Micro X-ray Computed Tomography Imaging and Ultrasonic Velocity Measurements of Hydrate-Bearing Porous Media

Abstract:

Naturally occurring gas hydrates contain signicant amounts of natural gas which might be produced in the foreseeable future. Thus, it is necessary to understand the pore-space characteristics of hydrate reservoirs, especially the pore-scale distribution of hydrate and its interaction with the sediment. The goal of our research is to examine the distribution of hydrate in the pore space and the influence of hydrate pore-scale distribution on seismic velocities and sonic logs. We conducted laboratory measurements to obtain information about the distribution of hydrate in the pore space of synthetic porous media (glass beads). We used Tetrahydrofuran (THF) as a guest molecule since THF hydrate is a proxy for naturally occuring hydrate. We used micro X-ray computed tomography (MXCT) to image hydrate distribution in the pore space. In addition, we investigated the influence of hydrate saturation and distribution on ultrasonic velocities simultaneously with the MXCT imaging. We installed a torlon vessel and a cooling system in the MXCT scanner which allows us to form hydrate in the MXCT scanner at atmospheric pressure and a temperature of approximately 2°C. Both, MXCT images and ultrasonic velocity measurements, indicate that THF hydrate forms in the pore space while residual brine coats the grain surfaces and fills small pores. Our observations are in accordance with the pore-filling model of the effective medium theory of hydrate-bearing sediments. Based on this knowledge, it may be possible to calibrate seismic and well logging data to calculate the amount of natural gas stored in a hydrate reservoir. This information will help to make decisions regarding the producibility of methane hydrates and to develop safe production schemes.