 

The data gathered from the logs and cores of the development wells are used to refine the correlation, and better understand areal and vertical changes in the reservoir quality. Core material may also be used to support log data in determining the residual hydrocarbon saturation left behind in a swept zone (e.g. the residual oil saturation to water flooding). 

Alcohol ethoxysulfates have been used in field tests as nitrogen (177) and carbon dioxide (178) foaming agents. Field use of alcohol ethoxysulfates is restricted to low temperature formations owing to its limited hydrolytic stabihty at low pH and elevated temperature (179). It has been reported that some foams can reduce residual oil saturation, not by oil displacement, but by emulsification and imbibition of the oil into the foam (180). 

Steam-foaming agents that efficiently mobilize heavy cmde oil by heat transfer can reduce the residual oil saturation. This can increase foam stabihty and improve the diversion of subsequently injected steam into oil saturated zones thereby increasing oil recovery (204). 

Earlier Kern River pilot results [59,82,83] showed that a steam foam formulation based on AOS containing 16-18 carbon atoms in the hydrophobe improves sweep efficiency and oil recovery of the steam drive but propagates relatively slowly and leaves the same residual oil saturation (ROS) as steam. 

Although residual oil saturation in the steam swept region can be as low as 10%, the average residual oil saturation in the formation remains much higher due to poor vertical conformance. Thus it is because of the creation of steam override zones that vertical conformance in steamfloods is usually poor. 

B. licheniformis JF-2 and Clostridium acetogutylicum were investigated under simulated reservoir conditions. Sandstone cores were equilibrated to the desired simulated reservoir conditions, saturated with oil and brine, and flooded to residual oil saturation. The waterflood brine was displaced with a nutrient solution. The MEOR efficiency was directly related to the dissolved gas/oil ratio. The principal MEOR mechanism observed in this work was solution gas drive [505]. 

Determining the formation of residual oil saturation from the chromatographic separation of the water-soluble tracer and the partitionable tracer... 

A chemical-enhanced oil-recovery technology can be used to remove oily contaminants from soil. Laboratory studies demonstrated that a variety of alkaline-surfactant combinations can be used with a polymer to reduce the residual oil saturation in waterflooding [1435]. 

G. L. Stegemeier and G. E. Perry. Method utilizing spot tracer injection and production induced transport for measurement of residual oil saturation. Patent US 5168927, 1992. 

In the next run, a core pack was saturated with 8.6 cp (at 50° C) Ranger-zone crude oil and water flooded to residual oil saturation. Polymer flood was then initiated and about 1.2% of the original oil in place (OOIP) was recovered. The results are shown in Figure 4. The pressure profiles show behavior essentially similar to the previous run except that the pressure drop across the core increased to 100 psi within 4 PV of injection of polymer. The steady state values of pH and viscosity were 7.0 and 0.7 cp. respectively. The oil ganglia retained in larger pores resisting displacement probably reduced the amount of polymer adsorbed and reduced the number of pores that the polymer molecules needed to seal off in order to block the core. This could explain the more rapid plugging of the core. Effluent pH and viscosities remained much lower than influent values. 

The interfacial tension results reported in this paper are part of a study to examine the benefits of using commercial foam-forming surfactants with steam-based processes for obtaining additional oil recovery. Low interfacial tension at elevated temperatures is needed to reduce residual oil saturation and to allow foams to form, or enhance their performance. 

The cores were then waterflooded with brine (2% NaCl) to obtain residual oil saturation. 

The results from OILEQUIL can be converted to an active layer thickness in several ways. First, the total thickness of the zone shown to contain product could be used. This would overestimate the thickness of the recoverable product layer, because oil at low saturations is immobile. Second, one could follow Van Dam (1967) in assuming a residual oil saturation of 20% (or another value appropriate to the soil and LNAPL involved) and pick out the thickness of the zone with greater than 20% product saturation from the OILEQUIL tabular output. In equation form,... 

The argument is that the permeability to water around the injection well is controlled by thp natural residual oil saturation after waterflooding. Any oil added by the injection water will not add to that residual to reduce the permeability to water. 

However, since the disposal formation usually has a 100% water saturation, there are also cases where residual oil saturation (created aiound the disposal well boiehole by entrained oil) is sufficient to reduce the injectivity below that required. 

