Polymer Retention and Adsorption in the Flow of Polymer Solutions Through Porous Media

Abstract

Summary A new experimental technique based on a surface treatment process was developed for determining mobility reduction as a result of polymer adsorption in flow of polymer solutions through porous media. The experimental method also allowed the direct determination of adsorptive and nonadsorptive polymer retention from flow experiments. The adsorptive mobility reduction for the flow of polyacrylamide (J333 TM) mobility control polymer through silica sand was found to be as high as 14% at the lowest experimental stress level of 3.75 dynes/cm2 [0.375 Pa]. This corresponded to an effective hydrodynamic thickness (EHT) of the adsorbed polymer layer of 0.57 um. Both the mobility reduction and the EHT decreased with an increase in shear stress. The amount of adsorptive polymer retention accounted for about 35.2% of the total retained polymer. polymer retention accounted for about 35.2% of the total retained polymer. Introduction Polymer molecules adsorb well at solid interfaces, such Polymer molecules adsorb well at solid interfaces, such as the rock surfaces found in petroleum reservoirs. The adsorbed polymer layers may represent both an additional resistance to flow as well as a loss of polymeric additive and are, therefore, of fundamental importance in EOR operations that involve the flow of polymer solutions through porous media. Polymer adsorption in the flow of polymer solutions through porous media is usually accompanied by a variety of additional complex phenomena. Viscoelasticity has been shown to contribute to increased flow resistance, particularly at high flow velocities. Consequently, the particularly at high flow velocities. Consequently, the effect of adsorption on permeability reduction is often obscured. In addition, phenomena such as mechanical entrapment and hydrodynamically induced retention can obscure the role adsorption plays in polymer retention and mobility reduction. The literature reveals that much effort has been devoted to quantifying the contribution of polymer adsorption to mobility reduction and polymer polymer adsorption to mobility reduction and polymer retention in both consolidated and unconsolidated porous media. The existing studies, however, do not quantify the resistance of the absorbed polymer layer to the flow of the polymer solutions. The phenomena contributing to the enhanced flow resistance (i.e., lower mobility) that polymer solutions experience in porous media have been discussed in numerous studies and review articles. Early investigators attributed all enhanced flow resistance (beyond that which shear viscosity alone would explain) to adsorption. Others have claimed that there was little correlation between adsorption and flow resistance. Smith compared heat-treated and untreated Berea sandstone cores and observed no correlation between the extent of adsorption and flow resistance. Similarly, Jennings et al. concluded that the retention (adsorptive plus mechanical) of polymer was not accompanied by an increased resistance polymer was not accompanied by an increased resistance to flow. The determination of flow reduction of the mobility control fluid requires knowledge of the degree of adsorptive retention of the polymer in the porous medium. This problem was addressed in the early work of Mungan and problem was addressed in the early work of Mungan and Mungan et al. These investigators attributed the difference between total polymer injected into a porous medium test section and the total polymer eluted from that section to adsorption. Later studies have postulated the existence of such polymer retention mechanisms as mechanical entrapment or hydrodynamically induced retention. Willhite and Dominguez have provided a clear account of the proposed mechanism by which entrapment might occur. Hydrodynamically induced retention is a second retention phenomenon that has been postulated. It is revealed by a dependence of retention postulated. It is revealed by a dependence of retention level on fluid velocity. These studies suggest that the magnitude of the contribution of hydrodynamic and mechanical retention to the total polymer retained in the porous medium also depends on the pore structure. porous medium also depends on the pore structure. Since the late 1960's, various attempts have been made to assess the importance of adsorptive retention relative to other modes of retention in porous media flow. These have generally-involved the comparison of flow retention with static adsorption (on a disaggregated sample) for a given adsorbent medium or the comparison of flow retention data from adsorbing and nonadsorbing porous media of different pore structures. porous media of different pore structures. Adsorbing vs. Nonadsorbing Porous Media. It is a well-established fact that native silica sand readily adsorbs hydrophilic polymers such as partially hydrolyzed polyacrylamide (HPAM). Thus a bed packed with native polyacrylamide (HPAM). Thus a bed packed with native silica sand would constitute an adsorbing porous medium with respect to HPAM. It is possible, with a technique introduced by Cohen and Metzner, to modify chemically the silica surface to render it nonadsorbing. This method uses a bifunctional silane compound, dimethyldiethoxysilane. p. 113