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在实验室对5块储层砂岩进行了模拟地层压力条件下的超声波速度测试。砂岩样品采自WXS凹陷的W地层,覆盖了从低到高的孔隙度和渗透率范围。实验选用了卤水和4种不同密度油作为孔隙流体,结合温度变化,实现了对流体粘度引致的速度频散研究。对实验结果的分析表明:(1)对于高孔隙度和渗透率的样品,无论是哪种流体饱和,观察到的超声波速度测试值和零频率Gassmann预测值的差异较小(约2-3%),基本上可以用Biot模型解释;对于中等孔隙度和渗透率的样品,低粘度流体(<约3mP?S)的频散效应也可以用Biot模型得到合理解释;(2)对于低、中孔隙度和渗透率样品,当流体粘度增加时,喷射流机制起主导作用,导致严重的速度频散(可达8%)。对储层砂岩的微裂隙纵横比进行了估计并用于喷射流特征频率的计算,当高于该特征频率时,Gassmann理论的假设条件受到破坏,实验室测得的高频速度不能直接用于地震低频条件下的W地层砂岩的Gassmann流体替换研究。
Five reservoir sandstones were tested for ultrasonic velocity under simulated formation pressure in the laboratory. Sandstone samples were taken from WXS depression W formation, covering low to high porosity and permeability ranges. The experiments selected brine and four kinds of different density oil as the pore fluid, combined with the temperature changes, to achieve the velocity dispersion caused by the viscosity of the fluid. Analysis of the experimental results shows that: (1) For samples of high porosity and permeability, the difference between the observed ultrasonic velocity test and the zero-frequency Gassmann prediction is small (about 2-3% ), Which can basically be explained by the Biot model. For samples with medium porosity and permeability, the dispersion effect of low viscosity fluids (<3 mP • S) can also be reasonably explained by the Biot model. (2) For low, medium Porosity and permeability samples, jet flow mechanics dominate as fluid viscosity increases, resulting in severe velocity dispersion (up to 8%). The micro-fissure aspect ratio of reservoir sandstone is estimated and used to calculate the jet’s characteristic frequency. When the characteristic frequency is higher than the eigenfrequency, the assumptions of Gassmann’s theory are destroyed. The measured high-frequency velocity in laboratory can not be directly used for earthquake Study of Gassmann Fluid Replacement for W Stratigraphic Sandstones at Low Frequency.