Geofluids

Volume 2019, Article ID 6074892, 11 pages

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

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

^{1}School of Mechanical and Automotive Engineering, Qingdao University of Technology, Qingdao, Shandong 266520, China^{2}Key Lab of Industrial Fluid Energy Conservation and Pollution Control, Ministry of Education, Qingdao, 266520 Shandong, China^{3}College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, China^{4}PetroChina Coalbed Methane Co., Ltd., Xian, Shanxi 710082, China

Correspondence should be addressed to Xinfu Liu; moc.621@rotcodcpu and Chunhua Liu; nc.ude.cpu@35009002

Received 10 April 2019; Revised 10 August 2019; Accepted 5 September 2019; Published 13 November 2019

Academic Editor: Umberta Tinivella

Copyright © 2019 Xinfu Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 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.

#### 1. 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. NGH sequesters hydrocarbon natural gases while the gas hydrate extracts dissolved natural gas from pore water [1, 2]. The reactions of dissolution and growth are highly reversible for NGH. Bonding of gas molecules in the cage of water molecules is accomplished by van der Waals force [3, 4].

Analysis on dissociation of offshore NGH will provide the theoretical basis to analyze technical issues of oceanic hydrate exploitation. There have been the basic researches on dynamic characteristics of NGH dissociation [5, 6]. Selim and Sloan [7] described gas hydrate dissociation under thermal stimulation in porous media. The proposed model viewed gas hydrate dissociation as a process of natural gas and water produced at a moving boundary. However, the proposed models were somewhat simple because of the limitation of NGH in marine sediments at that time. Clarke and Bishnoi [8–10] determined the intrinsic rate constants and activation energies for the dissociation of different NGHs. The proposed models neglected heat transfer and mass transfer because of the high stirring rate. He et al. [11] regarded gas hydrate decomposition as a gas-solid reaction without solid production layer with the temperature of above zero centigrade. The particle shrinking problem of NGH decomposition was studied from the crystal dissolution angle. In recent years, the models of depressurization dissociation described the effects of pressure drop, , on NGH dissociation by depressurization at some constant temperature. And the dissociation process in hydrate-bearing porous media was developed from Stefan’s equations [12–14] to the models combining multiphase flows and intrinsic kinetic process of NGH [15]. The basic researches mainly studied the effects of the driving drop of temperature and pressure on dissociation. This establishes the dissociation kinetic process of NGH. Ruffine et al. [16–20] focused on hydrate reservoir exploration and field production tests using different porous media or sediment systems. Boxall et al. [21–23] reported on the mechanism of hydrate dissociation in stirring reactors. The hydrate dissociation rate can be enhanced by increasing the experimental temperature or decreasing the experimental pressure. Meanwhile, the agglomeration of wetted hydrate particles in the initial hydrate dissociation stage was observed in detail by FBRM and PVM data.

The applied researches mainly involved studies on dynamics of NGH dissociation. And they were directed toward the dynamic simulation of hydrate dissociation in pipelines and real hydrate dissociation in porous media [24–26]. Sun and Mohanty [27] developed a simulator for methane hydrate formation and dissociation in porous media with the system of gas and ice consideration. And the simulation of intrinsic kinetics was carried out based on the Kim-Bishnoi model. However, no experimental results were found in their works. Liang et al. [28] and Kumar et al. [29] developed a mathematical model to describe the kinetics of methane hydrate dissociation below the ice point based on an ice-shielding mechanism. Tang et al. [30] carried out the experimental works of 1-D gas production from the hydrate-bearing core by depressurization with the simulator TOUGH-Fx/Hydrate. However, the effect of hydrate dissociation kinetics and the ice phase were neglected in their works. Nazridoust and Ahmadi [31] developed Users’ Defined Subroutines (UDS) using the FLUENT™ code. And an axisymmetric model of the core was solved for multiphase gas-liquid flows during the hydrate dissociation process. The axisymmetric model contained three separate phases: methane gas, water, and methane hydrate. However, their works assumed that gas hydrate was dispersed in the pores of the core. Kneafsey and Moridis [32] observed the changes of internal temperature and density during methane hydrate dissociation by depressurization in a Mount Elbert sandstone sample. However, the experimental results include only hydrate dissociation having occurred initially throughout the sample with the temperature maintained above the ice point. Windmeier et al. [33–35] recognized that mass transfer was the dominant factor near the dispersed hydrate-liquid interface during hydrate dissociation and developed the Consecutive Desorption and Melting (CDM) model. Chen et al. [36] and Shi et al. [37] developed an improved hydrate dissociation model by considering the changes in the dissociated surface induced by particle agglomeration. Therefore, it is necessary to further improve the model of dissociation under depressurization conditions, especially for offshore NGH in marine sediments, by considering heat transfer, mass transfer, and intrinsic kinetics.

