Polymers adsorption in porous media

Figure 5.5. Two methods (A and B) for evaluating polymer adsorption in porous media from the core effluent profiles (after Willhite and Dominguez, 1977).

Introductory overview of polymer adsorption in porous media... 

In surveying the literature on polymer adsorption in porous media as it pertains to oil recovery applications, it is evident that the level and nature of polymer adsorption depends mainly on ... 

AMPS adsorption is found to be lower than HPAM, shown by data from Szabo (1979) in Table 5.10, where the polymer used is HPAM if not marked with AMPS. Broadly, xanthan adsorption in porous media is rather less than that of HPAM and also tends to show less sensitivity to the salinity/hardness conditions of the solvent (Sorbie, 1991 Green and Willhite, 1998). However, this conclusion is not supported by the data shown in Figures 5.38 and 5.39, which show that the median adsorption for synthetic polymers (24 pg/g) is lower than that for biopolymers (35 pg/g). 

Cohen and Christ (unpublished) presented a new experimental technique for determining mobility reduction resulting from polymer retentions in porous media. Their method was designed to separate the contributions of adsorption and non-adsorptive retention to be measured in their flow experiments using HPAM. This was done by using a silane treatment of the silica in their sandpacks, which changed the surface such that it no longer adsorbs HPAM. 

Polymer adsorption at a liquid/solid interface is a very well-established phenomenon and has an enormous associated literature (Lipatov and Sergeeva, 1974 Parfitt and Rochester, 1983). On the evidence from porous medium flow experiments it appears that mechanical entrapment is also a reasonably well-established mechanism for polymer retention in flow through porous media. Hydrodynamic retention is a rate-dependent effect which is rather less well understood. However, this retention mechanism is not a very large contributor to the overall levels of polymer retention in porous media and, although interesting, is probably not a very important effect in field-scale polymer floods. The important point to note is that it must be understood sufficiently well in laboratory floods so that core flood results can be interpreted correctly concerning polymer adsorption and entrapment retention mechanisms. 

This section revisits adsorption as the main fundamental mechanism of polymer retention in porous media in most practical situations. That is, it is assumed that mechanical retention has been screened out and that hydro-dynamic retention is small. 

The general phenomenon of polymer adsorption/retention is discussed in some detail in Chapter 5. In that chapter, the various mechanisms of polymer retention in porous media were reviewed, including surface adsorption, retention/trapping mechanisms and hydrodynamic retention. This section is more concerned with the inclusion of the appropriate mathematical terms in the transport equation and their effects on dynamic displacement effluent profiles, rather than the details of the basic adsorption/retention mechanisms. However, important considerations such as whether the retention is reversible or irreversible, whether the adsorption isotherm is linear or non-linear and whether the process is taken to be at equilibrium or not are of more concern here. These considerations dictate how the transport equations are solved (either analytically or numerically) and how they should be applied to given experimental effluent profile data. 

In this section, the relevant laboratory tests that should be carried out on polymers in support of a proposed polymer pilot will be described. Virtually all of the actual technical points concerning polymer properties—such as compatibility/stability, filterability and formation plugging, polymer solution preparation, adsorption in porous media, in-situ rheology—have been discussed in detail elsewhere in this book. However, earlier the objective was to present an explanation and a view on the science of the various phenomena involved in polymer physics and chemistry both in bulk solution and in flow through porous media. Here, the intention is to abstract the much more limited subset of experiments that should be carried out in support of a practical polymer flood application in the field. In all of the discussion below, it is assumed that a range of commercially available off-the-shelf polymers... 

Malmberg, E.W. Smith, L. The Adsorption Losses of Surfactants in Tertiary Recovery Systems in Porous Media in Improved Oil Recovery by Surfactant and Polymer Flooding, Shah, D.O. Schechter, R.S. (Eds.), Academic Press New York, 1977, pp. 275-292. 

Hirasaki, G.J., Pope, G.A., 1974. Analysis of factors influencing mobility and adsorption in the flow of polymer solution through porous media. SPEJ (August), 337—346. 

Somasundaran, P. Adsorption from Flooding Solutions in Porous Media A Study of Interactions of Surfactants and Polymers with Reservoir Minerals U.S. Department of Energy Bartlesville, OK, 1989 DOE/BC/10848-15 (DE98000733) Contract No. AC19-85BC10848. 

Instantaneous adsorption. During the flow of polymer solutions through porous media, the hydrodynamic convection of macromolecules brings them into contact with solid surface in a mean time t which is of the order of a few seconds under our experimental conditions. Since the probability for a macromolecule to be adsorbed during the first contact with the pore surface free of adsorbed pol3mier is very high, the polymer adsorption can be considered as instantaneous. Indeed, this mean time is negligible compared to the residence time of macromolecules t inside the column. 

COHEN, Y. and CHRIST, F.R. "Polymer Retention and Adsorption in the Flow of Polymer Solutions through Porous Media", SPEJ, 1985, 25... 

Although many valuable statements concerning the adsorption of the polymers in porous media exist until now a detailed and comprehensive paper has not appeared. In this paper investigations of adsorption phenomena carried out during the past decade in the Petroleum Engineering Research Laboratory of the Hungarian Academy of Sciences will be summarized (16,17). 

If the polymer solution also contains surface active agents an additional factor can be the interaction of the polymer and tens id molecules and the mutual adsorption of the molecules and micelles. To determine how these factors influence the dynamic adsorption of polymers in porous media, more work is required, and we face the problem that all special natural system must be treated individually. 

It is widely recognized that during polymer solution flow in porous media, a portion of the polymer is retained [1-11]. It is evident from the cited papers that both the physical adsorption of polymer onto solid surfaces and the polymer retention by mechanical entrapment play a role in the total polymer retention. The question which of these mechanisms of polymer retention is more important has remained unanswered. 

In the following two subsections, some of the main features of the adsorption of HPAM and xanthan in porous media will be outlined. It is convenient to separate the discussion of these two polymer types because of their structural differences, as explained earlier in this work. HPAM is a flexible coil polyelectrolyte and, for the reasons discussed previously, shows much more sensitivity to solution conditions, pH, salinity, etc., than xanthan, which has a more rigid molecular structure. 

Before discussing the issues concerning the polymer experimental procedures, it is necessary to establish the conditions under which the more traditional field core data have been gathered (i.e. core permeabilities, relative permeabilities, etc.). Central to such consideration is the matter of core wettability and how the core has been conditioned or restored for the relative permeability experiments and, therefore, for the polymer flooding experiments. This very important matter will not be considered here, but it will be assumed that the wettability and conditioning of the reservoir core have been satisfactorily achieved. This is important for polymer properties, since the adsorption is thought to be greater in water-wet cores than in oil-wet systems. In the discussion below, it will be assumed in all cases that experiments in porous media use correctly conditioned field cores at residual oil (unless otherwise stated). The oil will be the (dead) field oil, and conditions of reservoir temperature, but not necessarily pressure, will be established in the core. 

Hirasaki, G. J. and G. A. Pope (1974). "Analysis of Factors Influencing Mobility and Adsorption in the Flow of Polymer Solution Through Porous Media." (08). 

Adsorption from Polymer Solutions and the Flow of Colloid-Polymer Dispersions through Porous Media, Preprint in U8th National Col-olid Symposium, the University of Texas at Austin, Austin, Texas, June 2U-26, I97U, p. 165 (with H. Chao, E. Chough, C. Earl, G. Lopatin, J. Wadman). 

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. 

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