Viscosified CO2 Process: Chemical Transport and other Issues

Abstract

ABSTRACT Supercritical CO2, viscosified by two orders of magnitude using a commercial silicon polymer and toluene as cosolvent, was used in the laboratory coreflood and the results reported previously. More corefloods in Beres cores with pressure taps along the core were conducted to investigate the transport of chemicals in the core. In an attempt to reduce the chemical costs, different silicon polymers and hydrocarbon cosolvents were tried. The effect of methane on the solubility of polymers was also investigated. Scoping economic runs were made by modifying an actual economic analysis of a field CO2 project, which showed that chemical costs have to be reduced significantly for the process to be economic. INTRODUCTION Several indirect methods have been used to solve the problems of mobility control with the CO2 process: the water-alternating-gas (WAG) process and the use of foam to reduce gas mobility1. These methods have their own problems of gravity segregation and reduced oil mobilization due to water shielding2 and the difficulty with the control of foam generation and propagation in the field. To obviate the problems of indirect methods for mobility control, direct thickening of the CO2 has been attempted. There have been several direct methods mentioned in the literature: use of entrainers3, dispersions4, in situ polymerization5, and polymers6. Early work on using polymers to thicken the CO26 showed that, among 53 commercially available polymers, none has solubility high enough in supercritical CO2 to increase CO2 viscosity. We circumvented the low solubility of polymers in the supercritical CO2 by improving the solvent characteristics of CO2 using a cosolvent8. We reported previously9 the results of our laboratory experiments using a polydimethyl siloxane polymer from General Electric (GE), having a 600K cSt designation, and toluene as a cosolvent, which is capable of forming a one-phase admixture with the selected polymer at temperatures and pessures of our interest. The work proved the technical feasibility of a thickened CO2 process. On the other hand, there were some questions remaining, both technical and economic. How do the chemicals transport in the porous media and does the polymer partition into the oil phase and increase the viscosity? Are there more efficient and more economic polymers and cosolvents? What is the effect of light hydrocarbons on the solubility of polymers in the CO2 phase? What is the economic prospect of this process and what could be done to improve it? The objective of this paper is to answer some of these questions.