致密砂岩层系多尺度波耗散规律及双重分形结构模型

Multiscale wave dissipation characteristics of tight sandstone strata and a double-fractal structure model

了解地层岩石的多尺度非均质性特征对不同频率弹性波响应的影响,对基于地震资料解释与反演地下介质属性至关重要.本文联合采用超声波实验(550 kHz)、声波测井(约10 kHz)和地震勘探(约30 Hz)手段,对鄂尔多斯盆地延长组致密岩石开展观测,估算了不同频率下致密砂岩中的纵波传播速度和衰减.超声波观测结果显示,随着围压增大,孔隙度对衰减的影响有变小的趋势,而黏土含量的影响随围压增大而增大.不同频率范围内,波速和衰减随孔隙度或黏土含量的变化趋势基本一致.测量结果中值及分布特征显示,从超声波到地震频段,纵波速度趋于下降,而衰减却有增大趋势.从超声波到声波频段,速度减小更为显著,中值减小了362 m/s;而测井速度与地震观测结果较为接近.衰减在声波和地震频率之间的差异更为明显.为解释多尺度观测数据,提出了描述孔隙介质中裂隙和黏土尺寸具有自相似分布特征的双重分形结构模型.模型预测与实验结果对比揭示了相关储层黏土包体和裂隙的分形特征.本研究可为进一步解释复杂岩石宽频带波频散和非弹性现象提供理论模型.

Acoustic wave velocity dispersion and attenuation(anelasticity)can be adopted in estimating the pore structure and fluid content of underground in-situ rocks.However,there are actual differences between the measured data at different frequency bands since the measurements involve different spatial scales with sorts of heterogeneities.In this study,the measurements of ultrasonic tests(550 kHz),sonic logging(about 10 kHz)and seismic exploration(about 30 Hz)are performed on the tight reservoirs of the Chang 7 member of Yangchang Formation in Ordos Basin,west China.Ultrasonic tests are performed on the 38 tight sandstones,and the P-wave attenuation is estimated by the spectral-ratio method.The mineral composition and pore structure characteristics are obtained by the X-ray diffraction and casting thin section analyses.Sonic log data are collected from the six wells,including density,porosity,shale content and full-waveform signal data.Post-stack seismic gathers crossing the three wells are acquired.The sonic wave attenuation is estimated by the statistical average method on the basis of the full-waveform sonic-log data,while the seismic attenuation is obtained by an improved frequency-shift method.Moreover,P-wave velocities at different frequencies are obtained.The ultrasonic data results show that P-wave velocity increases while attenuation decreases as the confining pressure increases.The effect of porosity on P-wave attenuation decreases but that of the clay content increases with increasing confining pressure.This may be due to the gradual closure of the microcracks,weakening the effects of wave-induced fluid flow between the stiff pores and microcracks.At the different frequencies,the ultrasonic,sonic logging and seismic results exhibit the same variation trends with respect to the porosity or clay content,i.e.,the velocity decreases and the attenuation increases with the increase of porosity or clay content.A comparative analysis shows that the medians of ultrasonic,sonic and seismic velocity data are 4883,4521,and 4432 m/s,and the corresponding attenuation medians(1000/Qp)are 16.16,17.97,and 25.56,respectively.P-wave velocity decreases more significantly when frequency decreases from ultrasonic to sonic ranges,with a median decline of 362 m/s,while the attenuation increases more significantly from sonic to seismic frequencies.Sonic logging and seismic velocity values are close and show similar normal distribution characteristics.To interpret the multiscale data,a double-fractal poroelasticity model by incorporating the self-similarity characteristics of crack systems and clay minerals is proposed.Numerical modeling is implemented through finite iterations,where a set of inclusions with the same scale are inserted into a porous host skeleton at each iteration.The simulations and observation results are in consistency,revealing the fractality of clay inclusions and crack systems.The attenuation at the seismic frequency band is mainly related to the wave-induced fluid flow mechanism occurring between the clay inclusions and intergranular pores,while the attenuation at the sonic and ultrasonic bands is caused by the joint effect of the two sets of fluid flow mechanisms associated with cracks and clay inclusions.The proposed model allows us to obtain the information about the characteristics(fractality)of fabric heterogeneities(cracks and clay)affecting wave anelasticity.