N-2 AND CO2 FOAM FRACTURING FLUIDS ON A FLOW-LOOP VISCOMETER

A study of the theological properties of mixed-gas foams was conducted on a flow-loop viscometer. The variables studied included different ratios of N-2 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 theologies of N-2 and CO2 foams. This finding was anticipated, because the theologies of N-2 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 N-2 70%-quality foams. Replacement of CO2 by N-2 in a 70%-quality foam decreases the half-life of the foam. The inclusion of only 20%-quality N-2 in a 70%-quality mixed-gas foam decreases the foam stability to equal a pure N-2 70%-quality foam. We saw no evidence that addition of N-2 to a CO2 foam improved its stability. Previously published foam-theology equations for N-2 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 N-2 or CO2 foams.