低渗透油藏地层压力保持水平对驱油效率的影响

低渗透储层由于岩性致密、渗流阻力大、压力传导能力差,导致开发过程中地层压力下降快、下降幅度大,从而造成岩石骨架变形而影响其物性和渗流能力。这种变化必然会影响到油气井的产能和油气田的开发效果。作为评价油气田开发效果的重要参数驱油效率一直是油藏工作者研究的热点问题,多年来对驱油效率影响因素的研究主要集中在岩石固有性质(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,遗憾的是未见到考虑地层压力保持水平因素的研究。通过室内流动实验,模拟再现了地层压力下降过程,研究了低渗透油气藏不同地层压力保持水平条件下驱油效率的变化规律。结果表明,水驱驱油效率是地层压力保持水、储层岩石初始空气渗透率的函数,其随着地层压力保持水平、储层岩石初始空气渗透率的下降而单调下降,表现出应力敏感性特征,且地层压力下降幅度相同时,储层岩石初始空气渗透率越低,水驱驱油效率降幅越大,驱油效率的应力敏感程度越强。分析认为储层岩石的弹塑性变形是驱油效率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气藏开发理念,为高效开发低渗透率油气藏提供了理论依据。

The Impact of Formation Pressure Maintenance on Displacement Efficiency in Low Permeability Reservoirs

In low permeability reservoirs, Because of lithological density, high filtration resistance and low pressure transmissibility, reservoir pressure decline would induce reduce well productivity and oil-gas fields development. Oil displacement efficiency has always been the focus of reservoir workers. Over the years, the study of factors affecting oil displacement efficiency has mainly been concentrated in the Inherent properties of rock,flow medium,water dynamic condition, etc. Unfortunately, the impact of formation pressure maintenance on displacement efficiency in low permeability reservoirs has never been studied. Through mercury-injection experiment, the formation pressure decline process was simulated, and the changing rule of the oil displacement efficiency was been was studied under the condition of different formation pressure maintenance. The results of experimental study have been shown that the displacement efficiency will be a yuan quadratic polynomial function of the formation pressure maintenance and the initial air permeability of the reservoir rock. Along with the formation pressure maintenance and the reservoir rock initial air permeability decline, the displacement efficiency appear monotonous decline rule which show the characteristics of stress sensitivity. If the formation pressure all go down by the same proportion, the reservoir rock initial air permeability will be lower , and the displacement efficiency will go down most sharply. Numerous analysis studies have shown that the reservoir rock elastic-plastic deformation is the root cause of the stress sensitivity of the oil displacement efficiency. Thus the development philosophy of low permeability reservoir has be put forward, which the lower the reservoir rock initial permeability , the earlier the waterflooding time. The study for the efficient development of low permeability reservoirs provide a theoretical basis.