碳封存CO₂注入井持续环空压力预测研究

Study on Prediction of Sustained Annular Pressure in CO₂ Injection Wells during the Carbon Storage Process

摘要: 为了准确预测碳封存CO₂注入井的持续环空压力，针对井下油管柱漏点引发的CO₂泄漏问题，在考虑压力–体积–温度状态方程和环空保护液压缩性的基础上，进一步引入CO₂相态变化产生的体积膨胀效应，建立适用于CO₂注入井的持续环空压力预测模型，研究持续环空压力、泄漏速率和环空CO₂相态的变化规律。研究结果表明，CO₂注入实例井的持续环空压力呈快速上升、增速放缓并最终趋于平稳的变化规律，泄漏速率小幅升高后快速衰减，最终缓慢趋于平衡。环空温压变化会显著影响沿井深CO₂的相态分布，使其由气态、超临界态逐步转向气–液–超临界态，最终转变为液态、超临界态。漏点当量直径、漏点深度及初始环空压力越大，持续环空压力达到稳定值的时间越短，漏点深度显著影响持续环空压力稳定值。研究结果对保障碳封存CO₂注入井的管柱、井筒屏障安全性及其长周期安全运行具有重要意义。

Abstract: To accurately predict the sustained annular pressure of CO₂ injection wells during the carbon storage process, aiming at the CO₂ leakage caused by leakage points in tubing strings, this paper takes into account the pressure-volume-temperature equation of state and the compressibility of annular protective fluid. Fully considering the significant volume expansion difference induced by CO₂ phase change, a prediction model for sustained annular pressure applicable to CO₂ injection wells is established, and the variation laws of sustained annular pressure, leakage rate, and annular CO₂ phase state are investigated. The sustained annular pressure of the field CO₂ injection well rises rapidly at first, then increases slowly, and eventually tends to be stable. The leakage rate rises slightly, followed by a rapid attenuation, and finally gradually reaches equilibrium. Changes in annular temperature and pressure significantly affect the phase distribution of CO₂ along the depth, making CO₂ evolve gradually from a gaseous state and supercritical state to a gas-liquid-supercritical state, and finally transform into a liquid state and supercritical state. Larger equivalent diameter of leakage point, deeper leakage depth, and higher initial annular pressure shorten the time for sustained annular pressure to reach a stable value, and the leakage depth exerts a remarkable influence on the stable value of sustained annular pressure. The research results are of great significance for ensuring the integrity of tubing strings and wellbore barrier safety, as well as the long-term safe operation of CO₂ injection wells during the carbon storage process.