低渗透/致密油藏驱替-渗吸数学模型及其应用

在亲水性低渗透/致密油藏注水开发过程中存在驱替和渗吸双重作用开发机理。为定量表征驱替和渗吸作用在低渗透/致密油藏注水开发中的贡献,明确影响低渗透/致密油藏注水开发过程关键因素,基于毛细管束模型,开展驱替和渗吸双重作用下驱油过程力学机制分析,明确驱替压力梯度、孔喉大小、润湿性、油水黏度、界面张力等因素对驱油速度的影响;基于孔喉分形分布特征,考虑束缚水和残余油赋存特征,构建岩心尺度驱替-渗吸数学模型,给出不同驱替压力梯度下岩心尺度驱油流量和采收程度随时间变化关系;构建驱替压力梯度-渗透率双对数图版,将低渗透/致密油藏注水开发分为驱替为主、渗吸为主和驱替-渗吸共同作用3类不同注水开发机理。驱替压力梯度-渗透率双对数图版可以判定低渗透/致密油藏任一点处注水开发机理类型,定量表征驱替与渗吸对注水开发驱油速率的贡献,为低渗透/致密油藏注水开发方案设计与调整提供一定依据。

A mathematical model of displacement and imbibition of low-permeability/ tight reservoirs and its application

There is a dual development mechanism of displacement and imbibition during the water flooding of hydrophilic low-permeability/tight reservoirs. To quantitatively characterize the contributions of displacement and imbibition during the water flooding of low-permeability/tight reservoirs and determine the relevant key influential factors, based on the capillary bundle model, this paper analyzes the mechanical mechanism of oil displacement under the dual effects of displacement and imbibition and clarifies the impacts of displacement pressure gradient, pore throat size, wettability, oil-water viscosity, and interfacial tension on the oil displacement speed. Based on the fractal distribution characteristics of pore throats, considering the occurrence characteristics of irreducible water and residual oil, this study constructs a core-scale mathematical model of displacement and imbibition, and presents the variations of core-scale displacement flow and recovery degree with time under different displacement pressure gradients. It also constructs a double logarithmic chart of displacement pressure gradient and permeability, and develops the displacement-based, imbibition-based, and displacement-imbibition combined water flooding mechanisms for low-permeability/tight reservoirs. The double logarithmic chart can determine the type of water flooding mechanism at any point in a low-permeability/tight reservoir, and quantitatively characterize the contributions of displacement and imbibition to the water flooding displacement speed, which provides a basis for the design and adjustment of waterflooding scheme for low-permeability/tight reservoirs.