Waterflood front

The low-tension polymer flood technique consists of combining low levels of polymer-compatible surfactants and a polymer with a waterflood. This affects mobility control and reduces front-end and total costs. [929]. 

In water-wet reservoirs being waterflooded, oil is displaced ahead of the water. The injection water tends to invade the small and medium-sized flow channels (or pores). As the water front passes, the remaining oil is left in the form of spherical, unconnected droplets in the center of pores or globules of oil extended through interconnected rock pores but completely surrounded by water. This oil is immobile and there is little oil production after injection water breakthrough at the production well (145). 

Achievement of low mobility ratios at the fronts between displacing and displaced fluids is of even greater concern in enhanced oil recovery than in waterflooding owing to the high costs and/or low viscosities of the injected fluids. One response to this concern has been the continuing effort to develop a fundamental understanding of so-called foam flow, which employs aqueous solutions of properly chosen surfactants at relatively low capillary numbers to reduce the effective mobility of low viscosity fluids (see 5,6 and papers on foam flow in this volume). 

During a waterflood, the injected water displaces the interstitial water as well as oil. There are two fluid boundaries one between the injected water and the interstitial water, and the other between the displaced interstitial water and the oil ahead. We want to find the water saturation at the boundary between the injected water and interstitial water. Because a nonadsorbing chemical travels at the same velocity as the water front, we can use Eq. 2.88 to find the water saturation at the front (the boundary between the injected and interstitial water), Swb- From Eq. 2.88, if Di is zero, we have... 

The waterflood front is given by the classical Buckley-Leverett theory by drawing the tangent to the f versus S. curve from (S c. 0), as shown in Figure 2.16. The corresponding equation is... 

In the waterflooding process, Craig et al. (1955) found that if the water mobility was defined at the jiverage water saturation behind the displacement flood front—that is, = A.(S )— the data on areal-sweep versus mobility ratio... 

Here, u is the volumetric velocity normal to the front. In waterflooding, if we assume that only water flows upstream and only oil downstream, it follows that Ud = u . Then the previous equation becomes... 

Figure 10.17 illustrates the pressure and saturation changes that occur during an alkaline waterflood. Shown are typical pressure gradients and oil saturations during a flood of a sand-packed column previously waterflooded to residual oil saturation. Oil and water flow simultaneously ahead of the alkaline water front. Note the sharp gradient in oil saturation that occurs at the front (Figure 10.17c), and the rise and fall in pressure gradient behind the alkaline water front (Figure 10.17b). The schematic (Figure 10.17d) illustrates what is observed microscopically.

For all these cases, the total amount of each chemical was the same. The core flood results are shown in Figure 13.21. We can see that the incremental oil recovery factors over waterflooding in Schemes 2 and 4 were obviously higher than that in Scheme 1. The alkali and surfactant concentration gradients from high to low can overcome the negative effects at the displacement front caused by dilution, alkali consumption, and surfactant adsorption. 

In Example 3.1, the waterflood displacement performance was estimated for this reservoir. The flood-front saturation was 0.4206, and the average water saturation behind the flood front, Sy, was determined to he 0.485. The relative permeability of water is given by Eq. 5.67. ... 

The polymer-flood-front properties and are found by drawing a tangent from the point (0, -0,174) to the fractional-flow curve with polymer as the displacing phase. The tangent, shown in Fig. 5.57, intersects the fractional-flow curve for the waterflood at/wi. wi Locations of 5 3, and S i, are shown in Fig. 5.57. The value of S f is the same as in Example 3.5. 

There is a small difference (0.0754 PV) between the arrival of the waterflood front and the oil bank. Locations of saturations behind tire polymer front are found with... 

Arrival of Waterflood Front, tpf. Because there is no initial gas... 

The drive water is moving faster than the saturations in die polymer bank and gradually overtakes the polymer bank. The drive water arrives at the end of the linear system at the same time as the polymer flood front, x. Fig. 5.59 shows the path traced by the rear of the polymer bank. The location of the rear of the polymer slug is almost a linear function of time for this example. Thus, for this case, the rear of the polymer slug appears to travel at a constant velocity. Fig, 5.59 also shows the waterflood front, oil bank, polymer flood front, and paths of selected saturations in the polymer slug. Saturations in the drive-water region are discussed later. 

