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 CO 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 CO 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 CO 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 CO 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%.

中文翻译：

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超临界二氧化碳在CO2压裂返排过程中的节流特性

利用二氧化碳压裂技术开发低渗透油气资源，并实现部分碳封存。然而，在压裂返排过程中，超临界CO容易在狭窄通道内发生节流现象，导致温度快速下降。这会导致干冰的形成，随后阻塞回流通道。本文主要通过数值模拟结合实验验证来分析干冰形成的根本原因。我们研究了关键操作参数的影响，包括初始压力、初始温度、孔径和出口压力。结果表明，温度下降主要是由高速CO膨胀过程中的冲击波引起的。具体而言，在入口压力为10 MPa、出口压力为5.1 MPa、入口温度为393 K、孔喉直径为10 mm的条件下，最高温度降低了155 K。出口压力的持续降低可以进一步导致温度下降，其值将低于CO三相点温度。这些调查是在代表性条件下通过多因素模拟进行的。在这些因素中，初始压力对温度变化的影响最为显着。数值模拟提供了在各种压力、温度和孔喉直径的条件下CO不形成干冰的最小出口压力。进一步建立了这些参数之间的具体拟合关系，得到了最小出口压力，相应的拟合误差在10%以内。