Simulation of surface pipeline plugging during liquid CO2 pre-fracturing and its inhibition method

A visual device that can evaluate the flow of fracturing fluid in pipeline was used to investigate the formation of hydrate and dry ice at the venting valve and in nearby pipeline. The forms of the potential blockage of the pipeline and their influence factors in the process using liquid CO2 for pre-fracturing were studied combined with the prediction CO2 phases at different conditions. Plug inhibitors for liquid CO2 fracturing pipeline were selected. The process of plug inhibitors displacing liquid CO2 was investigated by a numerical simulation method and the pumping procedure of liquid CO2 was also modified. The results show that, when the pipeline pressure with liquid CO2 is around 3.0 MPa, dry ice can be formed at the valve and in its nearby pipeline if liquid CO2 is directly released. When a water-based fracturing fluid is continuously injected after liquid CO2 injection, the blockage of the surface pipeline is mainly caused by the freezing of the water-based fracturing fluid and a small amount carbon dioxide hydrate. The preferred plug inhibitor should have a freezing point below -30 ℃ and is compatible with the water-based fracturing fluid and liquid CO2. The residual liquid CO2 can be displaced by pumping about 0.53 m3 plug inhibitor in the surface pipeline for around 1 min, and the temperature in the pipeline can be quickly increased to above 0 ℃. A pilot testing shows that a single set of fracturing facility can be used to conduct continuous pumping of liquid CO2 and water-based fracturing fluid. There was no pipeline blockage during the fracturing process and the operation time was shortened from 3-4 d to 0.5 d. The blockage risks of liquid CO2 fracturing can be significantly reduced. © 2020, Editorial Office of Journal of China University of Petroleum(Edition of Natural Science). All right reserved.