Abstract
The low viscosity of pure liquid CO₂ hindered the development of CO₂ fracturing technology. A modified silicone polymer was prepared as a CO₂ thickener to investigate the effect of temperature, pressure, shear rate and thickener content (wt.%) on the apparent viscosity and rheology of thickened liquid CO₂. In addition, CO₂ fracturing capacity was evaluated with the numerical simulation of extended finite element. The results displayed that an apparent viscosity of up to 1.3 mPa·s at 303 K and 18 MPa was attained over liquid CO₂ using the thickener of 3 wt.% and Toluene of 9 wt.% as additives. Compared to the commercial linear polydimethylsiloxane, a better apparent viscosity was obtained from the mixture of this prepared thickener, Toluene and CO₂. The apparent viscosity decreases with increasing temperature and shear rate. By contrast, an improving apparent viscosity was revealed with an increase in the pressure from 8 to 14 MPa and thickener content from 1 to 3 wt.%. The rheological index decreased with increasing thickener content with pressure but the rise in temperature led to an increasing rheological index. The mesh structure theory of the thickener, CO₂ and Toluene molecules was in this paper gives a good explanation for the discrepancy between CO₂ viscosity with the thickener content, temperature, pressure, or shear rate. Compared to pure CO₂, the numerical simulation of CO₂ fracturing demonstrated an excellent fracturing capacity by using the thickened CO₂ fracturing fluid in shale reservoirs. This investigation could provide the basic reference for the development of CO₂ fracturing technology.