Growth pattern of dispersed methane hydrates in brine-saturated unconsolidated sediments via joint velocity and resistivity analysis

Abstract

Although the different morphologies of methane hydrates have been identified in pore spaces, their growth mechanism has not yet been fully resolved. In this study, we performed two sets of experiments using dissolved methane in brine-saturated unconsolidated sediments composed of 95.5 wt% quartz sand and 4.5 wt% kaolin to simulate the formation of dispersed marine gas hydrates. The in situ ultrasonic velocities (VP and VS), electrical resistivity, and methane hydrate saturation (Sh) were simultaneously measured and were jointly analyzed and compared to elastic and electrical rock physics models of various hydrate morphologies in pore spaces to quantify the invisible dynamic evolution of the hydrates’ morphologies. The results indicate that the hydrate growth pattern is characterized by distinct stages as Sh increases. During the first set of experiments, the frame-strengthening morphology was dominant in all of the stages, the contact-cementing morphology occurred infrequently when Sh > 8%, and the pore-filling morphology gradually became identifiable when Sh > 25%. It is proposed that the frame-strengthening hydrates primarily grow on grain surfaces when Sh < 25%, then they extend toward the center of the pore, and finally they begin to plug the pores when Sh > 35%. In the second set of experiment, the geophysical responses were significantly affected by the presence of free gas, and the observed hydrate growth behavior was differed from that observed in the first set of experiments.