Change in Apparent Viscosity off CO2 Foam With Rock Permeability

Summary This paper summarizes new and previous high-pressure experiments measuring the mobility of CO2 foam in porous rock, including both sandstones and carbonates, with permeabilities ranging from < 1 md to hundreds of millidarcies. Foam mobility is defined here as the ratio of the combined flow rate per unit superficial area to the pressure drop required for simultaneous flow of dense CO2 and brine/surfactant through the sample and can be expressed in units of millidarcies per centipoise. The measured results, which can also be expressed in terms of the apparent viscosity of the CO2 foam, give a quantitative basis for the belief that this displacing fluid should be particularly effective in heterogeneous formations. From a macroscopic viewpoint, the CO2 foam acts like a single fluid with a viscosity equal to the ratio of the rock's brine permeability to the measured mobility. According to our results, the apparent viscosity depends on rock permeability in a nonlinear fashion, increasing from a minimum of about 2 cp for the tightest rocks to an apparent maximum of about 1, 000 cp for Berea. Individual mobility measurements were made at steady-state conditions; this is justified by consideration of oilfield operational constraints. Results of auxiliary experiments are given and the methods described. These include measurements of essential surfactant characteristics, such as durability against coalescence of the dense CO2 bubbles in brine/surfactant solution. Results given also show the influence of other factors, such as surfactant concentration and type, flowing fraction of the dense gas, and overall rate, on foam mobility.