低渗透储层流体非线性渗流机理及特征分析

基于微尺度渗流效应，分析了低渗透油藏非线性渗流机理，根据力学平衡方程和压汞实验数据，建立了低渗透储层流体非线性渗流数学模型，分析了非线性渗流特征，并引入动态阻力梯度概念，实现对低渗储层非线性渗流的动态表征。结果表明，边界层、流体屈服应力、边界流体体相流体间的表面力对低渗透渗流形成的附加阻力，是非线性渗流的主要原因；非线性渗流特征在不同的驱替压力梯度下一直存在，在低驱替压力梯度下明显，高驱替压力梯度下弱化；采用动态阻力梯度代替启动压力梯度对非线性渗流进行表征更加合理；随着驱替压力梯度的增加，动态驱替梯度曲线特征为先从一个比较高初始值瞬间降落，之后逐渐增大；动态阻力梯度与驱替压力梯度的比值先增大后减小。

Mechanism and Characteristics of Nonlinear Flow in Porous Media of Low Permeability Reservoir

The mechanisms of nonlinear seepage in low permeability reservoirs were analyzed based on the microseepage effect, and a mathematical model was constructed for these nonlinear seepages in low permeability reservoirs using the forcebalance equation and experimental data from mercury porosimetry experiments. This model was further used to analyze the characteristics of nonlinear seepage in low permeability reservoirs, and the concept of dynamic resistance gradients(DRGs) was introduced to enable the dynamic characterization of these nonlinear seepages. Based on the results of this study, the primary causes of nonlinear seepage are the additional resistances against low permeability seepage caused by the boundary layers, fluid yield stress, and surface forces between boundary layer fluids and bulk fluids. Nonlinear seepages are always present at all displacement pressure gradients, but the effects are more pronounced at low DPGs and weaker at high DPGs. It was also found that the characterization of nonlinear seepage became more realistic when the starting pressure gradient was replaced by the DRG. As the DPG increases, the dynamic displacement gradient initially undergoes an instantaneous decrease from a high initial value and then gradually increases. In addition, the ratio of the DRG to DPG initially increases with the DPG, but subsequently decreases with further increase in the DPG.