有效应力变化对致密砂岩孔隙结构及弹性波响应的影响规律

致密砂岩气藏具有裂缝发育和有效应力高的特征,研究不同有效压力下孔、裂隙介质地震波传播特征,有利于地震解释与地下储层的识别.但是前人的研究较少考虑岩石内部微观孔隙结构特征与孔隙、裂隙间流体流动的关系.本文首先通过选取四川盆地典型致密砂岩岩样,在不同有效压力下对岩石样本进行超声波实验测量.然后基于实验测得的纵、横波速度进行裂隙参数反演,得到不同有效压力下致密砂岩样本的裂隙孔隙度.再将裂隙孔隙度和样本岩石物理参数代入双重孔隙介质模型,模拟得到不同有效压力下饱水致密砂岩样本纵横波速度频散和衰减的变化规律.结果表明模型预测的速度频散曲线与纵波速度实验测量结果能够较好的吻合.最后统计分析了致密砂岩裂隙参数,得到了致密砂岩储层裂隙参数随有效压力及孔隙度变化特征.依据实际岩石物理参数建立模型,其裂隙参数三维拟合结果能够较好描述致密砂岩裂隙结构与孔隙度、应力的关联,可为实际地震勘探中预测储层裂缝性质提供基础依据.

Effect of effective pressure change on pore structure and elastic wave responses in tight sandstones

Tight sandstone gas reservoirs develop cracks and are subject to high effective stresses. The analysis of seismic wave propagation in media containing pores and cracks as a function of differential pressure is important for seismic interpretation and reservoir characterization. However, the predecessors seldom consider the relationship between the microscopic pore structure of the rock and the fluid flow between pores and cracks. In this paper, ultrasonic wave velocity measurements are performed on tight sandstones from the Sichuan Basin under different differential pressures. Crack parameter inversion based on experimental P- and S-wave velocities is performed, and the crack porosities of tight sandstones under different differential pressures are obtained. The crack porosities and the rock physical parameters are used in the double-porosity media model, and the variation of P-and S-wave velocity dispersion and attenuation under the different differential pressures of the water-saturated tight sandstone samples are modeled. The P-wave velocity dispersion curves predicted by the model agree well with the experimental data. By analyzing crack parameters of tight sandstones, the variations under different effective pressures and porosities are obtained. The 3D fitting results of crack parameters based on the actual rock physics parameters describe the relation between crack structure of tight sandstones and porosity and stress, which can support the prediction of reservoir crack properties in actual seismic exploration.