Microscopic vertical sweep

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. 

Now consider the role of polymer flooding in more realistic areal and vertical reservoir sections which are taken to be approximately two-dimensional (2-D) in nature. In a homogeneous areal system, the situation is similar in certain respects to the 1-D case discussed above. At lower M values, there is very good microscopic displacement and good areal sweep. 

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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