Vertical sweep efficiency

Polymer augmented waterflooding waterflooding in which organic polymers are injected with the water to improve areal and vertical sweep efficiency. 

One obvious mechanism in polymer flooding is the reduced mobility ratio of displacing fluid to the displaced fluid so that viscous Angering is reduced. When viscous Angering is reduced, the sweep efficiency is improved, as shown in Figure 1.2. This mechanism is discussed extensively in the waterflooding literature it is also discussed in Chapter 4. When polymer is injected in vertical heterogeneous layers, crossflow between layers improves polymer allocation in the vertical layers so that vertical sweep efficiency is improved. This mechanism is detailed in Sorbie (1991). 

Figure 8,3. Schematic diagram of the improvement in vertical sweep efficiency caused by polymer in a layered system.

An important application of polymers in several areas of the world is in the improvement of waterflooding performance in highly inhomogeneous reservoirs. In particular, polymers can reduce the harmful effects of high-permeability layers in a vertically stratified system and so reduce watercut and improve vertical sweep efficiency. Polymers can act by a combination of two mechanisms ... 

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

An Improvement of areal and vertical sweep efficiency by smaller mobility ratios is effectively achieved. Also in this phase of the flood no change in oil cut occurs. Oil has not yet reached the producer. 

The polymer in a polymer waterflooding process acts primarily as a thickener. It decreases the permeability of the reservoir and thus improves the vertical and lateral sweep efficiency. 

The use of surfactant-stabilized foams to counteract these kinds of problems was suggested several decades ago (7, 2) and has recently become actively pursued in laboratory and field tests (3—8). The use of foam is advantageous compared with the use of a simple fluid of the same nominal mobility because the foam, which has an apparent viscosity greater than the displacing medium, lowers the gas mobility in the swept or higher permeability parts of the formation. This lowered gas mobility diverts at least some of the displacing medium into other parts of the formation that were previously unswept or underswept. From these underswept areas, the additional oil is recovered. Because foam mobility is reduced disproportionately more in higher permeability zones, improvement in both vertical and horizontal sweep efficiency can be achieved. 

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 ... 

H. Surkalo, M. J. Pitts, B. Sloat, and D. Larsen, Polyacrylamide Vertical Conformance Process Improved Sweep Efficiency and Oil Recovery in the OK Field, Paper SPE 14115, Presented at the SPE 1986 International Meeting on Petroleum Engineering, Beijing, China, March 17-20 (1986). 

At the present time the improvement of areal and vertical (volumetric) sweep efficiency takes a great deal of room in secondary and tertiary oil recovery. One of the widely used and perspective methods is mobility control by diluted aqueous solutions of different polyacrylamides (1,2). In the middle of the sixties some authors (3,4) proposed that the viscosity enhancement and the non-Newtonian flow behavior of the solutions were responsible for the reduction of phase mobility. Mungan (5,6), Gogarty (7), Dauben and Menzie (8) have pointed out, however, that the sorption phenomenon plays a decisive role in the flow characteristics of the polymer solutions and carrier phases. In the papers devoted to... 

It is proposed that either vertical-horizontal well pattern or vertical-fishbone well pattern can be applied owing to the higher sweep efficiency and recovery over conventional vertical wells injection-production pattern, which could be realized by utilizing available vertical wells while correspondingly deploying horizontal wells or fishbone wells for further improvement of recovery at fractured reservoir. 

Thus, in summary, note that poor linear, areal and vertical waterflooding efficiency may occur for unfavourable M, and that the role of polymer is basically to remedy this situation by reducing M. However, for linear and areal (homogeneous) sweep improvement it is necessary only to reduce M to approximately unity. In cases where larger scale heterogeneities occur, e.g. vertical stratification or areal channel sands, it may be necessary to reduce M to a value considerably below unity. The desirable M for such cases will depend on the details of the particular system of interest. This may also be constrained by certain practical problems such as the magnitude of additional pressure build-up that is acceptable when using viscous polymers in the field. 

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. ... 

As of December 1989, the polymer project is now 50% complete based on mass of polymer injected and 30% complete based on volume Injected, representing about 16% of a pore volume. The average radius of polymer in the short string now extends about 325 feet (99 meters) around the wellbore, based on 200 feet (61 meters) of flooded sand (net vertical) and 80% sweep efficiency. None of the producers have shown polymer breakthrough. The nearest producer to the polymer flood front is 350 feet (107 meters) from the Injector. Polymer is expected to appear at this well In about seven months. 

When simulating the waterflood, the line drive configuration with vertical wells provided the quickest oil recovery and the best sweep efficiency, since the complete pattern was swept even at the edges. For a polymer flood, the use of three parallel, horizontal wells with a central injector provided the optimum configuration since the fastest oil recovery was achieved in comparison to the other configurations, as shown in Figure 11. By the year 2020, polymer flood with the horizontal well configuration recovered... 

Surprisingly, the vertical well configuration still provided the best sweep efficiency for the polymer flood since the edges of the horizontal well patterns were not completely swept However, the polymer injection rate was severely limited when using vertical wells and the parallel horizontal weU configuration was the more attractive alternative when injecting polymer into the reservoir to produce heavy oil at economic rates. 

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... 

The oil-solvent mixture and the meal is stripped of the solvent to recover solvent-free oil and meal. The solvent-enriched meal is conveyed to vertical desolventizer where heat and vacuum facilitate removal of solvent vapors. Desolventizer contains trays with sweeping arms to agitate the meal for improved efficiency. Some plants purge the cake with steam to remove the solvent, whereas others use hot air, although application of vacuum is most common. The solvent oil miscella are stripped of solvent in a three-stage evaporator. The hexane is reused for the extraction of oil. 

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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