渤海油田低渗透油藏黏性指进特性分析

Analysis of viscous fingering characteristics of low permeability reservoirs in Bohai oilfield

低渗透油藏一般采用注水开发,在水驱油开发过程中,由于油藏的非均质性和孔隙结构的微观随机性常会导致指进现象的产生,并随着注水开发进一步发展。准确揭示黏性指进的发展规律及其影响因素,对低渗透油藏提供合理的开发决策是十分必要的。针对低渗透油藏开发过程中的黏性指进问题,建立了考虑启动压力梯度的低渗透油藏水驱油黏性指进模型,引入了指进小瘤的概念,通过描述指进小瘤的运移距离来表征指进发展情况,在对该模型进行求解后,分析了渗透率、密度差、地层倾角、驱替速度和油水黏度比对黏性指进的影响。研究结果表明:低渗透油藏在驱油过程中,启动压力的存在使得指进现象严重,而重力能有效减缓指进;渗透率、密度差和地层倾角越大,指进程度越轻;驱替速度和油水黏度比越大,指进程度越严重。为了减缓指进,应采用从油藏底部注水的方式,并且驱替速度应该保持在一定范围之内。

Low-permeability reservoirs are generally developed by injecting water. In the process of waterflooding development, due to the heterogeneity of the reservoir and the microscopic randomness of the pore structure, it is common to cause fingering phenomenon, and it would become serious with continuing water injection. To make reasonable development decisions for low-permeability reservoirs, it is necessary to accurately understand the variation law of viscous fingering and its influencing factors. A viscous fingering model was established with a starting pressure gradient, for waterflooding low-permeability reservoirs, and a concept of small fingering tumors was introduced. The migration distance of small fingering tumors was used to characterize fingering change. After solving the model, the effects of permeability, density difference, formation dip, waterflooding rate and the ratio of oil-to-water viscosity were analyzed on viscous fingering. The results show that, in the process of waterflooding low-permeability reservoirs, the presence of starting pressure makes fingering phenomenon serious, but gravity can effectively slow down the phenomenon; the greater the permeability, density difference and formation dip, the less serious the fingering phenomenon; the greater the waterflooding rate and the ratio of oil-to-water viscosity, the more serious the fingering phenomenon. In order to slow down the fingering, it is suggested to inject water at the bottom of the reservoir, and the waterflooding rate should be kept reasonable.