Laboratory Study on Hydrate Production Using a Slow, Multistage Depressurization Strategy

Yanlong Li,Zhixue Sun,Nengyou Wu,Qiang Chen,Yurong Jin,Changling Liu,Chuqiao He,Qingguo Meng

doi:10.1155/2021/4352910

Abstract

Optimization of the depressurization pathways plays a crucial role in avoiding potential geohazards while increasing hydrate production efficiency. In this study, methane hydrate was formed in a flexible plastic vessel and then gas production processes were conducted at constant confining pressure and constant confining temperature. The CMG-STARS simulator was applied to match the experimental gas production behavior and to derive the hydrate intrinsic dissociation constant. Secondly, fluid production behavior, pressure-temperature ( P ‐ T ) responses, and hydrate saturation evolution behaviors under different depressurization pathways were analyzed. The results show that integrated gas-water ratio (IGWR) decreases linearly with the increase in depressurizing magnitude in each step, while it rises logarithmically with the increase in the number of steps. Under the same initial average hydrate saturation and the same total pressure-drop magnitude, a slow and multistage depressurization strategy would help to increase the IGWR and avoid severe temperature drop. The pore pressure rebounds logarithmically once the gas production is suspended, and would decrease to the regular level instantaneously once the shut-in operation is ended. We speculate that the shut-in operation could barely affect the IGWR and formation P ‐ T response in the long-term level.

Highlights

Natural gas hydrate (NGH) is an ice-like compound formed from water and gas molecules under relatively low-temperature and high-pressure conditions [1] NGH is considered as the most promising alternative fossil fuel due to its great energy potential [2]
To extract natural gas from hydrate-bearing sediments (HBS), four methods have been proposed based on thermodynamic conditions of NGH, i.e., depressurization [5, 6], thermal stimulation [7], CO2-CH4 exchange [8], and chemical injection
Sediment for HBS formation is packed inside the rubber sleeve

Summary

Introduction

Natural gas hydrate (NGH) is an ice-like compound formed from water and gas molecules under relatively low-temperature and high-pressure conditions [1] NGH is considered as the most promising alternative fossil fuel due to its great energy potential [2]. To extract natural gas from hydrate-bearing sediments (HBS), four methods have been proposed based on thermodynamic conditions of NGH, i.e., depressurization [5, 6], thermal stimulation [7], CO2-CH4 exchange [8], and chemical injection. These methods aim to promote hydrate in-situ decomposition and produce gas and water by using the methods that are widely applied in conventional oil-gas industry. The verified numerical model was employed to investigate the influence of depressurization pathways on gas-water production behaviors, as well as formation pressure-temperature (P‐T) and hydrate saturation responses

Experiments

Numerical Modeling and Verification

Numerical Simulation and Result Discussion

Findings

Summary and Conclusions