Microscopic displacement efficiency

The microscopic displacement efficiency is the fraction of the oil which is recovered in the swept part of the reservoir. If the initial oil saturation is S i, then... 

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). 

Melrose, J.C. Brandner, C.F. Role of Capillary Forces in Determining Microscopic Displacement Efficiency for Oil-Recovery by Water Flooding, /. Canadian Petrol. Tech. 1974, 13(1), 13. 

Xia, H.E., Wang, D.M., liu, Z.C., Yang, Q.Y., 2001. Mechanisms of viscous polymer solutions to increase microscopic displacement efficiency. ACTA PETROLEI SINICA 22 (4), 60-65. 

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. 

The microscopic displacement efficiency is affected by the following factors interfadal and surface tension forces, wetlabihty, capillary pressure, and relative permeability. 

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. 

When an alkaline solution is mixed with certain erode oils, surfactant molecules are formed. When the formation of surfactant molecules occurs in situ, the interfacial tension between the brine and oil phases could be reduced. The reduction of interfacial tension causes the microscopic displacement efficiency to increase, which thereby increases oil recovery. 

Oil recovery from a swept region can be expressed in terms of the ratio of oil removed to that originally in place (3). This ratio is the microscopic displacement efficiency, E, defined for normal nonwetting phase residual saturations, S... 

Fig. 11. Plots of microscopic displacement efficiency versus capillary number [vy/( )a].

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. 

The amount of oil that is recoverable from a reservoir by a displacement process depends on (1) the effectiveness with which the injected fluid displaces oil from the pores in the rock (microscopic displacement efficiency) and (2) the volumetric fraction of the reservoir contacted by the injected fluid (macroscopic sweep efficiency). This latter efficiency is governed by the mobility ratio but also in large measure by the geologic heterogeneity of the reservoir rock. Permeabilities vary both areally and vertically, and large changes typically occur in the vertical direction in a single well. As an example, Ffe. 5.72 shows permeability variation with depth for a shallow sandstone reservoir in eastern Kansas. ... 

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