致密砂岩衰减岩石物理图板分析:储层微裂隙预测

致密砂岩储层普遍具有孔隙度低、微裂隙发育的特点，岩石内部常含有强烈的结构非均质性.致密砂岩发育的微裂隙使储层具有良好的连通性，促成高饱和气的天然气成藏.针对川西某探区须家河组高含气饱和度致密砂岩，本文选取致密砂岩岩心样本，进行了不同围压下的超声波实验测量.考虑储层完全饱气情况下的粒间孔隙、微裂隙双重孔隙结构，采用Biot-Rayleigh双重孔隙方程，构建致密砂岩岩石物理模型，进而分析了裂隙含量对纵波频散和衰减的影响.基于地震波衰减，构建了致密砂岩多尺度岩石物理图板.采用谱比法和改进频移法估算致密砂岩样本及储层衰减，对超声和地震频带下的图板进行校正.将校正后的图板应用到研究工区，选取二维测线和三维区块，进行储层孔隙度和裂隙含量的定量预测.对比实际资料进行分析，结果显示，本文预测的孔隙度和裂隙含量与三口测井的孔隙度曲线和实际产气情况基本吻合，基于孔隙-裂隙衰减岩石物理模型有效地预测了优质储层的分布区域.

Analysis of attenuation rock-physics template of tight sandstones: Reservoir microcrack prediction

Tight-sandstone gas reservoirs have low porosity and contain developed microcracks, result in the strong matrix heterogeneity. Due to the development of microcracks in tight sandstone, the relevant good connectivity is favorable to the reservoir natural gas accumulation with high gas saturation. In this paper, for the tight sandstone reservoirs in the Xujiahe formation of western Sichuan Basin, the ultrasonic measurements are performed on the tight sandstone under different confining pressures. Based on the model of double-porosity structure of intergranular pores and microcracks, we build a rock physics model for the tight sandstone reservoirs with high gas saturation, by using the double-porosity media equation (Biot-Rayleigh). The effects of the microcrack content on the dispersion and attenuation of compressional waves are analyzed. Based on the seismic Q, the multi-scale rock-physics template of tight sandstone is built. We estimate the attenuation of the sample and the actual reservoirs by using the spectral ratio method and the improved frequency-shift method, respectively. Then, calibrations of the templates are performed at the ultrasonic and seismic frequencies. The templates are applied to the work area to quantitatively predict the porosity and microcrack content of reservoirs by using a 2D seismic line and a 3D dataset. The results indicate that the predicted porosity and microcrack content are consistent with the well-log data and actual gas production reports of the three wells. The distributions of high-quality reservoirs are effectively predicted based on the microcrack-pore attenuation rock physics model.