Abstract
To assess the dynamic stability of polymer slugs, flooding experiments have been performed in sandpacks and consolidated sandstone cores. The investigations have been carried out with a hydroxyethylcellulose (HEC) and a biopolysaccharide (BPS) (scleroglucane). Highly saline water (100 g/dm3 [0.2 lbm/L]) was used for the displacement tests, as well as for the preparation of the polymer solutions. preparation of the polymer solutions. From the eluate sampled during each flooding test, a concentration profile and a viscosity profile were recorded. A comparison between the two profiles gives an indication of the fraction of the polymer substance in the injected solution that does not contribute to the viscosity. From the recorded profiles, the influences of dispersion, adsorption/retention, and viscous fingering were established separately and quantitatively determined by means of material balances.
Introduction
In EOR, polymer slugs are used both for mobility control in the micellar/polymer process and for viscous flooding. To yield maximum success in polymer flooding - from a laboratory experimental, and not an economic, point of view - it is necessary that the mobility control effected by the polymer slug be maintained all the way from the injection well to the production wells. During its passage through the porous medium, the shape of the polymer slug, as indicated by the concentration profile, is altered and impaired as a result of interactions with the mobile phases and the rock matrix. phases and the rock matrix. In this paper, flooding experiments in linear models of porous media are described; the objective is to investigate the porous media are described; the objective is to investigate the stability of polymer slugs under dynamic conditions. The influence of the interactions that alter the slug has been determined separately and quantitatively established from these experiments. The fraction and distribution of the polymer substance in the injected solution that does not contribute to the viscosity is indicated too.
Theory
The action of a polymer slug is based on the adjustment of the mobility between the displaced phase and the displacing floodwater, 1 Hence, two transition zones characterize the mechanism of slug flooding:the transition from the displaced phase to the polymer slug andthe transition from the polymer slug to the polymer slug andthe transition from the polymer slug to the displacing floodwater.
If the mobility ratio at the leading edge of the slug is decreased to values near unity by the polymer solution, the effect of viscous fingering is suppressed, and the displacement is piston-like. At the trailing edge of the slug, on the contrary, an unfavorable viscosity ratio prevails; consequently, floodwater penetrates into the slug as a result of viscous fingering. In addition to these purely dynamic effects, the interaction between the rock matrix and polymer solution also causes an alteration of the slug during the passage through the porous medium. This change is described in terms of longitudinal dispersion and adsorption/retention. The term "longitudinal dispersion" designates the effect of microscopic inhomogeneities in the porous medium; this effect leads to a broadening of the mixing zone between the miscible fluids beyond that caused by molecular diffusion. "Adsorption" is defined here according to the Langmuir isotherm, whereas "retention" also includes the effects of filtration and time dependence of the adsorption equilibrium. The alteration of the polymer slug during passage through a porous model can be described polymer slug during passage through a porous model can be described by means of a comparison between the concentration profile of the slug emerging from the porous medium and that of the injected slug. Since the rheological properties of the mobile phases are also of importance for appraising the transport phenomena in the pore space, it is expedient to consider the viscosity profiles in pore space, it is expedient to consider the viscosity profiles in addition to the concentration profiles. Fig. 1 shows the originally injected rectangular profile (dashed line) and the concentration or viscosity profile obtained from the eluate of the flood test. The deviation of the leading edge of the profile from the rectangular shape is characterized by two parameters:the displacement of the 50% value by an amount "d"; andthe inclination of the profile line by an angle,.
The amount "d" is thereby a measure of the denudation of the leading zone by adsorption and retention, whereas the angle of inclination, is proportional to the influence of dispersion. The area denoted by "A/R, "the surface area corresponding to adsorption/retention, between the rectangular profile and the measured or extrapolated profile line is a measure of the polymer loss resulting from adsorption and retention. The effect of viscous fingering on the trailing edge is described in terms of the shift of the water breakthrough, "WD," from 1.0 PV after the start of waterflooding to lower values. The polymer initially delayed by viscous fingering is produced subsequently at lower concentration in the trailing edge but does not contribute to the viscosity of the displacing part of the slug.
SPEJ
P. 9
Digital image reduction for analysis of topological changes in the pore space of the rock matrix during chemical dissolution
