Performance Improvement of Thickened Liquid CO₂ by Introducing a Philic-CO₂ Silicone Polymer

As an excellent stimulation treatment for the shale reservoir, CO₂ fracturing liquid is injected into the reservoir to increase crude oil production due to the high mobility and high backflow. Nevertheless, many disadvantages in fracturing process, including high fluid loss volume, short fracture length and low viscosity of pure CO₂, hinders its application and development. In this investigation, a novel silicone thickener consisted of CO₂-Phobic Phenyl and CO₂-Philic Ester group is prepared. The thickening performance of a thickener in liquid CO₂ is measured by capillary viscosity measurement system at different conditions. Molecular simulation presents that a mesh structure can be formed among CO₂, solvent and silicone, and the density of mesh structure affects the viscosity of thickened liquid CO₂ Meanwhile, the filtration property and fracturing simulation are compared between pure CO₂ and thickened CO₂. With the capillary viscometer, the viscosity of thickened liquid CO₂ can be increased to 2 mPa·s at 3 wt% thickener at 303 K and pressure of 12 MPa. An increasing CO₂ viscosity was displayed as the thickener content and pressure increased, but a decrease of viscosity is shown as temperature and flow velocity rise. The elevated viscosity leads to a significantly reduced filtration volume. Fracturing simulation displays that the increased viscosity results in a longer fracturing time and length. By analyzing the fracturing simulation data, the thickener CO₂ can involve a farther reservoir which the pure CO₂ cannot reach. Compared to pure CO₂, a 55m fracture is revealed during the fracturing simulation while it is only 6m for pure CO₂. For the fracturing and filtration performance, the thickened CO₂ is obviously better than the pure CO₂. The addition of thickener can not only obviously improve the CO₂ viscosity to some degree, but also the excellent fracturing and filtration performance according to the extended finite element simulation and the thickened CO₂ is considered as an applicable fracturing fluid to develop the shale reservoir.