The throttling characteristics of supercritical carbon dioxide in the flowback process of CO2 fracturing

Carbon dioxide fracturing is used to develop oil and gas resources with low permeability and also achieve partly carbon sequestration. Nevertheless, during the process of fracturing flowback, supercritical CO2 easily undergoes a throttling phenomenon within narrow channels, leading to a rapid temperature decrease. This causes the formation of dry ice, which subsequently blocks the flowback channel. In this paper, combined with the experiment verification, the numerical simulation is primarily used to analyze the underlying causes of dry ice formation. We investigate the effect of key operational parameters, including initial pressure, initial temperature, pore diameter, and outlet pressure. The results reveal that the temperature drop is primarily caused by the shock waves during the high-speed CO2 expansion. Specifically, under the conditions of inlet pressure of 10 MPa, outlet pressure of 5.1 MPa, inlet temperature of 393 K, and pore throat diameter of 10 mm, the maximum temperature decreases by 155 K. Continuous reduction in the outlet pressure can further lead to temperature drop and its value will be below the temperature of CO2 triple point. These investigations are conducted through multi-factor simulations under representative conditions. Among these factors, the initial pressure has the most significant influence on the temperature variation. The numerical simulations provide the minimum outlet pressures at which CO2 does not form dry ice under conditions of various pressures, temperatures, and pore throat diameters. Further, a specific fitting relationship among these parameters is established to obtain the minimum outlet pressures, and the corresponding fitting error is within 10%.