Elastic wave velocity of marine sediments with free gas: Insights from CT-acoustic observation and theoretical analysis

The existence of free gas in pores will significantly affect the elastic wave velocity and attenuation coefficient of aqueous sediments. However, because the gas content and distribution are difficult to be accurately obtained through experiments, it is difficult to accurately characterize the relationship between elastic wave velocity and gas content, which may lead to the evaluation error of gas saturation in sediments. In this study, we applied the combined detection technology of X-CT and ultrasonic detection to simultaneously obtain the volume proportion of each phase and acoustic velocity of the selected marine sediments, respectively. On the basis of experiments, the applicability of effective medium theory (EMT), Biot-Gassmann theory by Lee (BGTL) and simplified three-phase equation (STPE) is verified. Meanwhile, the bulk modulus of pore fluid (Kfl) is recalculated by integrating Wood's and Domenico's equations by introducing calibration constant J to adapt to different reservoir conditions. The results show that the elastic wave velocities predicted by BGTL and STPE are more accurate compared with other gas estimation models. When J = −0.8, the acoustic velocity of sedimentary medium predicted by BGTL is more consistent with the experimental data. In addition, we apply the model to the elastic wave velocity data from ODP 204 Hole 1245 and obtain the free gas saturation. The results provide a theoretical and practical application reference for accurately estimating the free gas saturation in marine sediments.