弹性波散射衰减对致密砂岩非均质性的响应机制及实验研究

Tight sand oil and gas is a kind of strategic successor energy to China, but the heterogeneity is a serious constraint to the exploration, optimization and development of tight sand reservoirs. Therefore the study on the heterogeneity of tight sandstone reservoirs is the key of their exploration and development. However, because of the weak and complex geophysical characteristics of the tight sandstone reservoir caused by the low porosity and the change of the hydrocarbon saturation, the current researches on the heterogeneity of the compact sandstone are mainly focused on the macro and micro studies but few on mesoscale, which is the reason of difficulty of quantitative description of reservoir heterogeneity. Compared with the commonly used seismic reflection wave velocity, the attenuation of seismic wave is much more sensitive to the variation of reservoir parameters, and the scattering attenuation is the direct indication of the rock heterogeneity. To meet the technical needs of unconventional reservoir exploration and development, this project starts from the study of scattering attenuation of seismic waves. The relationship between the heterogeneity of the dense sandstone and the scattering attenuation of the elastic wave is studied by using the random medium scattering wave field simulation and the stress and strain data and the elastic wave data from the ultrasonic measurement. The study will be strong support for quantitative description of the reservoir heterogeneity.

致密砂岩油气是国家战略性接替能源的一种，但其非均质性特点对其油气储层的勘探、优化和开发效果产生严重制约和影响，因此对致密砂岩储层非均质性进行定量描述是致密砂岩勘探开发的关键。由于致密砂岩储层因低孔隙性和含油气饱和度变化所产生的地球物理特征变化微弱且关系复杂，目前对致密砂岩非均质性的研究主要集中于宏观及微观研究，对其在中观尺度上的非均质性研究较少，这是储层非均质性定量描述困难的主要原因。相对于通常使用的地震反射波波速而言，地震波衰减对于储层参数的反映要敏感得多，其中散射衰减更是岩石中非均质性的直接响应。常规地震反射波信息不能满足非常规油气勘探的技术需求，本项目从地震波散射衰减入手，通过随机介质散射波场模拟，结合超声实验测量岩芯的应力应变数据及弹性波数据，研究致密砂岩非均质性与弹性波散射衰减的关系，为利用地震数据对致密油气勘探储层非均质性进行研究及定量描述提供参考依据。

致密砂岩普遍具有低孔、低渗及微裂缝发育的地质特征，并且呈现出很强的非均质性。综合考虑致密砂岩孔隙结构及流体斑块状饱和非均质性，采用双双重孔隙介质结构模拟了致密砂岩的弹性波响应，分析了同时具备两类非均质性岩石中的波传播特征。调查分析了两组分别来自中国鄂尔多斯盆地苏里格气田及四川盆地广安气田的不同类型致密砂岩储层的岩芯超声波实验数据，给出了岩石样本的弹性波速度频散与衰减曲线。结果显示理论模型预测结果与完全饱和、部分饱和岩石的实验数据吻合良好。对两个地区数据进行对比分析，苏里格致密砂岩样本总体上比广安致密砂岩渗透率高，在各孔隙度范围内，特征模拟显示苏里格样本的裂隙尺寸明显大于广安样本。广安致密砂岩在低孔隙度范围内发育了更多、更小的颗粒裂隙／接触。此外计算了致密岩石的纵波尾波及固有衰减，结果显示衰减主要由散射造成。固有衰减随着渗透率和含气饱和度的增加而增加，而尾波衰减在部分饱和时强于完全饱和状态。使用三重孔隙介质模型和散射模型模拟固有及散射衰减，结果显示固有衰减主要是由微裂隙与粒间孔隙之间的流体流动造成的，而散射衰减可能是由不同矿物颗粒聚集体的非均质性引起的。利用地震波衰减与储层非均质性之间的响应规律为使用地震及测井资料指导致密油气储层勘探和开发提供参考依据，具有重要的理论意义和工程应用价值。