A Technique For The Laboratory Measurement Of Carbon Dioxide Unit Displacement Efficiency In Reservoir Rock
Authors: Watkins Robert W. 
Published: December 2007

Abstract
Past research in miscible flooding has indicated some residual oil may be left in carbon dioxide swept rock as a result of CO2-oil phase behavior, rock pore structure, or mobile water saturation. Determination of the magnitude of this residual oil saturation in the reservoir rock of interest is important when a field is evaluated for CO2 miscible flooding. However, miscible displacements in laboratory outcrop cores sometimes exhibit a length effect, suggesting that reservoir rock samples as long as ten feet or greater may be required for the determination of the true unit displacement efficiency. This paper describes the development of a technique allowing the measurement of CO2 unit displacement efficiency in short core system consisting of either actual reservoir rock or outcrop sandstones.
A number of core floods were performed under varying conditions in which continuously injected CO2 displaced a synthesized reservoir fluid from 15- to 30-cm long Berea sandstone cores and low permeability carbonate cores from a West Texas permeability carbonate cores from a West Texas reservoir. For secondary and tertiary recovery floods in both core types, residual oil saturations comparable to those reported for long sandstone systems were observed when solvent fingering was controlled. Length and rate effects previously observed are demonstrated to depend primarily on solvent sweep efficiency rather than the mechanism for achieving miscibility. The current as well as previous studies show that gravity, transverse dispersion, and viscosity gradation may each be effective stabilizing mechanisms under appropriate conditions. This work allows the relative importance of each to be estimated in a core flood and for the first time their combined influence correlated with the measured overall displacement efficiency. Two methods are described and shown to be effective for generating the viscosity graded CO2-oil transition zone required for flood stabilization when gravity and transverse dispersion are relatively unimportant, as in short, low permeability reservoir core samples. Based on these concepts, laboratory CO2 floods may be designed to prevent viscous fingering now enabling measurement of the true CO2 unit displacement efficiency in short laboratory systems.

Introduction
Carbon dioxide miscible flooding is one of the more promising processes for enhanced oil recovery. Evaluation of prospects for full-scale CO2 flooding requires a thorough understanding of recovery behavior. Unit displacement efficiency is an important factor in such an evaluation and, because unit displacement efficiency may depend on rock and fluid properties, core displacement tests in actual properties, core displacement tests in actual reservoir rock using realistic reservoir fluids may be necessary for the evaluation of each candidate reservoir. This paper describes a technique for flooding short pieces of reservoir rock containing reservoir fluids at reservoir temperature to determine the residual oil saturation left in the fully-swept rock.
Very few CO2 miscible displacement tests have been performed in the past using actual reservoir rock. Instead, most laboratory studies of CO2 flooding (as well as with LPG and other solvents) have been conducted using outcrop sandstone cores. Berea, Boise and Torpedo sandstones were chosen due to their availability and uniformity. These sandstones can be cut easily into lengths of approximately 10 ft, and much longer systems for displacement studies can be constructed by butting cores together. Reservoir rock, on the other hand, must be obtained from oil-field core samples. Lengths of reservoir rock for laboratory flooding are limited to several feet at most for stacked cores and to considerably less for single cores.
A wide variation in the apparent residual oil saturation left to miscible flooding, Sor, has been observed in laboratory displacements with both CO2 above the multi-contact miscibility pressure and with first-contact miscible fluids even when similar porous media were used. porous media were used.