Experimental study on the rheology of CO₂ viscoelastic surfactant foam fracturing fluid

Foam provides a highly attractive alternative to conventional non-Newtonian fluids for various oil and gas fracturing applications because of high viscosity and low liquid loss. However, much work has been carried out under 7MPa, a pressure much less than that of the practical fracturing value. In this paper, the rheology of CO₂ viscoelastic surfactant foam fracturing fluid was investigated under downhole condition on a large scale flow-loop experiment system. The testing results show that the foam fluid is shear thinning and its rheological properties can be described by the power-law model. Besides, the character of the fluid is different greatly from that of single phase non-Newtonian fluid because of its formation of foam and structure of external phase. It was also found that the viscosity of CO₂ viscoelastic surfactant foam fracturing fluid was proportional to the increment of foam quality and pressure, which, however, was inversely proportional to the increase of temperature and shear rate. Moreover, through analysis of the effects of different factors on the rheology characteristics, the correlations of rheological parameters, as well as the relation of frictional resistance coefficient λ versus generalized Reynolds number R e, were obtained.