Status of Miscible Displacement
Authors: Stalkup Fred I. 
Published: December 2007

Abstract
Methods for miscible flooding have been researched and field tested since the early 1950's. This paper reviews the technical state of the art and field behavior to date for the major miscible flood processes: first-contact miscible, rich-gas drive, vaporizing-gas drive, and carbon dioxide flooding. Important technological areas selected for review include phase behavior and miscibility, sweepout, unit displacement efficiency, and process design variations. Carbon dioxide flood technology is emphasized, and several technical issues are identified that still need to be resolved. Rules of thumb and ranges of conditions are discussed for applicability of each process. A comparison is made of the incremental recovery and solvent slug effectiveness observed in field trials of the different processes. From the limited data available, processes. From the limited data available, there is no clear-cut evidence that field results on average and for a given slug size have been appreciably better or poorer for one process compared with another.

Introduction
The search for an effective and economical solvent along with development and field testing of miscible-flood processes has continued since the early processes has continued since the early 1950's. Early focus was on hydrocarbon solvents, and three types of hydrocarbon-miscible processes were developed: the first-contact miscible process; the vaporizing-gas drive process, often called high-pressure gas drive; and the rich-gas drive process, often called condensing-gas drive.
First-contact miscible solvents mix directly with reservoir oils in all proportions and their mixtures always remain proportions and their mixtures always remain single phase. Other solvents are not directly miscible with reservoir oils, but under appropriate conditions of pressure and solvent composition these solvents can achieve miscibility in-situ by mass transfer of oil and solvent components through repeated contact with the reservoir oil. miscibility achieved in this manner is termed multiple-contact or dynamic miscibility. The vaporizing-gas drive process achieves dynamic miscibility by in-situ vaporization of intermediate molecular weight hydrocarbons from the reservoir oil into the injected gas. Dynamic miscibility is achieved in the rich-gas drive process by in-situ transfer of intermediate molecular weight hydrocarbons from the injected gas into the reservoir oil.
Propane or LPG mixtures typically were the solvents used in first-contact hydrocarbon miscible flooding, whereas natural gas at high pressure and natural gas with appreciable concentrations of intermediate molecular weight hydrocarbons were injection fluids in vaporizing-gas drive and rich-gas drive floods. The high cost of propane, LPG, or rich hydrocarbon gas propane, LPG, or rich hydrocarbon gas dictated that these solvents be injected as slugs which usually were driven with natural gas. Flue gas and nitrogen also have been found to achieve dynamic miscibility at high pressures with some oils by the vaporizing-gas drive mechanism.
Hydrocarbon miscible processes have received extensive field testing since the 1950's, primarily in the United States and Canada. Over 100 projects were initiated during this time period. The majority were small-scale pilot tests involving one or at most a few injection wells; however, a number of large projects were undertaken involving several thousand acres or more (more than 4 000 000 m2). A few projects tested flue gas injection.