致密气藏开发中气水相渗影响机制及曲线校正

The relative permeability curve has very important significants to dynamic analysis and numerical simulation. The problem is that the complicated micro pore structures of the low permeability reservoirs increases the specialty of flow phenomena, such as gas slippage,starting pressure, stress sensitivity, while these will restrict the flow laws of gas reservoir:on one hand,with the decresing of pressure, the pore structures and permeability will change, which will also cause the changing of gas slippage and starting pressure, and affect the effective permeaility of gas.On the other hand, the change of water saturation may enhance the stress sensitivity, and affect the gas slippage effect,and the starting pressure gradient may exists when the water saturation rises to certain degree. The gas slippage and starting pressure have mutual restriction with each other. But the experimental and micro mechanism study of the influence of water saturation on gas slippage and starting pressure is relatively few, and some disputes still exist. The aim of this project is to carry out further study on mobile water saturation, and on the function mechanism, influncing factors and interaction of stress sensitivity, gas slippage and starting pressure by the techniques of centrifugation, nuclear magnetic resonance, visual sandstone micro-model, then analyze their influnce on gas-water relative curve and revise it. The results can provide more reliable theoretical reference for establishing the mathmetical model which describes the special flow laws, and for accurate prediction of performance for low permeability gas reservoirs.

相渗曲线是油气田开发中动态分析以及油藏数值模拟等方面不可缺少的重要资料。致密气藏复杂的微观孔隙结构，加剧了气体渗流特殊性：如滑脱、应力敏感、启动压力等现象存在，而这些又制约了气藏的相渗特征：随压力下降，气藏孔隙结构和渗透率性发生变化，其又导致滑脱和启动压力变化，影响气体有效渗透率；而开发过程中含水饱和度变化，增强了应力敏感性，对滑脱效应也会产生影响，且当含水增大到一定程度，可能存在启动压力梯度，滑脱与启动压力二者此消彼长、相互制约。目前在含水对气体滑脱效应以及启动压力梯度影响方面的实验和微观机理研究相对较少，且存在部分争议。本项目旨在通过离心法、核磁共振、可视化砂岩微观模型等新技术对可动水，应力敏感、气体滑脱、启动压力梯度作用机理及其相互影响作深入研究，分析对气水相渗的影响并对相渗曲线进行校正。研究成果可以为建立更客观的致密气藏特殊渗流特征的数学模型，及准确预测动态提供可靠理论依据。

致密气藏特殊的微观孔隙结构导致其渗流规律极其复杂，影响因素众多，且各因素之间相互影响、相互制约。项目利用先进的实验设备如核磁共振仪、CT扫描仪、真实砂岩微观驱替装置对致密气藏中束缚水、可动水的大小及存在形式，以及气体渗流的滑脱效应、应力敏感、启动压力梯度等进行了深入研究，在此基础上分析了上述三种现象对气水相渗的影响，并利用气体滑脱因子对相渗曲线进行了校正。结果表明：致密气藏束缚水饱和度和可动水经不同驱替压力气驱后均有不同程度下降，且渗透率越低，束缚水饱和度越高，含气潜力越低，束缚水主要分布在小喉道、盲端、由小喉道包围的大孔隙及边壁处；渗透率、孔隙压力越低，气体滑脱效应越强，含水饱和度对气体滑脱的影响存在临界值，低于临界含水饱和度，随含水饱和度增加，滑脱效应增强，反之，滑脱效应则越弱；三轴应力下致密气藏存在一定的应力敏感性，比常规测试法测试结果要弱，渗透率越低，敏感性越强，敏感性强弱受加载方式、加载速度、轴向应力大小等影响；气体滑脱与启动压力效应两者此消彼长，在削弱气体滑脱效应后，观察到明显的启动压力梯度，其大小受含水饱和度和渗透率影响，与孔隙喉道半径成反比；由于气体滑脱，常规测试得到的气相相渗透率大于该饱和度下的气相绝对渗透率，而应力敏感和启动压力则使得气水两相相渗透率下降，端点处相对渗透率下降，束缚水和残余气饱和度上升；经气体滑脱因子校正后的气相相对渗透率曲线明显低于校正前，且含水饱和度越低，两者相差幅度越大。研究结果为致密气藏特殊渗流规律描述、动态预测及数值模拟提供了基础依据。