The Effect of Microscopic Heterogeneity on CO2-Foam Mobility: Part 1--Mechanistic Study

Summary The mixing and flow behavior of a CO2/surfactant-solution/oil system in porous media depends on the pore structure. Two-dimensional (2D) flow-visualization experiments in several glass micromodels, each with a different length scale of heterogeneity, were performed at reservoir conditions. The mechanisms associated with gas permeability reduction were investigated in terms of microscopic heterogeneity, presence of oil phase, surfactant concentration, and flow rate in secondary and tertiary miscible CO2 floods. Results showed that foam can be generated by upstream and downstream snapoff. Fluid diversion was obtained not only by lamellae and foam, but also by multiple interfaces of CO2, surfactant solution, and oil. depending on the pore structure of the micromodel. The most important factors in the in-situ generation of foam, however, were the mixing of fluids and the aspect ratio of the pore structure. Introduction CO2 field projects have frequently experienced early gas breakthrough, and production will stop when an uneconomically high CO2/oil ratio is attained. Both high-pressure gas-foams and chemical gels are currently being considered for use as high-permeability-zone blocking agents--i.e., in profile modification--in many on-going CO2 flooding operations. This paper presents our preliminary investigation into the mechanisms responsible for CO2 fluid diversion, increased CO2 sweep efficiency, and the mobilization of bypassed oil when using high-pressure CO2-foam as a blocking agent. The capillary-tube model was used in one of the first attempts to study the generation and propagation Of a single lamella or bubble. Fried reported that in his glass-tube model, larger foam bubbles subdivided into smaller bubbles as they passed through pore constrictions. At the same time, some of the oil in the region invaded by the foam became emulsified, and the newly formed oil particles soon became immobile. He reported that this foam subdivision and emulsification could have been responsible for the resulting flow blockage. JPT P. 872^