Pore volume, inaccessible

The efficiency and economics of oil recovery can be adversely affected by interactions between surfactant aggregates and polymer. Such interactions occur because of mixing at the boundary between surfactant and buffer solutions, and because residual surfactant adsorbed on the rock surface may later desorb into polymer solution. Mixing of polymer and surfactant may also occur throughout the surfactant bank because of the "polymer inaccessible pore volume" effect (1 ). Large polymer molecules are excluded from the smaller pores in the reservoir rock, and travel faster than the surfactant. Thus, polymer molecules enter into the surfactant slug. 

Polymer inaccessible pore volume results in a faster polymer velocity that is opposite to the polymer adsorption effect. The polymer inaccessible pore volume effect can be included in the Dp term. Lake (1989) explicitly added the term -c )ipv in Eq. 2.104 to include this effect. The unit of ( )ipv is fraction of porosity. 

When polymer molecular sizes are larger than some pores in a porous medium, the polymer molecules cannot flow through those pores. The volume of those pores that cannot be accessed by polymer molecules is called inaccessible pore volume (IPV). In an aqueous polymer solution with tracer, the polymer molecules will run faster than the tracer because they flow only through the pores that are larger than their sizes. This results in earlier polymer breakthrough in the effluent end. On the other hand, because of polymer retention, the polymer breakthrough is delayed. In other words, if only polymer retention is considered, the polymer will arrive in the effluent later than the tracer. 

The chromatographic separation of polymer and surfactant caused by the polymer s inaccessible pore volume cause the polymer to flow ahead of the surfactant thus, polymer is sacrificed for adsorption. Because some adsorption sites are covered by polymer molecules, fewer of the sites are available for surfactant adsorption this is called competitive adsorption. To consider competitive adsorption, we treat surfactant adsorption as a function of adsorbed polymer concentration. 

There is another phenomenon that is called polymer inaccessible pore volume (IPV). Laboratory data indicate that inaccessible pore volume is usually greater than the adsorption loss for polymers following a micellar solution (Trushenski et al., 1974). The competitive adsorption and IPV may make polymer penetrate the surfactant slug ahead of it. Therefore, surfactant-polymer interaction or incompatibility occurs not only in the surfactant-polymer process where the surfactant and the polymer are injected in the same slug, but also in the surfactant-polymer process where surfactant is injected before the polymer slug. 

Seright, R.S., Maerker, J.M., Holzwarth, G., 1981. Polymer Preprints (Polym. Prepr.) 22, 30-33. Shah, B., 1978. An Experimental Study of Inaccessible Pore Volume as a Function of Polymer Concentration during Flow Though Porous Media. M.S. thesis, University of Kansas, Lawrence. 

Adsorption measurements of nitrogen (-196°C) and n-hexane (0°C) on USHY, H-MOR, HZSM-5 and H-Erionite that were coked with n-heptane at 450°C, were carried out. The inaccessible pore volume in each case (Va), and the volume occupied by coke (Vr) were calculated. Typically, for low coke content the value is close to 1 (except for H-ERI), and at higher coke content it drops. The comparison of nitrogen and n-hexane results made it possible to discuss whether the coke inhibited the access of reactants to the active sites or not. 

Inaccessible Pore Volume and Surface Exclusion Chromatography... 

The effects of inaccessible pore volume (IPV) strictly defined as the pore volume where molecules cannot penetrate due to exclusion n the basis of size is very small (IPV << 1%) for microgel-free solutions (Rq r) pm)... 

For high permeability porous media with large particles like natural sands, the true inaccessible pore volume has a negligible effect on pol3nner velocity compared to that of surface or adsorbed layer exclusion phenomena. 

DAWSON, R, and LANTZ, R.B. "Inaccessible Pore Volume in Polymer Flooding", SPEJ, 1972, 448... 

LOTSCH, T., MULLER, T. and PUSH, G. "The Effect of Inaccessible Pore Volume on Polymer Core Flood Experiments", SPE 13590, Arizona, April 1985... 

SHAH, B. N., WILLHITE, G. P. and GREEN, D. W., "The Effect of Inaccessible Pore Volume on the Flow of Polymer and Solvent Through Porous Media", SPE 7586, presented to 53rd SPE Ann. Fall Conf.,... 

Before casting Equation 7.1 in dimensionless form, the inclusion of terms to describe adsorption and velocity enhancement of the transported species will be considered. These phenomena are, of course, more appropriate to polymer transport than tracer transport but the form of the equation is very similar. The velocity enhancement referred to concerns the effect of the excluded volume or inaccessible pore volume effect which the polymer shows (Chauveteau, 1982, Dawson and Lantz, 1972) and which is discussed in more detail below. However, the physical observation on polymer transport is that there appears to be a fraction of the pore space—either the very small pores (Dawson and Lantz, 1972) or regions close to the wall of the porous medium (Chauveteau, 1982)—which is inaccessible to polymer transport. This leads to an enhancement of the average velocity of the polymer through the porous medium. When there is both adsorption of transported polymer onto the rock matrix and a fraction of the pore volume is apparently inaccessible to the polymer, Equation 7.1 must be extended to ... 

Excluded/inaccessible pore volume effects in polymer transport through porous media... 

Figure 7.9. Original experimental demonstration of the inaccessible pore volume phenomenon using HPAM and salt (from Dawson and Lantz, 1972).

Measurement of transport parameters The main measurement of interest under this heading is of the excluded/inaccessible pore volume (IPV) of polymer relative to tracer as parameterised by the core permeability. If this quantity is known, then it should be included in the simulation studies since it may have some effect on the relative breakthrough times of polymer and tracer. However, it has been found that the IPV effect is usually dominated by the frontal retardation of the polymer as a result of adsorption/retention, and it is not generally of major importance in the assessment of the outcome of the polymer flood. Other measurements, such as of polymer dispersion coefficient and viscous fingering parameters, are primarily of importance for interpreting detailed core flood experiments since they do not scale in a simple way to the field and cannot therefore be used directly in the polymer field-scale simulations. 

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