Dispersion and attenuation of compressional waves in tight oil reservoirs: Experiments and simulations

We performed ultrasonic experiments in specimens from a tight oil reservoir. The P-wave attenuation of fluid-saturated specimens was estimated by the spectral ratio method. The results suggest that at ultrasonic frequencies, most specimens have stronger attenuation under gas-saturated conditions than at water- or oil-saturated conditions. The P-wave attenuation positively correlates with permeability. Scanning electron microscopy observations and the triple-porosity structure model were used to simulate the wave propagation. The P-wave velocity dispersion and attenuation are discussed on the basis of the Biot, Biot-Rayleigh double-porosity medium, and the triple-porosity structure models. The results suggest that the Biot and Biot-Rayleigh models cannot explain the attenuation, whereas the triple-porosity structure model is in agreement with the experimental data. Furthermore, we infer that microcracks are common in a porosity of 5%–10%, and the size of microcracks increases in samples with higher porosity. However, the volume ratios of microcracks and clay inclusions remain constant regardless of porosity variations. The size of microcracks is significantly larger than the clay inclusions, and the bulk modulus of microcracks is lower than the bulk modulus of clays.