A Comparison of Mixed-Gas Foams with N2 and CO2 Foam Fracturing Fluids on a Flow-Loop Viscometer

Summary A study of the rheological properties of mixed-gas foams was conducted on a flow-loop viscometer. The variables studied included different ratios of N2 and CO2 in 70%-quality foams, base gel concentration, crosslinking agent, and temperature. The rheological properties of mixed-gas foams are very similar to the rheologies of N2 and CO2 foams. This finding was anticipated, because the rheologies of N2 and CO2 foams are similar. The exact static stability behavior of these foams was not anticipated. Pure CO2 70%-quality foams have four to five times the static half-life as pure N2 70%-quality foams. Replacement of CO2 by N2 in a 70%-quality foam decreases the half-life of the foam. The inclusion of only 20%-quality N2 in a 70%-quality mixed-gas foam decreases the foam stability to equal a pure N2 70%-quality foam. We saw no evidence that addition of N2 to a CO2 foam improved its stability. Previously published foam-rheology equations for N2 and CO2 foams were found to be valid, and only minor adjustment to a few coefficients were needed to include the effects of mixed gases. A BASIC program is included which allows foam viscosity to be calculated as a function of quality, gas type and concentration, external liquid-phase power-law parameters n′ and K′, shear rate, and temperature. Fluid loss characteristics of mixed-gas foams were found to be the same as for N2 or CO2 foams.