When the polymer flood front arrives at the end of the linear system, the displacement process becomes a waterflood. The WOR jumps from 3.53 in the oil/water bank to 27.2 at the polymer flood front and then continues to increase. The remainder of the oil will be produced at high WOR. Oil recovery when the polymer front reaches the end of the linear system is identical to that in Table 5.22 at ( =0.8079, corresponding to polymer-flood-front breakthrough. Remember that in the case of slug injection where the PV of polymer injected equals Dp, the polymer flood front disappears just as the polymer reaches the end of the linear system. Incremental oil displaced at this time is 27,658 STB from the injection of 0.424 PV of polymer solution. Polymer required in the slug is... 

When the polymer front is overtaken by the drive water, the process becomes a waterflood. For example, when tpp=DJ2 (in Example 5.8), the polymer front is overtaken at =0.5/1.2377= 0.404. Fig. 5.61 shows the saturation distribution at this instant. Note that the oil bank created by the polymer front is present, with a saturation discontinuity from to 5 ,x at the rear of the oil bank. This saturation discontinuity is not stable. Because of miscibility, the velocity of this discontinuity is given by... 

Solution. During polymer injection (f/j 0.212), the polymer flood performs exactly as described in Examples 5.7 and 5.8. A waterflood front forms at saturation S, followed by an oil bank that has constant water saturation Sw. The oil bank is displaced by a polymer flood front, Sj. Table 5.20 presented properties of these fronts. 

Example 5,11—Estimation of Pressure Drop During a Continuous Polymer Flood in a Linear Reservoir. Determine the pressure drop for the polymer flood in Example 5.7 when the waterflood front, xpf, is located at a distance of 0.75 from the entrance of the system. Base permeability, the permeability to oil at interstitial water saturation, is 250 md. The ipjection rate is constant at 200 B/D. Recall that the linear segment of the reservoir being simulated is 500 ft wide and 20 ft thick. Injection and production wells are 1,000 ft apart. 

When the waterflood front is located at Xpf=0.75, the dimensionless injection time is given by... 

There are two integrals to be evaluated in Eq. 5.128 when flood fronts from both waterflood and polymer floods are in the system. The regions are treated separately with the same approach. 

Displacement of oil by waterflooding is increasingly dominated by viscous forces as the viscosity of the oil displaced increases. When the oil mobility is less than that of the injected water, the oil-water interface is not a piston-like front." Instead, instabilities occur at the flood front in the form of viscous fingers which tend to penetrate the less mobile oil bank. The magnitude of these protrusions, or the degree of channeling, increases... 

We have found that solutions of typical waterflooding polymers do not occupy all of the connected pore volume in porous media. The remainder of the pore volume is inaccessible to polymer. This inaccessible pore volume is occupied by water that contains no polymer, but is otherwise in equilibrium with the polymer solution. This allows changes in polymer concentration to be propagated through porous media more rapidly than similar changes in salt concentration. At the front edge of a polymer bank the effect of inaccessible pore volume opposes the effect of adsorption and may completely remove it in some cases. 

In accordance with Eqs. 2 and 3, the connate water bank will break through at / = (S - 5,)/fy, and the polymer front and associated saturation discontinuity will break through at / = (S + b)/fg. Fig. 5 shows the oil recovery curve constructed from Eqs. 2 through 4 and, for comparison, the recovery curve for a normal waterflood as calculated by the Buckley-Leverett method. 

Subsequently, close to 2 pore volumes of polymer solution, at a concentration of 1,500 ppm, were injected into the core after the initial waterflood (see Figure 1). Injecting the polymer formulation with an effective viscosity of 25 mPa s, at a rate of 2 ml/hr, caused a rapid pressure increase. The oil was banked ahead of the polymer front, and the water cut decreased to 40%. The oil... 

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