基于微观孔隙结构特征的致密砂岩部分饱和岩石物理模型研究

A patchy-saturated rock physics model for tight sandstone based on microscopic pore structures

储层的复杂微观孔隙结构对于弹性波诱导的孔隙流体发生流体流动的影响十分复杂。深入的调查流体流动机制有利于更加精确的描述储层弹性波传播特征与规律。在油气藏勘探开发过程中,实际储层大多是部分饱和的,因此,需要开展部分饱和储层弹性波传播机理的研究。本文基于等效介质理论,利用不同压力下干燥状态的超声波实验数据,获取岩石微观孔隙结构特征。将部分饱和理论模型与考虑不同裂隙纵横比喷射流模型进行结合,形成部分饱和-扩展Gurevich喷射流弹性波传播模型。通过数值模拟分析了含水饱和度、外部球体半径及粘度等参数对弹性波传播特征的影响。选取鄂尔多斯盆地庆阳地区致密砂岩样本,开展不同压力下部分饱和岩石的超声波实验观测,并利用理论模型对实测数据进行分析。相比于部分饱和模型,本文模型综合考虑了部分饱和效应与不同裂隙纵横比喷射流效应对弹性波传播的影响,所预测的纵波速度与衰减结果更符合于实验测量结果。

The wave-induced local fluid flow mechanism is relevant to the complex heterogeneity of pore structures in rocks.The analysis of the local fluid flow mechanism is useful for accurately describing the wave propagation characteristics in reservoir rocks.In the exploration and production of hydrocarbon reservoirs,the real stratum may be partially saturated with a multi-phase fluid mixture in general.Therefore,it is of great significance to investigate the wave velocity dispersion and attenuation features in relation to pore structures and fluids.In this work,the characteristics of fabric microstructures are obtained on the basis of pressure dependency of dry rock moduli using the effective medium theory.A novel anelasticity theoretical model for the wave propagation in a partially-saturated medium is presented by combining the extended Gurevich squirt-flow model and White patchysaturation theory.Numerical simulations are used to analyze wave propagation characteristics that depend on water saturation,external patchy diameter,and viscosity.We consider a tight sandstone from the Qingyang area of the Ordos Basin in west China and perform ultrasonic measurements under partial saturation states and different confining pressures,where the basic properties of the rock are obtained at the full gas saturation.The comparison of experimental data and theoretical modeling results shows a fairly good agreement,indicating that the new theory is effective.