Dynamic Characteristics of Offshore Natural Gas Hydrate Dissociation by Depressurization in Marine Sediments

Xinfu Liu,Chunhua Liu,Jianjun Wu

doi:10.1155/2019/6074892

Xinfu Liu, Chunhua Liu + Show 1 more

Open Access

https://doi.org/10.1155/2019/6074892

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Abstract

Dynamic characteristics of offshore natural gas hydrate (NGH) dissociation will provide the theoretical basis to analyze technical issues of oceanic hydrate exploitation. A mathematical model is developed to simulate offshore NGH dissociation by depressurization in marine sediments. Different phase combination statuses are involved in the process of NGH dissociation by taking ice melting and water freezing into account. The proposed methodology can analyze the processes of hydrate and water phase transitions, decomposition kinetics and thermodynamics, viscosity and permeability, ice-water phase equilibrium, and natural gas and water production. A set of an experimental system is built and consists of one 3-D visual reactor vessel, one isothermal seawater vessel, one natural gas and water separator, and one data acquisition unit. The experiments on offshore NGH dissociation by depressurization in 3-D marine sediments are carried out, and this methodology is validated against the full-scale experimental data measured. The results show that during the prophase, natural gas flow is preceded by water flow into the production wellbore and natural gas occupies more continuous flow channels than water under a large pressure gradient. Then, the natural gas flow rate begins to decline accompanied by an increase of water production. During the second phase, natural gas flow rate decreases slowly because of the decreased temperature of hydrate-bearing formation and low pressure gradient. The lower the intrinsic permeability in marine sediments, the later the water flow rate reaches the peak production. And the space interval of the production wellbore should be enlarged by an increase of the intrinsic permeability. The stable period of natural gas production enhances, and the water flow rate reduces with the increase of bottom-hole pressure in production wellbores. The main reason is the slow offshore NGH dissociation under the low producing pressure and the restriction of heat conductivity under the low temperature.

Highlights

Natural gas hydrates (NGHs) are ice-like crystalline compounds in which hydrocarbon gas molecules are encaged inside voids in interlocking bucky-ball-type cage structures of water molecules under suitable conditions of high pressure and low temperature
The flow rate of natural gas generated by NGH dissociation can be evaluated as follows: m ng where Ahd is the specific surface area of NGH dissociation in marine sediments, Fe is the equilibrium fugacity of natural gas, Fl is the local fugacity of natural gas, K0 is the kinetic dissociation constant which is equal to 1:25 × 105 mol/m2 · Pa · s, and ΔEhd is the activation energy of NGH dissociation given by −Ehd/R = 9:75 × 103 K
(1) during the prophase, NGH is undergoing initially rapid dissociation and natural gas flow rate increases effectively thanks to the large pressure gradient induced by pressure drawdown distribution

Summary

Introduction

Natural gas hydrates (NGHs) are ice-like crystalline compounds in which hydrocarbon gas molecules are encaged inside voids in interlocking bucky-ball-type cage structures of water molecules under suitable conditions of high pressure and low temperature. The dissociation process in hydrate-bearing porous media was developed from Stefan’s equations [12,13,14] to the models combining multiphase flows and intrinsic kinetic process of NGH [15]. The proposed dissociation model contains multiphase flows: gas, water, hydrate, and ice. Different phase combination statuses are involved in the process of NGH dissociation by taking ice melting and water freezing into account. Different phase combination statuses are involved in the process of NGH dissociation by taking ice melting and water freezing into account This methodology can analyze the processes of hydrate and water phase transitions, decomposition kinetics and thermodynamics, viscosity and permeability, ice-water phase equilibrium, and natural gas and water production. A set of an experimental system is built, and some experiments on offshore NGH dissociation by depressurization in 3-D marine sediments are carried out

Model Development of Offshore NGH Dissociation by Depressurization

Computation of Variables and Numerical Solution

Verification of the Model and Interpretation

Conclusions