含流体致密砂岩的纵波频散及衰减:基于双重双重孔隙结构模型描述的特征分析

P-wave velocity dispersion and attenuation in fluid-saturated tight sandstones:Characteristics analysis based on a double double-porosity structure model description

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匀性.致密砂岩储层与常规砂岩储层比较,具有明显的岩石物理性质、渗流力学性质方面的差异.致密砂岩内部的非均匀性对弹性波频散、衰减有显著影响,其中包括孔隙结构的非均匀性,即岩石内部孔隙参数的不均一性,以及孔隙内部不相混溶流体的非均匀分布;此外,非均匀性的尺度也决定了波出现显著频散与衰减的频段.综合考虑致密砂岩孔隙结构非均匀性及流体斑块状饱和的非均匀性,本文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.对两个地区致密砂岩岩芯数据进行对比分析,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.致密砂岩的速度频散与衰减结果受流体黏度、晶体破裂及流体斑块状饱和的共同影响.此外,孔隙度越大,部分饱和岩石中斑块状饱和机制对总衰减的贡献越低,与之相对,结构非均质性所占的比重则有所增强.

Tight sandstones have general geological characteristics of low porosity, low permeability and development of micro-crack systems, which present strong heterogeneity. There are obvious differences of rock physical and percolation mechanical properties between the tight sandstone reservoirs and the conventional sandstone reservoirs. The heterogeneity inside tight sandstones, including fabric heterogeneity （the inhomogeneity of pore properties within rock matrix） and the heterogeneous distribution of immiscible fluid mixtures inside pores, has a significant effect on elastic wave velocity dispersion and attenuation. Furthermore, the scale of heterogeneity determines the characteristic frequency bands of wave dispersion and attenuation. In a comprehensive analysis of the heterogeneities of pore structure and fluid patchy-saturation in tight sandstones, this paper adopts a double double-porosity structure to model elastic wave response features, and investigates the wave propagation characteristics in the rocks simultaneously containing the two types of heterogeneities. We analyzed the two different sets of ultrasonic measurement data of tight sandstones, one from the Sulige gas field of Ordos Basin, Northwest China and the other from the Guang＇ an gas field of Sichuan Basin, Southwest China, and presented the elastic wave velocity dispersion and attenuation curves for the corresponding rock specimens. The theoretical prediction results are shown to be in well agreement with the rock experimental data at the full-saturation and partial-saturation states. Comparative analysis on the core data of tight sandstones from the two gas fields shows that the Sulige specimens generally have higher permeability than Guang＇an specimens. In each different porosity range, characteristics analysis shows that the micro-crack sizes of Sulige specimens are larger than those of Guang＇an specimens. More grain contacts/cracks with smaller sizes are found to be developed in Guang＇an tight sandstones in the low-porosity range. The velocity dispersion and attenuation in tight sandstones are impacted by a combined effect of fluid viscosity, grain cracks and fluid patchy-saturation. It is also deduced in the partially-saturated rocks that the contribution of the mechanism of patchy-saturation to the total attenuation will decline as rock porosity increase, while on the other hand, the proportion of attenuation caused by the mechanism of fabric heterogeneity increases.