Displacement efficiency areal 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. 

Figure 8.15. Areal sweep efficiency at breakthrough versus mobility ratio for miscible and immiscible displacements in five-spot floods (from Craig, 1971).

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. ... 

Laboratory results are consistent with accepted theory with respect to improved areal sweep, permeability distribution and displacement efficiencies as would be predicted from the solution mobility measured in the core. 

If increased water viscosity could be economically realized, marked improvements in areal sweep efficiency, as discussed by Caudle and Witte and others, would be realized in field operations. Similarly, the improved mobility ratio would bring about increased recoveries by correcting permeability distribution problems as discussed by Stiles and Dykstra and Parsons, and improvements in displacement efficiency would be obtained as recognized by Buckley and Leverett and Welge. ... 

This unexpected reduction of water mobility by very dilute polymer solutions as determined in reservoir rock samples has been demonstrated beyond any doubt. It now remains to be seen if this reduction in mobility produces a prt r and cmresponding influence in oil recovery. The three reservoir efficiency factors of areal sweep, permeability distribution and displacement efficiency were studied independently to determine whether the recoveries would be improved according to the solution viscosity or ac-... 

It is common to further divide the volumetric displacement efficiency into a product of areal and vertical sweep efficiency (Lake 1989) ... 

However, Eq. 2 is a signilic ant simplification of the overall, trae displacement efficiency, sinc e aU three efficiencies are nonlinear functions of each other. Eq. 2 is a leftover from the 1970-80s, when a first rough approximation of the displacement efficiency was attempted by assuming the three terms to be independent of each other. Modern, 3D, SL simulators have eliminated the need to separate the volumetric sweep efficiency into an areal and vertical component because today s SLs are fully 3D and, therefore, capture the volumetric sweep direetly. The periodic updating of SLs accounts for the nonlinear dependence between E and Ey The relationship is linear for the period in which the SLs are kept fixed—the global timestep size—but when the SLs are updated, so is the dependency between the two terms. The power of Eq. 1 rests in the characteristic that the main physics governing the... 

E = overall sweep efficiency = areal displacement efficiency E = local displacement efficiency Ey = volumetric displacement efficiency / , = water fractional flow... 

Steam-based processes in heavy oil reservoirs that are not stabilized by gravity have poor vertical and areal conformance, because gases are more mobile within the pore space than liquids, and steam tends to override or channel through oil in a formation. The steam-foam process, which consists of adding surfactant with or without noncondensible gas to the injected steam, was developed to improve the sweep efficiency of steam drive and cyclic steam processes. The foam-forming components that are injected with the steam stabilize the liquid lamellae and cause some of the steam to exist as a discontinuous phase. The steam mobility (gas relative permeability) is thereby reduced, and the result is in an increased pressure gradient in the steam-swept region, to divert steam to the unheated interval and displace the heated oil better. This chapter discusses the laboratory and field considerations that affect the efficient application of foam. 

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

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