High 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. 

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. 

Polymer Flooding. Even in the absence of fractures and thief 2ones, the volumetric sweep efficiency of injected fluids can be quite low. The poor volumetric sweep efficiency exhibited in waterfloods is related to the mobiUty ratio, Af, the mobiUty of the injected water in the highly flooded (low oil saturation) rock, divided by the mobiUty of the oil in oil-bearing portions of the reservoir, (72,73). The mobiUty ratio is related to the rock permeabihty to oil, and injected water, and to the viscosity of these fluids by the following equation ... 

Silicate gel enhances the sweep efficiency of a waterflood, gasflood, or steamflood operation by reducing the permeability of the high-permeability zones. Weak acids may be added to control gel generation rate [377]. 

Volumetric sweep efficiency is determined by the permeability and wettability distribution in the reservoir and by the properties of injected fluids. Waterflooding characteristically exhibits poor volumetric sweep efficiency. The more expensive the injection fluid, the more important it is to have a high volumetric sweep efficiency so that the injected fluid contacts and thus mobilizes a larger volume of oil. High permeability streaks or layers (thief zones) and natural or induced rock fractures can channel the injected fluid through a small portion of the reservoir resulting in a low volumetric sweep efficiency. 

The displacing fluid may be steam, supercritical carbon dioxide, hydrocarbon miscible gases, nitrogen or solutions of surfactants or polymers instead of water. The VSE increases with lower mobility ratio values (253). A mobility ratio of 1.0 is considered optimum. The mobility of water is usually high relative to that of oil. Steam and oil-miscible gases such as supercritical carbon dioxide also exhibit even higher mobility ratios and consequent low volumetric sweep efficiencies. 

High gas mobility and low sweep efficiency are typical problems encountered in oil recovery processes using gas injection. 

The high-pressure and temperature micromodel system has been used in this study to investigate the formation, flow behavior and stability of foams. Micromodel etching patterns were made from binary images of rock thin sections and from other designs for a comparison of pore effects. These experiments show how simultaneous injection of gas and surfactant solution can give better sweep efficiency on a micro-scale in comparison to slug injection. 

The micelles present also help to solubilize the released oil droplets hence, this process is sometimes referred to as micellar flooding. The emulsions can be formulated to have moderately high viscosities that help to achieve a more uniform displacement front in the reservoir this uniform front gives improved sweep efficiency. Thus, a number of factors can be adjusted when using a microemulsion system for enhanced oil recovery. These are discussed in detail in Chapter 7. 

Gas injection can also recover oil by reducing oil viscosity and residual oil saturation, even when miscibility is not achieved. Reduction in viscosity is more significant if the oil viscosity is large, and this process is attractive in viscous or semiviscous reservoirs, especially when accompanied by some other improved recovery mechanism. Residual oil saturation in three-phase flow in water-wet rock is very low (essentially zero), even at very low capillary numbers. Two main problems in such a process are the low relative permeabilities and sweep efficiencies. This process can be implemented in a highly dipping reservoir to take... 

In the Xia-er-men field operated by Henan Oilfield, Sinopec, the produced water was used to make a polymer solution. Because of the high viscous oil and very heterogeneous reservoir, a normal polymer solution was not good enough to reach desired sweep efficiency. Profile control was tried instead. Because of small injection volume, however, water soon bypassed the injected gel. Therefore, a large volume of weak gel (deep profile control) was tested in a pilot. 

As described in Chapter 4, the higher the displacing fluid viscosity, the higher the sweep efficiency (recovery factor). Chapter 4 proposed that the displacing fluid mobility should be equal to the displaced oil mobility corrected by the oil saturation. In Daqing, however, the polymer solution viscosity of three to five times the oil viscosity was used so that a high oil recovery factor can be obtained in heterogeneous reservoirs. 

Yang and Me (2006) reported that Scheme 1 was better because a high-concentration polymer slug has a higher mobility ratio so that the sweep efficiency was better. When the polymer molecular weight was higher, the advantage of Scheme 1 was more obvious. 

Other mechanisms, which are not discussed here, are more or less related to emulsification and reduced IFT due to in situ generation of soap. One application based on these mechanisms is to inject alkaline solution and gas, simultaneously or alternately, to improve sweep efficiency. As we know, there is a viscous fingering problem for gas injection only. Injection of an alkaline solution in a reservoir with active crude oil will generate 0/W and W/0 emulsions. The high viscous emulsions and foam formed through gas injection will reduce the viscous fingering problem. In this case, CO2 cannot be injected because it will neutralize the alkaline solution. 

Improved microscopic sweep efficiency and displacement efficiency as a result of polymer viscoelastic property. Oil in the dead ends is pulled out, and the oil films on the pore walls are peeled off owing to the high-velocity gradient. 

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