Study On The Safety Of Supercritical CO₂ Conveying Pipeline Network Under Operating Conditions

Different from gas and liquid transportation,as the most economical phase of large-scale CO₂ conveying,supercritical CO₂ has serious suffocation hazards when it is released.In addition,in the actual operation of the CO₂ pipe network containing impurities,there are problems such as pipeline corrosion,leakage,multiphase flow,overpressure,blockage and other problems,which damage the integrity of the pipe network and threaten the safety of the pipe network.Therefore,it is of more practical significance to analyze the safety problems in the operation of supercritical CO₂ conveying pipeline network containing impurities.This paper takes the actual impurity-containing supercritical CO₂ pipeline network in Yanchang oilfield as the research object,analyzes and proposes that the main factors affecting the safe operation of supercritical CO₂ pipeline network are the physical properties of impurity-containing CO₂,hydrate formation and internal corrosion of pipe wall caused by impurity water.The PR equation of state is used to calculate the phase characteristics of CO₂ containing impurities,and the premise to ensure the safe transportation of the pipe network is to prevent the fluid from entering the gas phase region and two-phase region containing CO₂ containing impurities.The vd W-P phase equilibrium model is used to calculate the phase equilibrium curve of impurity-containing CO₂ hydrate,and it is found that the transport parameters of supercritical CO₂ pipe network fall in the generation interval impurity-containing CO₂ hydrate in a large range,threatening the conveying safety of the pipeline network.At the same time,due to the special physical properties and transcritical properties of supercritical impurity-containing CO₂,the flow parameters of pipeline network fluctuate greatly under different working conditions.Aiming at the safety problems in the operation of the pipeline network,a research model for the safety of the supercritical CO₂ conveying pipeline network is established by using the conservation equation and hydraulic and thermal models of the pipeline network in OLGA multiphase flow software.The two cases of CO₂ release experiments in literature at home and abroad are simulated and calculated,and the CO₂ temperature and pressure data in the tube at the release experiment are compared and analyzed.The results show that the maximum average error of the pressure calculation value is 12.22%,and the maximum average error of the temperature is 13.14%.The errors meet the practical needs of the engineering.The model is more accurate,and it can be used for the safety study of the supercritical CO₂ conveying pipeline network.The OLGA model is used to simulate the first-station pressurized steady-state and transient conveyance of the simplified supercritical impurity-containing CO₂ transport network,as well as the pressurized steady-state transport in the middle of the simplified network.The results show that under steady-state conveying conditions,supercritical CO₂ enters and maintains a dense-phase flow state after 5.3km from the inlet of the mainline pipe.And a small amount of CO₂ hydrate is formed and continues to exist around 22km of the mainline pipe,and there is a continuous water film threatening with corrosion in the pipeline 22.3km before.These will threaten the steady-state operation safety of supercritical CO₂ conveying pipeline network.Under the transient operating conditions of the first pressurized CO₂ transportation pipeline network such as flowed fluctuations,pressure fluctuations,shutdown and restart,and pipeline discharge,the CO₂ phase state in the pipe will change significantly outside the two-phase zone,which will cause the corresponding changes in the position and content of hydrates formation and of water film content in the pipe,it will induce many unsafe problems such as erosion and corrosion of the pipe wall.Research on the safety of steady-state operation of the pipeline network with intermediate pressurization has shown that a certain quality of hydrate will enter the centrifugal pump along the pipeline,aggravating the problem of blade erosion.The outlet pressure of pipeline network after middle pressurization is basically achieved at an injection pressure that above 13.95MPa,and the CO₂ in the entire pipeline network maintains a safe dense-phase flow.But due to the temperature rise caused by pressurization,a continuous water film on the pipe wall appears within 19.5km of the branch line,and CO₂ hydrate starts to form at 19.5km,which increases the risk of pipeline network operation.