Development of Novel Silicon-Based Thickeners for a Supercritical CO2 Fracturing Fluid and Study on Its Rheological and Frictional Drag Behavior

Abstract

To address low viscosity of CO2 in supercritical CO2 fracturing, the authors studied silicon-based thickeners for a supercritical CO2 fracturing fluid. Two silicon-based thickeners, namely, PMSQ-VAc and PDMS-VAc, were prepared by solution polymerization and emulsion polymerization, respectively; for each of the thickeners, molecular structural characterization, observation of dissolution in supercritical CO2 with a pco.dimax cs1 high-frequency microscope camera, and exploration of viscosity-enhancing mechanisms were performed; a differential pressure method for fluid flow in a high-pressure delivery pipe was used to calculate viscosity and frictional drag coefficient of the supercritical CO2 fracturing fluid, and rheological behavior and frictional drag behavior of the fracturing fluid under different experimental conditions were evaluated. The test results show that, by solution polymerization, the PMSQ-VAc thickener could be prepared from vinyl acetate and polymethylsilsesquioxane at a mass ratio of 13.95:7.51, with a yield of 84%, a molecular weight of 49,170 g/mol, and a viscosity of 12.86 mPa·s; by emulsion polymerization, the PDMS-VAc thickener could be prepared from dimethylcyclosiloxane, vinyltriethoxylsilane, and vinyl acetate at a mass ratio of 2:1:3, with a yield of 93%, a molecular weight of 76,190 g/mol, and a viscosity of 21.47 mPa·s; in the absence of any cosolvent, both silicon-based thickeners had a good solubility and viscosity-enhancing effect in supercritical CO2, among which PDMS-VAc had a better solubility and viscosity-enhancing effect, enabling the viscosity of supercritical CO2 to be up to 10.12 mPa·s; after being incorporated with the silicon-based thickeners, the supercritical CO2 fracturing fluid exhibited rheological behavior in agreement with shear behavior of pseudoplastic fluid and thus belongs to non-Newtonian fluid; the viscosity of the supercritical CO2 fracturing fluid varied in a nonlinear manner with increasing temperature and pressure, i.e., increasing and then decreasing, and with increasing thickener concentration, the viscosity increased and then tended to be stable; the frictional drag coefficient of the fracturing fluid in pipe flow decreased gradually with increasing flow rate and pipe diameter and increased gradually with increasing viscosity of the fracturing fluid; two fracturing fluid systems consisting of silicon-based thickeners and supercritical CO2 barely or weakly damaged the reservoir core. The silicon-based thickeners are practicable to some extent during supercritical CO2 fracturing and likely to become high-performance, low-cost, pollution-free supercritical CO2 thickeners.