低渗透油藏地层压力保持水平对油水渗流特征的影响

Impact of formation pressure maintenance on oil-water seepage characteristics in low permeability reservoirs

油水渗流规律的研究是低渗透油藏水驱开发的关键。相对渗透率曲线能直观反映油水渗流特征,其影响因素研究主要涉及岩石固有性质(润湿性、孔隙结构)、流动介质(油水粘度比)、动力条件(驱替压力梯度及速度)等方面,极少见到地层压力对相对渗透率曲线影响的研究。通过室内流动实验,模拟低渗透油藏地层压力下降过程,建立了不同地层压力保持水平下的相对渗透率曲线,分析了地层压力保持水平对油水渗流特征的影响规律。结果表明,地层压力保持水平下降,孔隙结构非均质性增强,油相相对渗透率下降,水相相对渗透率上升,等渗点左移,油水两相区变窄,残余油饱和度增加,即低渗透油藏渗流规律也存在着应力敏感性特征。分析认为,储层岩石弹性或塑性变形是低渗透油藏油水渗流特征应力敏感性的根本原因,因而提出了储层岩石初始渗透率越低,越应尽早注水保持地层压力开发的低渗透油藏效益开发理念。

In low permeability reservoirs,it is essential to exactly know the rule of fluid flow in porous media during waterflooding development. The relative permeability curve may intuitively reflect the characteristics of the oil and water seepage in porous media. The study of factors affecting the relative permeability curve mainly focuses on the inherent properties of rock(wettability and pore structure),flow medium(oil and water viscosity ratio),water dynamic condition(displacement pressure gradient and velocity),etc. The impact of the formation pressure on the relative permeability curve in the low permeability reservoirs has seldom been studied. Through mercury-injection experiment,the decline process of the formation pressure was simulated,and the relative permeability curves in the low permeability reservoirs under different levels of formation pressure maintenance were established. The variation of the oil and water seepage in the low permeability reservoirs was analyzed. The results of experimental study show that when the heterogeneity of pore structure becomes stronger and the formation pressure goes down,the oil-phase relative permeability declines and the water-phase relative permeability rises,and also the isotonic point shifts to the left and the two-phase region turns narrow,resulting in the increase of residu-al oil saturation. That is,the stress sensitivity exists in the oil-water seepage in the low permeability reservoirs. Analyses have shown that the elastic or plastic deformation in the reservoir rocks is the root cause of the stress sensitivity of the oilwater seepage characteristics. Thus the development philosophy of the low permeability reservoir has been put forward:the lower the initial permeability of the reservoir rocks is,the earlier the waterflooding time should be.