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. 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.

#### 2. Model Development of Offshore NGH Dissociation by Depressurization

In terms of characteristics and behavior, offshore NGHs are crystalline, ice-like substances belonging to a class of compounds called clathrates. The chemical compounds have natural-gas molecules bound within almost spherical water cages through physical rather than chemical bonds. The offshore NGH dissociation simulator contains three components (NGH, natural gas, and water) and four separate phases (gas, water, hydrate, and ice). Processes of hydrate and water phase transitions, decomposition kinetics, and thermodynamics are considered in the NGH dissociation simulator. And then a three-dimensional (3-D) hydrate-bearing marine sediment is developed based on the conservation of mass and energy [38–40].

Furthermore, some assumptions are made as follows: (1) Darcy’s law is valid for a two-phase (gas and water) flow; (2) the water phase contains only free water, and the ice phase contains only the component of water; (3) the effect of salt concentration on synthesis and decomposition of the hydrate is neglected; (4) the molecular diffusion and hydrodynamic dispersion are neglected in mass transportation. Therefore, the equations of mass balance for the phases of gas, water, ice, and NGH can be, respectively, given by

The subscripts of , , , and denote the separate phases of ice, NGH, natural gas, and water, respectively. The parameters of , , , and are the flow rate of decomposition, density, saturation, and porosity, respectively. The relationship of saturation of these phases satisfies that , where is the gravitational acceleration, and are the permeabilities of natural gas and water, respectively; and are the flowing pressures of natural gas and water, respectively; and are the injected natural gas and water at the boundary, respectively; and and are the viscosities of natural gas and water, respectively.

and can be evaluated from the given formulae: where is the bottom-hole pressure of the production wellbore [41–44]; and are the radii of the wellbore and the supply boundary, respectively; is the Dirac function of ; , , and are the coordinates of the Cartesian coordinate system; and , , and are the coordinates of the production wellbore.

The equation of the energy balance for the multiphase flows can be determined as follows: where , , , , and are the coefficients of effective specific heats of ice, NGH, natural gas, marine sediment, and water, respectively; and are the effective specific heats at constant volume and pressure, respectively; , , , , and are the effective thermal conductivities of ice, NGH, natural gas, marine sediment, and water, respectively; and are the enthalpy changes in ice melting and NGH dissociation, respectively; and is the energy conducted through marine sediments.

is the energy conducted through the bottom hole and well head and can be evaluated from the give formula:

The pressures of natural gas and water are related to the equation of capillary force:

The permeabilities of natural gas and water are usually described in terms of the intrinsic permeability in marine sediments and relative permeability, as follows: where is the intrinsic permeability in marine sediments, and are the relative permeabilities of natural gas and water, respectively; and is the exponential parameters for permeability.

By neglecting the change in porosity (porous media is almost incompressible), the storage terms of water and natural gas can be developed as follows: where is the coefficient of compressibility of water in marine sediments [45], is the molar mass of natural gas, is the quality of natural gas, is the universal gas constant, is the volume of natural gas, and is the compressibility factor of natural gas.

The compressibility factor of natural gas, , illustrates the ratio of actual volume to ideal volume under the condition of identical quality. This compressibility factor is known as a function of pseudoreduced density, , and pseudoreduced temperature, , for the pure natural gas. An explicit factor, which is an accurate mathematical approximation [46, 47], is developed on the basis of experimental results and is given by where is the specific gravity of natural gas in marine sediments.

The boundary conditions of this proposed model are given by and the initial conditions of this proposed model are given by where , , , and are the initial distributions of pressure of natural gas, saturation of NGH, saturation of water, and temperature, respectively, and is the temperature at the bottom of the wellbore.

Dissociation kinetics of NGH is based on the Kim-Bishnoi model [48, 49]. Then, the flow rate of natural gas generated by NGH dissociation can be evaluated as follows: where is the specific surface area of NGH dissociation in marine sediments, is the equilibrium fugacity of natural gas, is the local fugacity of natural gas, is the kinetic dissociation constant which is equal to , and is the activation energy of NGH dissociation given by .

The flow rate of water generated by NGH dissociation can be given by

The decomposition rate of NGH dissociation can be determined as follows: where and are the molar masses of NGH and water and is the hydration number and is given as 5.75 in the actual construction of NGH.