Micro-scale experiments involve the microscopic observation of flowing foams in etched-glass micromodels. Here the pore dimensions are typically on the order of hundreds of micrometers. Such experiments provide valuable and rapidly obtainable qualitative information about foam behaviour in constrained media under a variety of experimental conditions, including the presence of a residual oil saturation... 

One generally attempts to reduce the capillary forces restraining the oil and/or alter viscosity of the displacing fluid in order to modify the viscous forces being applied to drive oil out of the pores. The ratio of viscous forces to capillary forces actually correlates well with residual oil saturation and is termed the capillary number. One formulation of the capillary number is,... 

Two sets, i.e., four experiments, of core flow studies are compared. Sets No. 1 and No. 2 were tertiary miscible and immiscible CO2 floods without mobility control. The same core from each set, after plain CO2 injection, was restored to waterflood residual oil saturation and flooded with 0.05% AEGS 25-12 surfactant in brine. There was almost no difference between the oil saturation distributions in the cores between experiments, with the average Sorw values of 37 1 saturation percent in both sets of experiments. CO2 was injected continuously in all experiments at a nominal rate of 1 ft/day. No attempt was made to preform a foam, or to inject alternate slugs of surfactant solution and CO2. 

IMMISCIBLE CO2/QIL WITH LOWERED CO /BRINE IFT. SET NO. 2. EXPT. NO. 4. The core used in experiment No. 3 was restored to water flood residual oil saturation and flooded with 0.05 wt% AEGS 25-12 surfactant in brine, and then CO2. Initially, the CO2 began to sweep the entire core cross section and build an oil bank similar to the one observed in experiment No. 3. Then the CO2 buoyed up and overrode the water and oil banks. Figures 15a-d. The result is both poor sweep and poor displacement of oil. 

Sorw = residual oil saturation after exhaustive water flood, dimensionless... 

Results of the Miscible Displacement Tests. The results of the tests are shown in Figures 9-12. Referring first to Figure 9, the CO2 flood of a waterflooded light oil reservoir, it can be seen that the waterflood, represented by the data prior to CO2 injection, recovered 0.5 PV of oil. This amount represented 76% of the oil initially in place and produced a residual oil saturation... 

Berea sandstone cores (25.4 cm by 3.8 cm) used in the experiments with Wilmington and Delaware-Childers crude oils were fired at 427°C. After firing, the cores were saturated with brine, mounted in a Hassler type core holder, and placed in a temperature-controlled oven. After initial absolute permeability was determined, the cores were left either oil-free or saturated with oil and waterflooded to residual oil saturation. 

The first two tests were performed in the usual manner of saturating the core with Wilmington oil and then waterflooding to residual oil saturation, resulting in oil saturation of 45 and 49%, respectively, before caustic injection. After injection of caustic slugs, the effective permeabilities were reduced 27 and 25%, respectively. 

The ratio of viscous forces to capillary forces correlates well with residual oil saturation and is termed the capillary number (N ). One formulation of the capillary number is... 

Wettability. Wettability of the porous medium controls the flow, location, and distribution of fluids inside a reservoir (7, 28). It directly affects capillary pressure, relative permeability, secondary and tertiary recovery performances, irreducible water saturations, residual oil saturations, and other properties. 

The oil bank that forms will exist at an oil saturation that is greater than the residual oil saturation. At the front of the bank, residual oil is taken up, while at the back, the capillary number must remain high to minimize oil entrapment. In this way, the oil bank grows larger and forms slightly ahead of the injected chemicals. 

This developed miscibility process results in a miscible fluid, that is capable of displacing all the oil which it contacts in the reservoir... The efficiency of this displacement is controlled by the mobility (ratio of relative permeability to viscosity) of each fluid. If the displacing fluid (i.e. carbon dioxide) is more mobile than that being displaced (i.e. crude oil) then the displacement will be relatively inefficient. Some of the residual oil saturation will never come into contact with carbon dioxide. Both laboratory and field tests have indicated, that even under favourable condition, injection of 0.15-0.6 10 m of carbon dioxide is required for recovery of an additional barrel (0.16 m ) of oil". Here our goal is to obtain a mass ratio of CO2 to incremental oil of 1 to 4, on the basis of the Bonder s data. 

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. 

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