Macroscopic sweep efficiency

The macroscopic sweep efficiency s the fraction of the total reservoir which is swept by water (or by gas in the case of gas cap drive). This will depend upon the reservoir quality and continuity, and the rate at which the displacement takes place. At higher rates, displacement will take place even more preferentially in the high permeability layers, and the macroscopic displacement efficiency will be reduced. 

This is why an offtake limit on the plateau production rate is often imposed, to limit the amount of by-passed oil, and increase the macroscopic sweep efficiency. 

This must be combined with the macroscopic sweep efficiency to determine the recovery factor (RF) for oil (in this example). 

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). 

Miscible processes are aimed at recovering oil which would normally be left behind as residual oil, by using a displacing fluid which actually mixes with the oil. Because the miscible drive fluid is usually more mobile than oil, it tends to bypass the oil giving rise to a low macroscopic sweep efficiency. The method is therefore best suited to high dip reservoirs. Typical miscible drive fluids include hydrocarbon solvents, hydrocarbon gases, carbon dioxide and nitrogen. 

Improved macroscopic sweep efficiency because of the viscous polymer drive. 

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. ... 

The macroscopic displacement efiiciency is made up of two other terms, the areal, E, and vertical, E, sweep efficiencies ... 

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. 

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