The Effect of Phase Equilibria on the CO2 Displacement Mechanism

Abstract

Carbon dioxide flooding under miscible conditions is being developed as a major process for enhanced oil recovery. This paper presents results of research studies to increase our understanding of the multiple-contact miscible displacement mechanism for CO2 flooding. Carbon dioxide displacements of three synthetic oils of increasing complexity (increasing number of hydrocarbon components) are described. The paper concentrates on results of laboratory flow studies, but uses results of phase-equilibria and numerical studies to support the conclusions.Results from studies with synthetic oils show that at least two multiple-contact miscible mechanisms, vaporization and condensation, can be identified and that the phase-equilibria data can be used as a basis for describing the mechanism. The phase-equilibria change with varying reservoir conditions, and the flow studies show that the miscible mechanism depends on the phase-equilibria behavior. Qualitative predictions with mathematical models support our conclusions.Phase-equilibria data with naturally occurring oils suggest the two mechanisms (vaporization and condensation) are relevant to CO2 displacements at reservoir conditions and are a basis for specifying the controlling mechanisms. Introduction Miscible-displacement processes, which rely on multiple contacts of injected gas and reservoir oil to develop an in-situ solvent, generally have been recognized by the petroleum industry as an important enhanced oil-recovery method. More recently, CO2 flooding has advanced to the position (in the U.S.) of being the most economically attractive of the multiple-contact miscibility (MCM) processes. Several projects have been or are currently being conducted either to study or use CO2 as an enhanced oil-recovery method. It has been demonstrated convincingly by Holm and others that CO2 can recover oil from laboratory systems and therefore from the swept zone of petroleum reservoirs using miscible displacement. However, several contradictions seem to exist in published results.. These authors attempt to establish the mechanism(s) through which CO2 and oil form a miscible solvent in situ. (The solvent thus produced is capable of performing as though the two fluids were miscible when performing as though the two fluids were miscible when injected.) In addition, little experimental work has been published to provide support for the mechanisms of multiple-contact miscibility, as originally discussed by Hutchinson and Braun.One can reasonably assume that the miscible CO2 process will be related directly to phase equilibria process will be related directly to phase equilibria because it involves intimate contact of gases and liquids. However, no data have been published to indicate that the mechanism for miscibility development may differ for varying phase-equilibria conditions.This paper presents the results of both flow and phase-equilibria studies performed to determine the phase-equilibria studies performed to determine the mechanism(s) of CO2 multiple-contact miscibility. These flow studies used CO2 to displace three multicomponent hydrocarbon mixtures under first-contact miscible, multiple-contact miscible, and immiscible conditions. Results are presented to support the vaporization mechanism as described by Hutchinson and Braun, and also to show that more than one mechanism is possible with CO2 displacements. The reason for the latter is found in the results of phase-equilibria studies. SPEJ P. 242

Development and Optimization of Gas-Assisted Gravity Drainage (GAGD) Process for Improved Light Oil Recovery

“…mole% of methane, 30 mole% of n-butane and 45 mole% of n-decane. This standard gasoil system has been reported to have a slim-tube minimum miscibility pressure (MMP) of 1700 psia at 160 o F (Metcalfe and Yarborough, 1979). This miscibility pressure is further reproduced with phase diagram measurements (Metcalfe and Yarborough, 1979) and analytical model predictions (Monroe et al, 1990;Orr et al, 1993).…”

“…Metcalfe and Yarborough (1979) have studied the phase behavior by performing various experiments on reservoir fluids using CO 2 as the displacing phase and have concluded that more than one mechanism (vaporizing and condensing) is possible for a CO 2 -reservoir fluid system and that reservoir temperature and displacement pressure determine the type of mechanism (vaporization, condensing or vaporizing/condensing) that will control the displacement process.…”

“…This standard gasoil system has been reported to have a slim-tube minimum miscibility pressure (MMP) of 1700 psia at 160 o F (Metcalfe and Yarborough, 1979). This miscibility pressure is further reproduced with phase diagram measurements (Metcalfe and Yarborough, 1979) and analytical model predictions (Monroe et al, 1990;Orr et al, 1993). The IFT measurements in this gas-oil system at 160 o F and at various pressures were carried out using the capillary rise technique due to its suitability to measure low gas-oil interfacial tensions with good repeatability as judged from our earlier experiments in n-decane-CO 2 system.…”

Evaluating Reservoir Production Strategies in Miscible and Immiscible Gas-Injection Projects

“…cell (7,7,2) cell (9,9,2) b) Viscosity of the gas in the second layer As Fig. 3.39 indicates, the breakthrough gas into each cell has the highest viscosity, since it is rich in intermediate components of the oil.…”

A reduction method for phase equilibrium calculations with cubic equations of state

“…The first exemplification is given here for the MY10 mixture, taken from Metcalfe and Yarborough (1979). The MY10 synthetic mixture contains 10 normal-alkanes, with composition and BIPs of methane (taken from Firoozabadi and Pan, 2002) with the remaining components given in Table 1.…”