基于多点动态监测的低黏度致密油溶解气驱机理研究

When oil reservoir pressure is below bubble-point pressure, it is considered as solution-gas-drive-reservoir. Earlier works show that the drive mechanism will lead to decrease of oil recovery. However, the applicant finds out that several low viscosity tight oil reservoirs reveal the contrary phenomenon, which indicates that the recovery of solution-gas-drive-reservoir will increase. Thus, in terms of researches on mechanism of solution-gas-drive tight oil reservoir, they will bring advantages to recognize the particularity of solution-gas-drive-reservoir and improve the theory of tight oil reservoir development. When solution-gas-drive reservoir reaches two-phase flow regime, viscosity sistance of crude oil will increase, oil permeability will decrease and free gas drive force will augment. These three parameters have a comprehensive impact on the driving force，driving mode and displacement efficiency of solution-gas-drive-reservoir. Therefore，physical properties of reservoir, PVT values of fluids and production methods might be the reasons leading to recovery of solution-gas-drive low viscosity tight oil reservoir. In order to obtain critical gas saturation, production and recovery of oil and gas, the project simulates in-situ condition of low viscosity tight oil reservoir and monitors pressure and gas saturation at multiple points of a long core plug during solution-gas-drive process. Proceed comparative experiments focusing on parameters as viscosity, permeability and method of depletion to analyze the characteristic of critical gas saturation at different conditions and summarize the mechanism of solution-gas-drive tight oil reservoir and main controlling factor of recovery.

当油藏压力低于饱和压力时，油藏进入溶解气驱阶段。很多研究表明，常规油藏进入溶解气驱后其采收率降低。但申请者近期研究发现在低黏度致密油藏中存在反常现象：进入溶解气驱阶段的采收率更高。为此针对致密油藏溶解气驱的机理开展研究，将有助于认识致密油藏溶解气驱的特殊性、进一步完善致密油藏开发理论。溶解气驱油藏进入两相流动后，根据相态及相渗曲线特征，原油黏性阻力增加、油相渗透率降低、自由气驱动力增大，综合影响溶解气驱油藏驱动力、驱动方式及驱动效果，因此储层物性、流体PVT以及开发方式可能是影响低黏度致密油溶解气驱采收率的主要因素。项目重点从影响因素入手，通过模拟低黏度致密油藏地层条件，对长岩芯在溶解气驱过程中的压力和含气饱和度进行多点监测，获取临界气相饱和度及产量、采收率等参数。主要从渗透率、黏度及衰竭方式三个方面进行对比实验，分析不同条件下临界气相饱和度特征，研究致密油溶解气驱机理及采收率的主控因素。

针对低黏度致密油藏溶解气驱阶段产量增加的反常现象，本项目基于低黏度致密油藏典型的储层物性、流体性质，设计了长岩心实验，模拟该类油藏的溶解气驱过程，并综合监测实验过程中产量、压力、饱和度等参数的变化情况，从储层特征、流体性质及开发方案三个方面进行对比实验，分析得知致密油藏渗透率越小开发时间越长、到达峰值产量的时间越长、临界气相饱和度出现越晚，采收率越低；致密油黏度越大，到达峰值产量的时间越长、产量越低；不同的衰竭方式进行开发产量也会有较大的差异。同时与常规储层实验进行对比，致密砂岩油藏溶解气驱过程中临界气相饱和度出现时间更早，气体的弹性膨胀能作用时间更久。由于常规砂岩油藏的物性特征导致在溶解气驱阶段孔隙内更容易形成油气两相渗流，因此产油量较低。而致密砂岩油藏溶解气驱阶段主要依靠析出气体的弹性膨胀能驱油，产量较高，因此导致了IPR曲线上表现出的特殊性。低黏度致密油藏溶解气驱阶段的实验研究，进一步完善了致密油藏溶解气驱的理论体系，为提高致密油藏经济效益提供理论支撑。在低粘度致密油开发过程中可以充分利用天然能量，以及保持较长的溶解气驱阶段，可最大限度的节约开发成本，达到提高采收率的效果。