Summary
Naturally occurring gas hydrates contain significant 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 hydrates and their interaction with the sediment. The goal of this study is to determine how the hydrates are distributed in the pore space and the implications of this porescale distribution for hydrate saturation estimates from seismic and acoustic velocities.
Laboratory measurements were conducted to obtain information about the distribution of hydrates in the pore space of synthetic sediments (glass beads). Tetrahydrofuran (THF) was used as a guest molecule since THF hydrate is a proxy for naturally occuring hydrate. We performed micro X-ray computed tomography (MXCT) on laboratory formed glass-bead samples. MXCT images indicate that THF hydrates form in the pore space with little to no contact to the grain surfaces. We observed salt precipitation at grain contacts and in small pore space. These hydrate-bearing sediments appear to follow a pore-filling model but contained salt cement. 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.
Introduction
Gas hydrates are clathrate structures of natural gases. They require low temperatures and high pressures for stability. These conditions are met in shallow sediments in Arctic permafrost regions and beneath the seafloor along continental slopes. The estimated amount of natural gas, mainly methane, stored in hydrate reservoirs exceeds the amount of natural gas stored in conventional resources by at least one order of magnitude (Meyer, 1981; Dobrynin et al., 1981; Collett et al., 2009). The widespread occurence of gas hydrates in permafrost and shallow marine sediments is well established (Collett et al., 2009). Anderson et al. (2008) and Dallimore et al. (2008) demonstrated that gas-hydrate production can be developed with existing oil and gas production technology. For successful production of methane gas from hydrate reservoirs we need to obtain knowledge about physical properties of gas-hydrate bearing sediments.
The most common geophysical methods used to characterize and quantify gas hydrates in nature are seismic surveying and well logging. In order to calibrate and interpret these field measurements, laboratory studies are necessary to determine the bulk physical properties of hydrate-bearing sediment. Currently, it is possible to predict the existence of gas hydrates from geophysical measurements. However, the techniques used to estimate hydrate saturation based on either seismic data or well logs require further development (Collett and Lee, 2012). Thus, the amount of hydrates stored in a reservoir remains uncertain in some cases. Information about the distribution of gas hydrates in the rock is necessary to determine hydrate saturation.
