Dissolution and thickening mechanisms of ester-modified polysiloxanes in liquid CO2

CO 2 dry fracturing technology offers advantages such as environmental friendliness, mitigation of water-sensitive damage, and improved flowback efficiency, making it an ideal choice for the development of low-permeability and tight unconventional reservoirs. However, the inherently low viscosity of liquid CO 2 limits its proppant-carrying and fracture-propagating capacity. The introduction of efficient thickeners is currently the most effective method. In this work, an ester-modified polysiloxane polymer (PDH) was successfully synthesized via the copolymerization and hydrosilylation, its dissolution and thickening mechanisms in liquid CO 2 were systematically investigated using high-pressure phase equilibrium experiments combined with molecular dynamics simulations. Experimental results showed that both the cloud pressure and thickening performance of PDH in liquid CO 2 improved significantly with increasing thickener and co-solvent concentrations. At a thickener dosage of 5 wt% and co-solvent (ethanol) dosage of 5 wt%, PDH exhibited a cloud pressure of only 10.91 MPa and increased the viscosity of liquid CO 2 by 59.02 times. Simulation results indicate that strong dipole–quadrupole interactions between the polar ester groups and CO 2 enhance compatibility can reduce intermolecular distances and increase interaction energies. Furthermore, the higher rigidity of PDH chains and the formation of a stable solvation layer effectively restrict CO 2 mobility, leading to significantly lower mean square displacement and diffusion coefficients compared to PDMS. These features enable PDH to achieve high solubility and pronounced thickening effects at relatively low pressures. It can provide theoretical support for the synthesis and screening of CO 2 thickeners in the future.