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.