#### 3. Computation of Variables and Numerical Solution

Some additional relationships need to be considered to close the equations of the NGH dissociation in porous marine sediments. Viscosity of natural gas is known as a function of temperature and pressure and can be evaluated with modification of the Amooey [50] model.

Relative permeabilities of natural gas and water and capillary pressure are determined with the modified Corey model [51, 52]. where is the entry capillary pressure.

The equilibrium equation of NGH can be read as [53] where is the pressure of the NGH phase equilibrium.

The effect of pressure on the ice-water phase equilibrium is usually low since the magnitude of pressure in porous marine sediments varies from 10^{6} Pa to 10^{7} Pa [54]. Furthermore, possible phase combination statuses can be natural gas+NGH+water+ice, natural gas+NGH+water, natural gas+NGH+ice, natural gas+water+ice, NGH+water+ice, NGH+ice, natural gas+ice, NGH+water, etc. And the existence of the ice phase complicates the process of NGH dissociation.

NGH dissociation in marine sediments is an endothermic process of phase transition. The latent heat for per kilogram of NGH in the process of phase transition can be evaluated from the given formula [55, 56]: where and are the constants given by and and is the drop of latent heat in the process of phase transition.

Different phase combination statuses are involved in the process of NGH dissociation by taking ice melting and water freezing into account. The traditional approaches of simulation are not the most suitable as far as NGH dissociation in marine sediments are concerned. The main reasons are that the unknowns are usually the same at different times for each grid block and the numerical solution must consider very small incremental times. Therefore, the fully implicit numerical methodology is introduced to discrete the partial differential equations and track the phase transition between the ice and water phases. The nonlinear algebraic equations are discretized and solved using the Newton-Raphson method. And the following steps for the calculation procedures are proposed for NGH dissociation. (1)The supply boundary and phase status of each grid block are determined for the NGH dissociation system in porous media of marine sediments(2)The boundary conditions and initial conditions are input, respectively, at time 0. And at each time step, the existence of the different phases for each grid block must be determined from the results of the previous time step(3)The primary variables of the NGH dissociation model are chosen at the current time step and the equations related to their grid blocks determined. When the phase status of grid blocks is switched to water and ice, the corresponding equations of grid blocks include equations (1), (2), (3), (4), (5), and (6).(4)The physical parameters of NGH are first solved explicitly, followed by explicit solution of natural gas and water. Then, the pressure of phase equilibrium, decomposition rate, relative permeability, and capillary pressure of NGH are solved implicitly, followed by the implicit solution of natural gas and water(5)The matrix of coefficients for temperature, pressure, and saturation are built up for different separate phases(6)The preconditioned conjugate gradient method is used to solve the system by discretizing the equations. Determine the iteration convergence conditions and update the next time step given by (7)Iterate by returning to step 3 until the target accuracy is obtained for the different variables

#### 4. Verification of the Model and Interpretation

Performance of the mathematical model is demonstrated by comparing the numerical results of offshore NGH dissociation by depressurization with experimental results. A 3-D marine sediment is considered to clarify the dynamic process of offshore NGH dissociation. This 3-D marine sediment includes a NGH bearing zone located between an impermeable cover and bottom layers under depressurization.

The process flow diagram of offshore NGH formation and dissociation system by depressurization is given in Figure 1. This experimental system consists of one 3-D visual reactor vessel supplying the system with gas (natural gas and N_{2}) and fresh water, one isothermal seawater vessel, one natural gas and water separator, and one data acquisition unit. The 3-D visual reactor vessel can be operated up to 9.0 MPa and is enclosed in the isothermal seawater vessel with a seawater condition. The isothermal seawater is provided by a circulating pump at the temperature range from -10°C to 50°C. The production well is located at the corner of the 3-D visual reactor vessel. The formation of offshore NGHs and their dissociation in the case of depressurization, ice melt, and water freeze can be observed in the 3-D visual reactor vessel. Nine inlet pipes, thirty-six temperature sensors, and nine pressure sensors are evenly installed and arranged on the upper side of the 3-D visual reactor vessel in the manner of , , and , respectively. Nine inlet pipes are laid to supply high-pressure natural gas or N_{2}. Thirty-six temperature sensors are laid to detect the distribution variation of NGH saturation with time and space. The data acquisition unit can record all the information varying with time, which includes the spatial distributions of pressure, temperature, and saturations of natural gas, NGH, water, and ice. Then, the offshore NGH formation and dissociation in 3-D marine sediment, thermal stimulation, and inhibitor injection can be achieved with this experimental system.