基于离子变化的高温高压CO2-烃-地层水热力学实验及理论研究

The mutual soluble ability of CO2-hydrocarbon-formaion water system is far beyond that of normal hydrocarbon gas reservoirs at the high pressure and high temperature conditions. With the decrease of pressure and temperature, the CO2 dissolved in formation water will exsolution and enter the vapor phase, inducing the increase of CO2 content and water content in vapor phase, and reducing calculation precision of gas properties for such gas reservoirs. So far, the influence of the mutual solubility of CO2-hydrocarbon-formation water system on phase equilibria and numerical simulation software Eclipse and CMG have been neglected, leading to inaccuracy of development performance prediction for such CO2-conent natural gas reservoirs and CO2 sequestration.. To solve this scientific problem, the methods of static volumetric difference and flash separation are deployed to test the solubility of CO2-content natural gas in different salinity formation water and water content in vapor phase under different pressure and temperature conditions, respectively. A thermodynamic model considering the CO2-H2O cross association and H2O self association is proposed to study the phase equilibia of CO2-hydrocarbon-formation water system which considers different types of salts and salinity as well as solution and exsolution of CO2 in formation water by means of PR equation of state modified by the statistical associating fluid theory and particle swarm optimization (PSO) methods for determination of binary interaction parameters. This work provides a new idea for an accurately fast acquirement of phase equilibria parameters of CO2-content natural gas reservoirs, and is of great significance to study the mechanism of CO2 trapping in geological storage sites.

高温高压含CO2气藏中CO2-地层水互溶能力远超过烃类气藏。开采过程中，随地层压力下降，地层水中溶解的CO2不断逸出，气相中水蒸汽含量不断增加。目前含CO2气藏的开发和CO2埋存的相态模拟中均未考虑CO2-地层水互溶的影响，导致动态预测存在不准确性。针对此问题，本项目采用静态容积差法和闪蒸分离法分别测试不同温度、不同压力、不同CO2含量的天然气在不同地层水中的溶解度和气相中的水蒸汽含量，然后借助统计缔合流体理论增加一个考虑CO2-H2O-离子交互作用和H2O自缔合作用的缔合项，以改进PR方程，建立考虑离子类型、离子及CO2含量变化下CO2-H2O-离子和H2O自缔合的热力学模型，其中的二元相互作用系数通过粒子群优化理论确定。在此基础上，开展考虑离子变化和地层水中CO2溶解与逸出的高温高压CO2-烃-地层水的热力学理论研究。该研究为准确求取此类气藏流体相态参数提供新方法，对揭示CO2埋存机理有重要意义。

高含CO2气藏在开采过程中，随地层压力下降，地层水中溶解的CO2不断逸出，造成气相中水蒸汽含量不断增加。目前含CO2气藏的开发和CO2埋存的相态模拟中均未考虑CO2-地层水互溶的影响，导致动态预测存在不准确性。本项目针对此基础科学问题，采用静态容积差法和闪蒸分离法分别测试不同温度、不同压力、不同CO2含量的天然气在不同地层水中的溶解度和气相中的水蒸汽含量。然后利用PRSV方程结合MHV1混合规则，能够较精确地预测CO2在水中的溶解度，在此基础上，基于统计缔合流体理论增加一个考虑CO2-H2O交互作用和H2O自缔合作用的缔合项，改进了iCPA-PR模型，拟合得到更好的预测结果。接着，利用PSO（粒子群算法）算法拟合二元相互作用系数改进了PR-HV模型，能较准确地预测CO2、CH4在不同离子溶液（NaCl、CaCl2）中的溶解度。最后，以人工神经网络机器算法为基础，建立二元酸性气体饱和含水量预测模型，得到较为精确的预测酸性气体饱和含水量。该研究成果对准确求取高含CO2天然气藏流体相态参数、开发方案设计以及CO2地下有效埋存具有重要的理论意义和应用价值。