Fluid displacement efficiency

For flow through porous media studies, the sandpacks used as porous media were flushed vertically with carbon dioxide for an hour to replace interstitial air. Distilled water was pumped and the pore volume (PV) of the porous medium was determined. By this procedure, the trapped gas bubbles in the porous media can be easily eliminated because carbon dioxide is soluble in water. For determining the absolute permeability of the porous medium, the water was pumped at various flow rates and the pressure drop across the sandpack as a function of flow rate was recorded. After the porous medium was characterized, the mixed surfactant solutions of known surface properties were injected. This was followed by air injection to determine the effect of chain length compatibility on fluid displacement efficiency, breakthrough time and air mobility in porous media. 

Table II shows the effect of chain length compatibility on oil recovery, fluid displacement efficiency, breakthrough time and effective gas mobility in porous media. For gas/liquid systems (e.g. Foams), a maximum in fluid displacement efficiency, a...

Sharma, M.K., Shah, D.O. and Brigham, W.E., The Influence of Temperature on Surface and Microscopic Properties of Foaming Solutions in Relation to Fluid Displacement Efficiency in Porous Media., AIChE J.,... 

Further studies showed that a decrease in surface tension was obtained with increasing temperature. As the temperature was raised, the kinetic energy of the molecules increased, resulting in a decrease in attractive forces between the molecules which in turn, reduced the surface tension of the surfactant solutions. It was concluded that, as the surface tension decreased the fluid displacement efficiency in porous media increased. 

The effect of temperature on fluid displacement efficiency in the presence of SDS and Suntech IV systems is shown in Figure 8. It was... 

The effect of oil viscosity on fluid displacement efficiency for both pure and commercial surfactants is shown in Figure 10. It was cbserved that the oil viscosity has an effect on the fluid displacement... 

WRIGHT, R. J. and DAWE, R. A., "Fluid Displacement Efficiency in Layered Porous Media Mobility Ratio Influence", Rev. Inst. Fr. du P trole, 38, pp 455-474, July 1983. 

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). 

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). 

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). 

The theoretical fluid displacement of a piston pump equals the total volume swept by the piston on each delivery stroke. Because of leakage past the piston and the valves and failure of the valves to close instantly, this theoretical displacement is not attained in actual practice. The volumetric efficiency, defined as the ratio of the actual displacement to the theoretical displacement, is usually in the range of 70 to 95 percent. 

The overall recovery efficiency E of any fluid displacement process is given by the product of the macroscopic, or volumetric, displacement efiiciency and the micro-... 

The macroscopic displacement efficiency is a measure of how well the displacing fluid has come in contact with the oil-bearing parts of the reservoir. The microscopic displacement efficiency is a measure of how well the displacing fluid mobilizes the residual oil once the fluid has come in contact with the oil. 

Another factor affecting the microscopic displacement efficiency is the fact that two or more fluids are usually flowing in an EOR process. When two or more fluid phases are present, the saturation of one phase affects the permeability of the other(s), and relative permeabilities have to be considered. Figure 1 is an example of a set of relative permeability curves plotted against the wetting phase saturation (water in this case). 

In the previous section, it was noted that the microscopic displacement efficiency is largely a function of interfacial forces acting between the oil, rock, and displacing fluid. If the interfacial tension between the trapped oil and the displacing fluid could be lowered to 10 to 10 dyn/cm, the oil droplets could be deformed and could squeeze through the pore constrictions. A miscible process is one in which the interfacial tension is zero that is, the displacing fluid and the residual oil mix to form one phase. If the interfacial tension is zero, then the capillary number Nyc becomes infinite and the microscopic displacement efficiency is maximized. 

Surfactants play an important role in the formation and stability of foams. Investigators have determined foam stability by measuring the half-life (e.g. t 2) the foam. Half-life is the time required to reduce foam voLume to half of its initial value. It has been demonstrated that the foam stability (i.e.half-life) decreased with increasing temperature, whereas the foaminess of the surfactant solution increased with temperature. It is likely that these properties of foam depend on the molecular structure and concentration of the surfactant at the gas/liquid interface. Comparison of the results of static foam stability with that of the dynamic behavior of foam in porous media revealed that the foam stability is not required for efficient fluid displacement or a decrease in the effective air mc >ility in a porous medium. Moreover, the ability of the surfactants to produce in-situ foam was one of the important factors in the displacement of the fluid in a porous medium. 

Below the bubble-point, pressure gas percolates out of the oil phase, coalesces and displaces the crude oil. The gas phase, which is much less viscous and thus more mobile than the oil phase, fingers through the displaced oil phase. In the absence of external forces, the primary depletion inefficiently produces only 10 to 30 percent of the original oil in place. In the secondary stage of production, water is usually injected to overcome the viscous resistance of the crude at a predetermined economic limit of the primary depletion drive. The low displacement efficiencies, 30 to 50 percent, of secondary waterfloods are usually attributed to vertical and areal sweep inefficiencies associated with reservoir heterogeneities and nonconformance in flood patterns. Most of the oil in petroleum reservoirs is retained as a result of macroscopic reservoir heterogeneities which divert the driving fluid and the microscopically induced capillary forces which restrict viscous displacement of contacted oil. This oil accounts for approximately 70 percent, or 300 x 10 bbl, of the known reserves in the United States. 

This imbibition method is dependent on both the chemical properties of the rock surface and the geometry of the pore network, particularly the pore throat size and shape. Since pore stmctnre can affect the measurement results, the reduction of the interfacial tension between pore fluids by surfactants can affect fluid movement through the pore structure. This method, therefore, is not strictly a measnre of surface wettability, but is a measure of the efficiency of fluid displacement that is normally most strongly affected by the wettability state of the pore surfaces. Within this framework of understanding, wettability alteration of the pore surfaces using snrfactants can be stndied with this measurement method. 

Thus, the role of polymer here is to improve the mobility ratio between the fluids by either increasing aqueous-phase viscosity or reducing the aqueous-phase effective permeability (see Chapter 5) or both. This lowered mobility ratio will improve both the linear and areal displacement efficiency, thus leading to improved oil recovery. 

Mobility control is a generic term describing any process where an attempt is made to alter the relative rates at which injected and displaced fluids move through a reservoir. The objective of mobility control is to improve the volumetric sweep efficiency of a displacement process. In some processes, there is also an improvement in microscopic displacement efficiency at a specified volume of fluid injected. Mobility control is usually discussed in terms of the mobility ratio, M, and a displacement process is considered to have mobility control if 1.0. Volumetric sweep efficiency generally increases as M is reduced, and it is sometimes advantageous to operate at a mobility ratio considerably less than unity, especially in reservoirs with substantial variation in the vertical or areal permeability. 

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