流体饱和致密砂岩的实验测量与岩石物理建模应用

与孔隙几何相对简单的高孔隙度、高渗透率砂岩不同,致密砂岩的岩石骨架和孔隙结构通常表现出不同尺度上的非均质性。为了研究这种非均质性和饱和流体对致密砂岩性质的影响,测量了具有9块不同孔隙度的致密砂岩在不同围压、干燥和水饱和条件下的超声波速度。利用速度随有效压力的变化曲线估计软孔隙的纵横比分布,进而利用喷射流模型进行流体替换效应解释。基于超声测试数据和局部流机制,建立了考虑软孔隙随纵横比分布的喷射流模型,实现了致密砂岩跨频段岩石物理建模,利用此模型分析了超声频段和地震频段内饱和致密砂岩的流体敏感性。基于超声频段测量的流体敏感度被高估,实际上地震频段的流体敏感度可能与超声分析结果存在较大差异,因此实现实验室低频直接测量对于准确的岩石物理建模和应用具有关键性的意义。

Experimental measurement and rock physics modeling of fluid-saturated tight sandstone

Unlike traditional high-porosity,high-permeability sandstones with relatively simple pore geometries,the rock skeleton and pore structure of tight sandstones often exhibit heterogeneity at different scales.In order to reveal the effects of pore structure and different fluids on the elastic properties of tight sandstones,we measured the ultrasonic velocity of tight sandstones with nine different porosity under different confining pressures,drying and water saturation conditions.The aspect ratio distribution of soft pores is estimated by using the curve of velocity versus effective pressure,and then the squirt flow model is used to explain the fluid substitution effect.The results show that the Gassmann fluid replacement equation cannot explain the velocity variation of water-saturated tight sandstone under different pressures,and the squirt flow model can predict this velocity change.Based on the ultrasonic test data and local flow mechanism,a jet flow model considering soft pore and aspect ratio distribution is established to realize the rock physics modeling of tight sandstone cross-range.The model is used to analyze the fluid sensitivity of saturated tight sandstone in ultrasonic frequency band and seismic frequency band.The fluid sensitivity based on ultrasonic frequency band measurement is overestimated.In fact,the fluid sensitivity of the seismic frequency band may be quite different from the ultrasonic analysis result.Therefore,the realization of laboratory low frequency direct measurement is of key significance for accurate petrophysical modeling and application.