Variations and model of the rheological parameters of low damage BCG − CO₂ fracturing fluid

A BCG − CO₂ fracturing fluid composed of a new thickener and CO₂ causes low amounts of formation damage, and it can be used to stimulate low-permeability gas reservoirs with water sensitivity. However, at present the rheological properties of this foam fracturing fluid are unclear, which has limited its field application to some degree. The rheological parameters and a model of the BCG − CO₂ fracturing fluid were investigated. The rheological properties of the BCG − CO₂ fracturing fluid were studied using a foam rheological test system. The unfoamed condition test results indicated that when the shear rate was lower than 1000/s, the effective viscosity demonstrated a rapid decreasing trend. The effective viscosity of the BCG − CO₂ fracturing fluid was 4 to 26 mPa·s under foamed conditions. The comparison results showed that the viscosity of the BCG − CO₂ foam was suitable as a low damage fracturing fluid. The measured data were fitted using the H-B and power law models. The power law model, meanwhile, was fit for describing the relationship between shear rate and shear stress. The foamed condition rheological index n^″ first decreased and then increased with increasing foam quality, and the rheological coefficient k^″ first increased and then decreased with increasing foam quality. For the different foam qualities, the rheological index n^″ first increased and then decreased with increasing temperature. The rheological coefficient k^″ showed a decreasing trend with increasing temperature. When an exponential function, in which the foam quality and temperature were independent, was used for fitting, it was not accurate in predicting the rheological properties. The fitting results based on a quadratic polynomial surface model were better. The average calculation errors of the rheological parameters were all less than 3.32%.