Summary
The objective of this paper is to report some experimental investigations on the effect of polymer adsorption on gas/water flow in non-Darcy regimes in homogeneous porous media, in contrast to previously available analyses focused mainly on the Darcy regime. Our investigation concentrates on gas flow either at low mean pressure, when Klinkenberg effects (or gas slippage) must be considered, or at high flow rates, when inertial effects are significant.
The experimental study reported here consists of water and nitrogen injections into various silicon carbide model granular packs having different permeabilities. Experiments are carried out at different water saturations before and after polymer adsorption over flow regimes ranging from slip flow to inertial flow. In good agreement with previous works, in the Darcy regime, we observe an increase in irreducible water saturation and a strong reduction in the relative permeability to water, while the relative permeability to gas is slightly affected. At low mean pressure in the gas phase, the magnitude of the Klinkenberg effect is found to increase with water saturation in the absence of polymer, whereas for the same water saturation, the presence of an adsorbed polymer layer reduces this effect. In the inertial regime, a reduction of inertial effects is observed when gas is injected after polymer adsorption, taking into account water-saturation and permeability modifications. Experimental data are discussed according to hypotheses put forth to explain these effects. Consequences for practical use are also put under prospect.
Introduction
Water/oil or water/gas flows in porous media are strongly modified in the presence of an adsorbed polymer layer on the pore surface. Several studies, performed in the Darcy regime, showed a phenomenon of disproportionate permeability reduction (DPR). The relative permeability to water (krw) is reduced more than the relative permeability to gas (krg) or to oil (kro). Although this effect was observed over most of the water-soluble polymer/weak gel systems and rock materials, the origin of this effect is still controversial in the literature. Several physical processes have been put forth to explain the selective action of the polymer.Mennella et al. (1998) studied water/oil flows in the presence of an adsorbed polymer layer in random packs of monodisperse spheres. They concluded that the DPR was caused by a swelling/shrinking effect depending on the kind of fluid flowing throughout the packs. They also explained the DPR by pore-scale topological modification (pore-size reduction). Similar studies (Dawe and Zhang 1994; Sparlin and Hagen 1984) were carried out on different systems such as micromodels.Some authors (White et al. 1973; Schneider and Owens 1982; Nilsson et al. 1998) have interpreted the effect of polymer by assuming that a porous medium is composed of separate oil/water pore networks. With this representation, the DPR can be explained by the fact that water permeability is affected by the hydrosoluble polymer present in the pore network occupied by water, while oil permeability is not.Many studies attributed the DPR to a wall effect (Zaitoun and Kohler 1988, 2000; Barreau 1996; Zaitoun et al. 1998), which decreases the pore section accessible to water. The physical origin of this mechanism is adsorption—almost irreversible—on the solid surface. An adsorbed polymer layer on pore walls induces steric hindrance, lubrication effects, and wettability modification, all of which are in favor of a stronger reduction of water permeability than of oil permeability. The physical relevance of this mechanism was tested on numerical simulations at the pore scale (Barreau et al. 1997).Liang and Seright (2000), following Nilsson et al. (1998), proposed to complete the explanation of DPR by a "gel-droplet" model. In this scenario, gel droplets formed in pore bodies cause a higher pressure drop at the pore throat in the wetting phase than in the nonwetting one.
These reported studies mainly have been dedicated to the polymer action on oil/water systems, and much less attention has been paid to gas/water flow. However, all available results in this last configuration lead to the same behavior, and the same type of physical explanation (wall effect) was proposed (Zaitoun and Kohler 1989; Zaitoun et al. 1991).
If published results dealing with the effect of polymer on permeability reduction observed in the Darcy regime are quite numerous, very little work has been dedicated to the non-Darcy regimes. Elmkies et al. (2002) reported laboratory experimental data showing that adsorbed polymer on natural porous-media cores decreases the inertial effects during gas flow.
In this paper, we focus our attention on the influence of adsorbed polymer on gas/water core flow in non-Darcy regimes. Gas injection was performed on unconsolidated cores having different permeabilities, at different water saturations, before and after polymer treatment, and at low mean pressure to investigate Klinkenberg effects, as well as at high flow rates, when inertial effects become important.
Exploring the impact of long-term evaporation on the relationship between capillary pressure and water saturation in unsaturated porous media