Experimental study on the leakage characteristics and flow prediction of freely propagating random fractures in CO2 transportation pipelines

Understanding the risk of pipeline leakage is crucial in the carbon capture, utilization, and storage (CCUS) process. Attention should be paid to the characteristics of fracture leakage resulting from external force-induced defects in transportation pipelines. Existing experimental and numerical simulation studies generally simplify leak orifices into circular or rectangular shapes, and no research has yet addressed the behavior of freely expanding fractures. A novel experimental setup was developed to investigate the leakage behavior of fractures under CO₂ flow and record the pressure and temperature variations during the leakage process. This paper analyzes the leakage characteristics of three different scales of random fractures and provides a detailed calculation of leakage rates. Results show that the maximum leakage rate through the slit was 11.34 kg/s. Despite the extended leakage duration, the internal fluid temperature within the pipeline remained nearly constant, and the lowest recorded temperature in the leakage zone was only −18.025 °C, indicating minimal leakage risk. The medium-scale fracture posed the greatest low-temperature risk, with an observed leakage zone temperature of −61.834 °C. The full-scale fracture exhibited the greatest high-pressure impact risk based on visible cloud analysis. The lowest internal pipeline temperature was −46.96 °C, which significantly affected the internal temperature of the pipeline. The rapid cooling rate also presented the highest potential risk for low-temperature embrittlement. Comparing experimental leakage volumes to the predictive model, the slit's experimental value was 101.14 kg, the mid-scale fracture was 122.908 kg, and the full-scale fracture was 109.004 kg. The corresponding predicted values were 104.248 kg, with an error of 3.07 %, 126.09 kg with an error of 2.59 %, and 111.542 kg with an error of 2.33 %. Leakage flow rates can compensate for the lack of experimental CO₂ leakage data in classical flow calculation models. In the analysis of the leakage process, the leakage coefficient C_d was derived to be 1.9. The relative errors between experimental and predicted flow rates were found to be 3.07 %, 2.59 %, and 2.33 %, respectively. These small error values provide a basis for calculating leakage flow rates and controlling risk time in CO₂ transportation pipelines where fracture formations occur. The lack of fundamental data on leakage rates from fracture-like orifices has been addressed, contributing to improvements in predictive models. This provides a method to enhance the accuracy of leakage rate predictions in real-world scenarios involving non-circular orifices and offers a scientific basis for understanding the risk characteristics of pipeline fractures.