Residual permeability reduction factor

Crosslinked polymer-like bulk gel used in water shut-off has very poor flowability the viscosity is very high (>10,000 mPa s). Uncrosslinked polymer is used to increase water viscosity. A movable gel is used in between it has the intermediate viscosity, and more importantly, it can flow under some pressure gradient. Colloidal dispersion gel (CDG) is a typical gel used in these situations. The mechanisms of a movable gel are (1) it has high viscosity to improve mobility ratio like an uncrosslinked polymer solution (2) it has a high resistance factor and high residual permeability reduction factor and (3) it has viscoelasticity so that the remaining oil in the rocks can be further reduced. 

Eqs. 5.36 to 5.38 can be used to describe the irreversible and reversible processes of polymer permeability reduction. If they are used to describe an irreversible process, an additional parameter called residual permeability reduction factor, F]j , mnst be nsed to track the history of Fk, so that... 

Note Eq. 5.40 defines the term residual permeability reduction factor. In the literature (Jennings et al., 1971 Bondor et al., 1972 Sorbie, 1991 Green and Willhite, 1998 UTCHEM-9.0, 2000), the term residual resistance factor (Frr) is used to represent the residual permeability reduction factor (F]j ). Their residual resistance factor is defined as ... 

Resistance is related to mobility, which includes the effects of both permeability reduction and viscosity increase. Obviously, the viscosity effect is not included in the residual resistance factor defined in Eq. 5.41 because water viscosity is used before and after polymer flow. Such a name convention is confusing. Therefore, we suggest the terms permeability reduction factor and residual permeability reduction factor be used. If the process were considered reversible, there would be no need for the term of residual permeability reduction factor. To include both permeability reduction and viscosity increase, we define another parameter, resistance factor (F,) ... 

For adsorbing polymers and weak gels, permeability reduction factors and residual permeability reduction factors increase with decreased permeability (Seright, 2006). 

The residual permeability reduction factor in the test area was about 2. 

The residual permeability reduction factors using the Hall plot in the test area were 1.5 to 2.5. 

In ASP flooding, alkaline, surfactant, and polymer have different effects on relative permeabilities. Table 13.2 shows our attempt to summarize these effects compared with waterflood. From Table 13.2, we can see that the effect of alkaline flood in terms of emulsification is similar to the polymer effect, whereas its effect in terms of IFT is similar to the surfactant effect. Less rigorously, we may say that only polymer reduces k, and only surfactant reduces IFT. In ASP flooding, the viscosity of the aqueous phase that contains the polymer is multiplied by the polymer permeability reduction factor in polymer flooding and the residual permeability reduction factor in postpolymer water-flooding to consider the polymer-reduced k effect. Then we can use the k curves (water, oil, and microemulsion) from surfactant flooding or alkaline-surfactant flooding experiments without polymer. 

Although excluded-volume effects are well-accepted in polymer chromatography, the preceding arguments are not without controversy in the petroleum literature because of the way that the apparent polymer viscosity in porous media was determined. In our work, we simply report the resistance factor (i.e., the brine mobility before polymer injection divided by the polymer-solution mobility). This is a well-defined parameter that derives directly from the Darcy equation and measurements of pressure drops and flow rates. Advocates of the depletion-layer effects use a different method to determine apparent polymer viscosity in porous media. Specifically, they flush water through the core after polymer injection to determine the permeability reduction or residual resistance factor. The resistance factor during polymer injection is then divided by the residual resistance factor to determine the apparent polymer viscosity in porous media. Unfortunately, several experimental factors can lead to incorrect measurement of high residual resistance factors, which, in turn, lead to calculation of unexpectedly low apparent polymer viscosities in porous media. 

From the pressure gradients determined for two flooding rates we are able to deduce the resistance factor RF, which serves as a measure of the flow resistance of the polymer solution in the pore space, as well as the residual resistance factor RRF. The factor RRF serves as a measure of the permeability reduction due to polymer material absorbed and retained in the pores. The values obtained for the VS/VA/AM-copolymers are particularly good. 

The results from a typical injectivity test are given in Figure 12 where resistance factor (RF) and residual resistance factor (RRF) are plotted versus the number of pore volumes of polymer and brine injected. Resistance factor is the mobility of brine (k/p) divided by the mobility of polymer and is a measure of the reduced Injection rate the polymer produces in a given reservoir rock. Residual resistance factor is the mobility of brine before polymer injection divided by the mobility of brine after polymer injection. Thus, a permeability reduction of 99 percent corresponds to an RRF of 100. 

The proper design of a profile modification treatment requires measurement of the permeability reduction caused by a given gel system. Core tests have shown that relative gel strength measurements with the capillary viscometer correlate with permeability reduction--Increasing gel strength develops higher residual resistance factors. 

It is convenient to describe the permeability reduction in terms of the initial brine permeability. In practice, this is done by defining the residual resistance factor (Eq. 5.12) as the ratio of the brine mobility before contact with polymer, to the brine mobility after aU mobile polymer has been displaced from the pore space. 

The permeability reduction usually persists for a large number of PV s of fluid throughput. In laboratory tests with relatively low fluid throughput, little change in brine permeability occurs, as shown in Table 5.8, However, prolonged fluid inyection eventually erodes the permeability reduction, as indicated in Fig. 5.29, where the residual resistance factor declines markedly with PV s throughput. 

The sand pack had three pressure taps, one in the center and two 10-in. eidier side. The absolute permeability of the core (100 percent water saturated) was lowered by about 15 percent after Kelzan had contacted the sand. However, the measured relative permeability to water at residual oil was reduced by a factor of 3 after Kelzan had contacted the sand. This reduction was permanent in that it persisted after flushing with a large volume of normal water. The reduction in relative permeability to polymer water at residual oil was the same between Taps 1 and 2 and between Taps 2 and 3. That is, there was no indication of plugging near the inlet end of the sand pack. Pressure drop and flow rate measurements during flow of the 268-ppm Kelzan solution in a core containing no oil indicated an effective viscosity of 3 cp. 

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