微纳米孔隙介质超临界CO2驱替CH4非线性渗流特征研究

Facing the energy security and environmental treatment, it’s a frontier research for researchers to improve unconventional gas recovery by using super-critical CO2 flooding. This method not only makes use of the natural storage conditions for unconventional gas reservoir in the implementation of CO2 sequestration (CCS), but also realizes the low carbon economy, and protects national energy supply and security. The nano-micro pore throat network system is widely distributed in the unconventional natural gas reservoir. So the displacement mechanism of the super-critical CO2 in the nano-micro pore is the key to improve gas recovery technology for unconventional gas reservoir..This project will be developed by the research on experiment under high temperature and high pressure on the super-critical CO2 displacement efficiency and the competitive adsorption mechanism, which involved the phase characteristics test, isothermal adsorption characteristics measurement, displacement and seepage flow experiment. Molecular dynamics method is employed to simulate the transfer, adsorption and separation process of the CO2 molecular in the nano-micro organic pore, which can reveals the competitive adsorption mechanism of super-critical CO2/CH4. The mathematical characterization is analyzed by the displacement mechanism. The nonlinear seepage model of the process of super-critical CO2 flooding CH4 will be obtained considering on the adsorption, dispersion and seepage, which can be modified by the frontier experiment. The project can finally provide a solid theoretical basis and guidance for improving unconventional gas recovery by using super-critical CO2 flooding.

面对能源安全和环境治理问题，利用超临界CO2排驱非常规储层中的天然气提高天然气采收率，不仅能够利用非常规气藏自身的天然封存条件实施CO2埋存，并且有利于实现低碳经济，保障国家能源供应及安全。非常规天然气储层广泛分布微纳米孔喉网络系统，超临界CO2在微纳米孔隙介质中排驱天然气的机理及流动特征是提高采收率技术的关键。.本项目通过高温高压下相态分析、等温吸附测试、超临界CO2岩心驱替及渗流等实验来观察分析超临界CO2在微纳米有机质孔中的排驱效率及竞争吸附现象；基于分子动力学方法模拟有机质纳米狭缝中分子的传递、吸附及分离特征，进一步系统的揭示超临界CO2/CH4竞争吸附机理；对排驱机理进行数学表征，建立微纳米有机质孔超临界CO2驱替CH4非线性渗流模型，与实验结果对比，对模型进行修正，最终为超临界CO2高效开采非常规天然气提供坚实的理论基础与指导。

利用超临界CO2排驱非常规储层中的天然气提高天然气采收率，不仅能够实施CO2埋存，并且有利于实现低碳经济，保障国家能源供应及安全。非常规天然气储层广泛分布微纳米孔喉网络系统，超临界CO2在微纳米孔隙介质中排驱天然气的机理及流动特征是提高采收率技术的关键。.甲烷和二氧化碳的产出曲线分为三个阶段，第一阶段，甲烷的产出量和产出速率均大于二氧化碳；第二阶段气体的产出量有所减小，曲线也相对平缓，曲线斜率明显下降，这一阶段为驱替的主要阶段，经历时间也比较长，这阶段中二氧化碳的产出量仍小于甲烷；第三阶段，甲烷的产出增长缓慢，二氧化碳产出显著增大，驱替置换作用不再明显，当往岩样中注入二氧化碳气体，产气曲线发生明显的突变。随着串联岩心渗透率级差增加，CO2/CH4置换比（CO2储存体积/产出CH4体积的比值）逐渐增大，CO2驱替开采CH4的采收率逐渐降低。.分子动力学模拟过程中，为了证明模型的有效性，将模拟得到的拟合密度与美国国家标准与技术研究院（NIST）密度值比较，模拟得到的CH4密度数据与NIST密度数据吻合较好，证明了模拟的有效性。静止吸附层绝对吸附量与流动吸附层绝对吸附量存在差异，压差越大，吸附量差越大。在同一压差下，随着压力的增大，压力对各个吸附层绝对吸附量的影响逐渐减小，其中对第一吸附层绝对吸附量影响最大，然后逐层递减。CO2对CH4的吸附选择性大于1，则CO2的吸附性大于CH4，说明CO2可以驱替CH4；并且随着压力的增加，CO2对CH4的吸附选择系数降低。.建立了纳微米孔隙介质多尺度非线性渗流模型。选取孔喉直径在2-40nm的区下志留统龙马溪组钻井取心样品，进行了室内微观渗流模拟实验。微观渗流模拟实验所测得的数据点，与采用纳微米级孔隙气体流动模型计算所得渗流特征曲线进行了对比，可以看出，实验数据与模型计算结果拟合的很好，说明本模型具有较高的精确度，可用于工程实际。