Two-Phase Flow Through Porous Media: Effect of an Adsorbed Polymer Layer

Abstract

SPE Members Abstract Polyacrylamide and polysaccharide adsorption on natural water-wet sand and sandstones is shown to modify markedly the flow behavior of both water and oil. At residual oil saturation, the quantity of adsorbed polymer per gram of rock is found to be almost the same as at full water saturation, but the permeability reduction to water is increased by the presence of trapped oil droplets which reduce the flow cross-section for the water phase. The presence of adsorbed polymer always induces an increase in the water irreducible saturation due to polymer hydration water which is not expelled by contact with oil. However the oil relative permeability is shown to be little affected by adsorbed polymer over the remaining saturation range. On the basis of a polymer over the remaining saturation range. On the basis of a capillary two-phase flow model, this phenomenon is interpreted as the result of the competition between the reduction of flow cross-sections and the enhancement of the water-wettability of the rock, with both effects having the same origin, i.e. the polymer adsorption process. polymer adsorption process Introduction High-molecular-weight water-soluble polymers are widely used as thickening agents in the oilfield industry. They can be injected into the near-wellbore zone of a reservoir as in well treatments, or deeper into the formation as in chemical flooding where polymer slugs provide the mobility control of the injected fluids. During the propagation of a polymer slug through a reservoir, a fraction of the polymer is permanently fixed on the rock by a process called "adsorption." The polymer adsorption process has two characteristics which make it different from adsorption of other chemicals:it is practically irreversible;it leaves on the surface of the rock an adsorbed polymer layer whose thickness is of the same order of magnitude as pore size. Both characteristics seem to be closely related to the high molecular weight of this class of products. Since each macromolecule is constituted of several products. Since each macromolecule is constituted of several thousands of subunits, the linkage of only some of them on the surface of the solid fixes the whole macromolecule. Moreover, the free subunits favor the formations of loops and tails which give the adsorbed polymer molecules a size comparable to free ones. Even if the bonding force between a polymer subunit and an adsorption site of the rock is weak, the sum of several bonding forces generally results in a strong attachment of the polymer molecule to this rock. This phenomenon explains why polymer molecule to this rock. This phenomenon explains why nonionic or weakly anionic polymers adsorb on solids whose average surface charge is negative. Electrostatic repulsions between polymer molecules and pore walls are not sufficient in this case to destroy all the bonding forces which tend to fix the polymer to the solid. polymer to the solid. A recent study has shown that, by a slow process, there is a permanent exchange between adsorbed and free polymer molecules in the surrounding solution. However, as soon as free molecules are removed by waterflooding, this process stops and adsorbed molecules appear to be permanently fixed on the solid. This phenomenon explains why, both in laboratory core flow experiments and in field cases, we observe that the portions of the reservoir which have "seen" polymer remain affected over long periods of time by the presence of adsorbed polymer, even after the washing out of this zone. As reported before, the hydrodynamic size of these high-molecular-weight polymers in solution is in the range of a um. The average pore size of most reservoir rocks is also of the same order of magnitude, ranging between a fraction of a um in the tighest formations to more than 10 um in the more permeable ones. It is thus expected that the presence of an permeable ones. It is thus expected that the presence of an adsorbed polymer layer at the pore wall will have, in most cases, a marked incidence on the flow of fluids through the pore channels. The effect of adsorbed polymer on water flow has been intensively studied in the past within the framework of polymer flooding studies. Adsorption of polymer in a core is generally found to decrease its permeability to water. This effect is characterized by a number called "permeability reduction" or "i residual resistance factor," which is equal to the ratio of initial-to-final permeability. P. 297

Aperture-Tolerant, Chemical-Based Methods to Reduce Channeling

“…21,14,15,[18][19][20] In other words, these materials damage lowpermeability rock more than high-permeability rock. Depending on the magnitude of this effect, these polymers and gels can harm production flow profiles in wells.…”

“…21,33,48,52 Additional work of this type is needed for other gels (especially pore-filling gels) if extensive RPM/DPR applications are to be applied in hydrocarbon zones that produce at intermediate fractional flows.…”

“…16 Also, suspensions of gel particles and adsorbed polymers provide resistance factors and residual resistance factors that increase with decreasing permeability. 14,15,[18][19][20][21] Not surprisingly, suspensions of small gel particles are more effective at restricting flow through small pores than through large pores. Straight-forward calculations using the Darcy equation reveal that this behavior can be detrimental for sweep improvement-both during polymer flooding and gel treatments.…”

“…During this series of floodingexperiment studies, one must also be concerned about the magnitude of residual resistance factors as a function of rock permeability. 15,[18][19][20][21] This latter comparison should be made using cores that were completely filled with polymer or gel (i.e., NOT using misleading parallel corefloods). If residual resistance factors in low-permeability rock are significantly greater than in high-permeability rock, polymer or gel treatments will impair sweep efficiency (see Fig.…”

Development of Polymer Gel Systems to Improve Volumetric Sweep and Reduce Producing Water/Oil Ratios

“…Liang et al (1995) 3 Lubrication effects. Sparlin and Hagen (1984) Zaitoun and Kohler (1988) 4 Gels constrict water pathways more than oil pathways in a given pore (wall effects).…”