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

Abstract

A BCG − CO 2 fracturing fluid composed of a new thickener and CO 2 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 main purpose of this study was to ascertain the rheological parameters and a model of the BCG − CO 2 fracturing fluid. The rheological properties of the BCG − CO 2 fracturing fluid were evaluated by using a foam rheological test system. The unfoamed condition test results showed that when the shear rate was lower than 1000 s −1 , the effective viscosity exhibited a rapid decreasing trend. The effective viscosity of the BCG − CO 2 fracturing fluid was 4 to 26 mPa Á s under foamed conditions. The comparison results showed that the viscosity of the BCG − CO 2 foam was suitable as a low damage fracturing fluid. The measured data were fitted using the H-B and power law models in this study. The power law model, by contrast, was fit for describing the relationship between shear rate and shear stress. The foamed condition rheological index n 0 first decreased and then increased with increasing foam quality, and the rheological coefficient k 0 first increased and then decreased with increasing foam quality. For the different foam qualities, the rheological index n 0 first increased and then decreased with increasing temperature. The rheological coefficient k 0 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%. The findings of this study can help develop a better understanding of the heat transfer and flow coupling process in wellbore and fracture in fracturing treatment.