Development and Testing of a New Foam-Gel Technology to Improve Conformance of the Rangely CO2 Flood
Authors: Friedmann F. , Hughes T.L. , Smith M.E. , Hild G.P. , Wilson A. , Davies S.N. 
Published: December 2007

F. Friedmann, Chevron Petroleum Technology Company, T.L. Hughes, Schlumberger Cambridge Research, M.E. Smith, Chevron Petroleum Technology Company, G.P. Hild, Chevron USA Production Company, A. Wilson and S.N. Davies, Schlumberger Cambridge Research

Abstract
Thirty six thousand four hundred barrels of CO2 gel led foam were successfully placed via an injector well into the Rangely Weber Sand Unit in Colorado. The treatment objectives were 1) to improve volumetric conformance in this CO2 flood by reducing excessive CO2 breakthrough through fractures and 2) to increase oil recovery from the associated producers. Local reservoir characteristics indicate the need for a large-volume treatment to achieve these goals. The required treatment volume is beyond the economic limits of standard gel systems. Foam-gel technology is one way to economically achieve in- depth conformance improvement at Rangely by replacing 60% to 80% of the liquid phase by the less expensive CO2 phase.
A new gelled foam system was specifically designed for application to Rangely conformance problems. The field-tested surfactant/gel system was designed according to the following criteria:to produce strong and robust gel led foams under the harsh pH conditions of a CO2 flood,to provide enough gelation delay to achieve the injection of a large foam volume with manageable injectivity reduction andto considerably reduce the unit cost of the treatment fluid relative to standard non-foamed gel systems.
This paper describes the methodology used to design and test the optimum gel led foam system for Rangely. Laboratory results are presented to support the chemicals system selection, including gelation kinetics experiments, surfactant selection and core floods with supercritical CO2 at field conditions. The candidate well selection process is described, including injection profile surveys, offset well response and bypassed reserves calculation. Data taken during the injection phase of the program, including injectivity history and on-site quality control monitoring of the chemical system behavior are given.
Finally, a preliminary assessment of the impact of the treatment on CO2 cycling rates and incremental oil production is presented.

Introduction
Oil recovery efficiency in CO2 floods can be substantially reduced if premature CO2 breakthrough occurs at offset producers through fractures. Injector polymer gel treatments have successfully improved volumetric sweep in some CO2 field projects. CO2 diversion from the fracture network to the adjacent matrix rock resulted in the recovery of additional oil reserves. However, the volume of injector polymer gel treatments can be limited by cost which in turn limits the potential impact on reservoir sweep. If there is significant crossflow between the thief zone and the remainder of the reservoir, larger treatment volumes impact sweep over a large volume of the reservoir thereby, improving incremental oil recovery.
Gelled foam technology is one way to economically increase the treatment volume when reservoir characteristics dictate the need for in-depth conformance improvement. Fluid costs can be substantially reduced if 60% to 80% of the expensive aqueous phase can be replaced by a cheap readily available phase such as CO2. P. 883^