The following factors affect the macroscopic displacement efficiency: heterogeneities and anisotropy, mobility of the displacing fluids compared with the mobility of the displaced fluids, the physical arrangement of injection and production wells, and the type of rock matrix in which the oil or gas exists.
Heterogeneities and anisotropy of a hydrocarbon-bearing formation have a significant effect on the macroscopic displacement efficiency. The movement of fluids through the reservoir will not be uniform if there are large variations in such properties as porosity, permeability, and clay cement. Limestone formations generally have wide fluctuations in porosity and permeability. Also, many formations have a system of microfractures or large macrofractures. Any time a fracture occurs in a reservoir, fluids will try to travel through the fracture because of the high permeability of the fracture, which may lead to substantial bypassing of hydrocarbon.
Many producing zones are variable in permeability, both vertically and horizontally, leading to reduced vertical, Ei, and areal, Es, sweep efficiencies. Zones or strata of higher or lower permeability often exhibit lateral continuity throughout a reservoir or a portion thereof. Where such permeability stratification exists, the displacing water sweeps faster through the more permeable zones so that much of the oil in the less permeable zones must be produced over a long period of time at high water-oil ratios. The situation is the same, whether the water comes from natural influx or from injection systems.
The areal sweep efficiency is also affected by the type of flow geometry in a reservoir system. As an example, linear displacement occurs in uniform beds of constant cross section, where the entire input and outflow ends are open to flow. Under these conditions, the flood front advances as a plane (neglecting gravitational forces), and when it breaks through at the producing end, the sweep efficiency is 100%—that is, 100% of the bed volume has been contacted by the displacing fluid. If the displacing and displaced fluids are injected into and produced from wells located at the input and outflow ends of a uniform linear bed, such as the direct line-drive pattern arrangement shown in Fig. 10.5(a), the flood front is not a plane, and at breakthrough, the sweep efficiency is far from 100%, as shown in Fig. 10.5(b).
Figure 10.5(a) Direct-line-drive flooding network.
Figure 10.5(b) The photographic history of a direct-line-drive fluid-injection system, under steady-state conditions, as obtained with a blotting-paper electrolytic model (after Wyckoff, Botset, and Muskat6).
Mobility is a relative measure of how easily a fluid moves through porous media. The ratio of effective permeability to fluid viscosity. Since the effective permeability is a function of fluid saturations, several apparent mobilities can be defined. When a fluid is being injected into a porous medium containing both the injected fluid and a second fluid, the apparent mobility of the displacing phase is usually measured at the average displacing phase saturation when the displacing phase just begins to break through at the production site. The apparent mobility of the nondisplacing phase is measured at the displacing phase saturation that occurs just before the beginning of the injection of the displacing phase.
Areal sweep efficiencies are a strong function of the mobility ratio. The mobility ratio M, is a measure of the relative apparent mobilities in a displacement process and is given by
A phenomenon called viscous fingering can take place if the mobility of the displacing phase is much greater than the mobility of the displaced phase. Viscous fingering simply refers to the penetration of the much more mobile displacing phase into the phase that is being displaced.
Figure 10.6(b) shows the effect of mobility ratio on areal sweep efficiency at initial breakthrough for a five-spot network (shown in Fig. 10.6(a)) obtained using the X-ray shadowgraph. The pattern at breakthrough for a mobility ratio of 1 obtained with an electrolytic model is included for comparison.
Figure 10.6(a) Five-spot flooding network.
Figure 10.6(b) X-ray shadowgraph studies showing the effect of mobility ratio on areal sweep efficiency at breakthrough (after Slobod and Caudle7).
The arrangement of injection and production wells depends primarily on the geology of the formation and the size (areal extent) of the reservoir. For a given reservoir, an operator has the option of using the existing well arrangement or drilling new wells in other locations. If the operator opts to use the existing well arrangement, there may be a need to consider converting production wells to injection wells or vice versa. An operator should also recognize that, when a production well is converted to an injection well, the production capacity of the reservoir will have been reduced. This decision can often lead to major cost items in an overall project and should be given a great deal of consideration. Knowledge of any directional permeability effects and other heterogeneities can aid in the consideration of well arrangements. The presence of faults, fractures, and high-permeability streaks can dictate the shutting in of a well near one of these heterogeneities. Directional permeability trends can lead to a poor sweep efficiency in a developed pattern and can suggest that the pattern be altered in one direction or that a different pattern be used.
Sandstone formations are characterized by a more uniform pore geometry than limestone formations. Limestone formations have large holes (vugs) and can have significant fractures that are often connected. Limestone formations are associated with connate water that can have high levels of divalent ions such as Ca2+ and Mg2+. Vugular porosity and high-divalent ion content in their connate waters hinder the application of injection processes in limestone reservoirs. On the other hand, sometimes a sandstone formation can be composed of small sand grains that are so tightly packed that fluids will not readily flow through the formation.
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