压力及流体对岩石孔隙结构与黏弹性的影响规律研究:理论模型及实验观测

A study on the effects of pressure and fluid on rock pore structure and anelasticity:Theoretical model and experimental measurement

地质成因和构造/热应力导致地壳岩石中的孔隙结构(裂隙和粒间孔)的变化.影响岩石黏弹性的因素包括压力、孔隙度、孔隙中包含的流体和孔隙几何形状等.相对于岩石中的硬孔隙,岩石黏弹性(衰减和频散)受软孔隙(裂隙)的影响更大.本文选取三块白云岩样本,进行了不同围压和流体条件下的超声波实验测量.利用CPEM(Cracks and Pores Effective Medium,裂隙和孔隙有效介质)模型获得了岩石高、低频极限的弹性模量,并通过Zener体(标准线性体)模型将CPEM模型拓展到全频带而得到CPEM-Zener模型,用该模型拟合岩石松弛和非松弛状态下的实验数据,本文得到平均裂隙纵横比和裂隙孔隙度以及纵波速度和品质因子随频率的变化关系.结果表明,饱水岩石的平均裂隙纵横比和裂隙孔隙度均高于饱油岩石,随着压差(围压和孔隙压力的差值)的增加,饱油岩石中的裂隙首先闭合.并且压差在70 MPa以内时,随着压差增大,岩石的平均裂隙纵横比和裂隙孔隙度在饱水和饱油时的差值增大,此时流体类型对于岩石裂隙的影响越来越显著,此外,对饱水岩石,平均裂隙纵横比随压差增加而增大,这可能是由于岩石中纵横比较小的裂隙会随压差增大而逐渐趋于闭合.在饱水和饱油岩石中,裂隙孔隙度和裂隙密度都随着压差增加而减小.通过对裂隙密度和压差的关系进行指数拟合,本文获得压差趋于0时的裂隙密度,且裂隙密度随孔隙度增大而增大,增大速率随压差增加而降低.针对饱水和饱油的白云岩样本,CPEM-Zener模型预测的纵波频散随压差增大而减小,此变化趋势和实验测得的逆品质因子随压差的变化关系基本一致,由此进一步验证了模型的实用性.本研究对岩石的孔隙结构和黏弹性分析以及声波测井、地震勘探的现场应用有指导意义.

Geological origins and tectonic/thermal stresses result in variable pore structures(cracks and intergranular pores)in crustal rocks.The factors impacting on rock anelasticity include pressure,porosity,pore fluid,pore geometry,and so on.Anelasticity(wave attenuation and velocity dispersion)is more affected by soft pores(cracks)than by stiff(equant)pores.We perform ultrasonic measurements on the three dolomite samples under variable pressure and fluid type.The CPEM(Cracks and Pores Effective Medium)model is adopted in this study to compute the bulk and shear moduli of high and low frequency,and moreover,we obtain the CPEM-Zener model by extending the CPEM model to full frequency range,and incorporating the Zener(standard linear solid)model.The average crack aspect ratio,crack porosity and the phase velocities and quality factors are obtained as a function of frequency by fitting the experimental data of the relaxed and unrelaxed states.The results show that the average crack aspect ratio and crack porosity in the water-saturated rocks are higher than the oil-saturated cases,so that cracks containing oil tend to close earlier as the differential pressure(the difference between confining and pore pressures)increases.Moreover,the difference between the average crack aspect ratio and crack porosity of water-saturated rocks and those of oil-saturated rocks increases with increasing differential pressure(differential pressure less than 70 MPa),therefore,as the differential pressure increases,the effect of fluid type on crack in rocks increases.In addition,the average crack aspect ratio increases as the differential pressure increases for water-saturated rocks,this phenomenon is possibly related to the close of cracks with smaller aspect ratios.The values of crack porosity and crack density gradually decrease for the oil-and water-saturated dolomite specimens with increasing differential pressure.The initial crack density at 0 differential pressure can be obtained by performing exponential fitting on the relation between crack density and differential pressure.Crack density increases with increasing porosity,however,this growth rate decreases with increasing differential pressure.The P-wave dispersion derived from the CPEM-Zener model decreases with increasing differential pressure for fully water-and oil-saturated dolomites.This trend is generally consistent with the relationship between experimentally-measured inverse quality factor and differential pressure,which validates the applicability of the model.These results are useful for analyzing pore structure and rock anelasticity,and the related field applications about sonic logging and seismic exploration.