Gas hydrate in-situ formation and dissociation in clayey-silt sediments: An investigation by low-field NMR

Xiaoxiao Sun,Xuwen Qin,Jianchun Xu,Jingli Wang,Hongfeng Lu,Zijie Ning

doi:10.1177/0144598720974159

Abstract

The hydrate reservoir in the Shenhu Area of the South China Sea is a typical clayey-silt porous media with high clay mineral content and poor cementation, in which gas hydrate formation and dissociation characteristics are unclear. In this study, the CO2hydrate saturation, growth rate, and permeability were studied in sandstone, artificial samples, and clayey-silt sediments using a custom-built measurement apparatus based on the low-field NMR technique. Results show that the T2spectra amplitudes decrease with the hydrate formation and increase with the dissociation process. For the artificial samples and Shenhu sediments, the CO2hydrate occupies larger pores first and the homogeneity of the sandstone pores becomes poor. Meanwhile, compared with the clayey-silt sediments, CO2hydrate is easier to form and with higher hydrate saturation for the sandstone and artificial samples. In hydrate dissociation process, there exists a protection mechanism, i.e. the dissociation near the center of hydrates grain is suppressed when gas pressure drops suddenly and quickly. For permeability of those samples, it decreased with hydrate forms, and increases with hydrate dissociation. Meanwhile, with the same hydrate saturation, permeability is higher in hydrate formation than in dissociation.

Highlights

Natural gas hydrates (NGHs), composed of water and natural gas, are solid clathrate compounds found under relatively high-pressure and low-temperature conditions, such as sub-marine sediments and underneath permafrost ( Klauda and Sandler, 2005; Makogon et al, 2007; Sun et al, 2018 )
The estimated global in place (GIP) of natural gas hydrate gas is on the order of 1015 to 1018 m3, which is almost twice as much as the world’s proven conventional fossil energy reserves (Lerche and Bagirov, 2004; Li et al, 2016; Sloan and Koh, 2007)
For the artificial sample that consists of fine sand and clay, it shows three unique peaks resulting from the large pores between the fine sand and the micropores mainly distributed in the clay

Summary

Introduction

Natural gas hydrates (NGHs), composed of water and natural gas, are solid clathrate compounds found under relatively high-pressure and low-temperature conditions, such as sub-marine sediments and underneath permafrost ( Klauda and Sandler, 2005; Makogon et al, 2007; Sun et al, 2018 ). Compared to the NGHs in sandstone reservoirs with relatively high permeability (more than 100 mD) and saturation (about 70%) in Japan (generally coarse sand reservoir) and the USA (conglomerate reservoir), the mean median grain size of the sediments is about 12 lm in the NGH reservoir of the Shenhu Area, which leads to low permeability (2–5 mD) and saturation (about 33%) (Li et al, 2018; Ye et al, 2018). The hydrate reservoir in the Shenhu Area of the SCS is mainly composed of fine-grained clay silt with low permeability and low NGH saturation, making exploitation and development difficult

Methods

Results

Conclusion