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

Key words:  liquid CO₂  pre-fracturing  blockage  inhibitor  numerical simulation

Abstract:

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.