Hydrate Reformation Research Articles

Numerical modeling on hydrate formation and evaluating the influencing factors of its heterogeneity in core-scale sandy sediment

Natural gas hydrate (NGH) has been regarded as a fossil fuel reserve for the future on account of its tremendous potential. The numerical modeling on NGH formation/dissociation mechanism contributes to better understanding its accumulation and distribution feature, and optimizing the development program. This paper aims to develop a new simulator for the NGH formation in the core-scale sandy sediments based on the computational fluids dynamic (CFD) methods. The mathematical model is established based on the kinetic reaction model of hydrate formation, the permeability reduction model by the NGH, model of heat and mass transfer in porous media. The hydrate formation model is programmed by C language, and used as a subroutine for Fluent software which is adopted to solve the governing equations of the multiphase flow. The simulator scheme is verified by comparison with the experiment and numerical simulation in literature. What's more, this study reproduces the same fluctuant tendency of temperature as the experiment during the 1.0 h–2.0 h for the first time. Different reaction surface models of NGH formation/dissociation are evaluated by the developed codes. The effects of the reaction surface of hydrate (RSH) model and the initial fluids distribution on the hydrate formation process are simulated and analyzed. The variation of the RSH in NGH formation/dissociation should be taken into consideration when modeling the hydrate re-formation in the exploitation of NGH. The initial distribution of water and gas has a great impact on the hydrate formation in the sealed reactor. The hydrate distribution is ununiform, even when assuming the water and methane are mixed uniformly in a homogeneous porous media. This study provides new insight for the parametric estimation of the RSH model in the hydrate formation and dissociation modeling.

Numerical analysis of the gas recovery performance in hydrate reservoirs with various parameters by stepwise depressurization

Natural gas hydrates have drawn worldwide attention as a significant clean energy source in the 21st century. Depressurization is known to be the most effective method for hydrate exploitation owing to its feasibility and economic benefits. Recently, stepwise depressurization has received extensive attention owing to its advantages of effective prevention of ice generation and hydrate reformation during gas recovery. In this work, we optimized the 2-D simulator to model the evolution of temperature, pressure, hydrate saturation, and gas production behavior in hydrate sediments during decomposition process via stepwise depressurization. The effect of various reservoir characteristics on gas production was analyzed by introducing the sensitivity factor. The results demonstrated that higher specific heat capacity of the reservoir would promote gas production in the early stage of the reaction; however, this effect is reversed in the later stage. A higher thermal conductivity of the hydrate cores can dramatically enhance the gas production rate; however, this advantage is weakened with increasing thermal conductivity. Reducing the gas relative permeability hinders the hydrate decomposition process, and the water relative permeability has minimal effect on gas production. Based on the sensitivity analysis, the thermal conductivity and gas relative permeability are the main factors affecting hydrate decomposition. • Numerical simulation of gas production by step-wise depressurization is presented. • Higher thermal conductivity enhanced gas production via stepwise depressurization. • Gas permeability and thermal conductivity are main factors of gas recovery.

Hydrate blockage observation and removal using depressurization in a fully visual flow loop

Hydrate blockages impose a serious flow assurance problem on gas transportation pipelines. Regular inspections for the formation of hydrate blockages are imperative along with the use of an effective blockage removal method. In this study, the processes of hydrate blockage formation and removal were investigated using a fully visual flow loop. Comprehensive analysis of the pressure drop was performed along with a discussion of the acquired visual images of the hydrates to describe the phenomena. Hydrate blockage first occurred in no through-flow zone of the pipeline, specifically in deadleg. The results showed that higher LL (liquid loading: the initial liquid volume fraction) caused a blockage in the inlet, while lower LL caused a faster and more widespread blockage. Moreover, shut-in and restart operations also caused blockages due to the temperature decrease and annealing of hydrate. The experiments showed that blockages formed more quickly as a result of lower LL during the shut-in operation. This necessitated the reduction of shut-in duration and maintenance of the temperature at phase equilibrium condition. In addition, the depressurization method was used to decompose bulk hydrate in order to remove the blockage. The results confirmed that stepwise depressurization could be applied to prevent hydrate reformation at a higher LL and water conversion rate, while a lower backpressure could be used at lower LL to efficiently remove blockages.

Prediction and management of hydrate reformation risk in pipelines during offshore gas hydrate development by depressurization

The hydrate reformation in marine gas hydrate production system can significantly affect the accuracy control of the production pressure differential and even cause pipeline blockage problems. In this paper, a transient temperature and pressure calculating model was established to predict the risk of hydrate reformation in production pipelines during offshore gas hydrate development, which considering the coupling interactions between the real-time drainage of electric submersible pump (ESP), the downhole preheating of electric induction heater (EIH), the wellbore multiphase flow and heat transfer. Using the model, the dynamic evolution laws of gas–liquid two-phase flow behaviors in different pipelines of mixed transportation, drainage and gas production were studied. Furthermore, the region of hydrate reformation in production pipelines was predicted quantitatively. The results indicate the risk of hydrate reformation in the drainage pipe is the highest relative to other pipes, and the main flow pattern in this pipe is bubbly flow. Besides, the conventional hydrate management method through inhibitor injection was applied to prevent the hydrate reformation. It can be found that the required Ethylene glycol (MEG) minimum concentration reaches 22% in order to completely eliminate the risk of hydrate reformation. To this end, a new hydrate management strategy by adding additional pumps and heaters was proposed, and the prevention effect of hydrate reformation in different scenarios was discussed in detail. The simulated results demonstrate that this method can effectively address the potential hydrate reformation risks without the use of inhibitors. This work will provide theoretical references for the hydrate flow assurance operations in marine gas hydrate production testing.

Gas Production from Methane Hydrate Reservoirs in Different Well Configurations: A Case Study in the Conditions of the Black Sea

Gas hydrates are considered “near-future energy resources” due to their vast existence all around the world. Recently, China has tested gas production from methane hydrate-bearing clayey silts via horizontal well technology for the first time. This study aims to investigate the effect of well configuration on gas production from the Black Sea gas hydrates. Mainly, two different hydrate-bearing models (single-layer homogeneous hydrate-bearing sands and multilayer hydrate-bearing turbidite) were constructed in this study. Gas production numerical simulations were held using the TOUGH + HYDRATE simulator. Gas production via the horizontal well in single-layer homogeneous hydrate-bearing sand was higher compared to the vertical well case at early stages. However, fast depressurization via the horizontal well caused a rapid decline in reservoir temperature and hydrate dissociation rate. Then, local hydrate reformation was detected near the wellbore. In the later periods of gas production, higher gas production with the vertical well was obtained. The clay layers in the heterogeneous methane hydrate reservoirs (alternating sand and clay layers in turbidite) behave like a barrier to depressurization with a horizontal well.

Depressurization-induced changes in memory effect of hydrate reformation correlated with sediment morphology

Research on the memory effect can improve the rate and efficiency of hydrate reformation, which has important applications in gas hydrate technology. Depressurization is the most widely used gas hydrate dissociation method; however, studies on the mechanism of depressurization on memory effect are still lacking. In this study, the memory effects were compared and analyzed after the hydrate sediment was subjected to different dissociation patterns. The different dissociation patterns included depressurization (different backpressures: 4.5, 3.5, 2.5, 1.5, and 0.1 MPa) and single thermal stimulation dissociation pattern (STSP). The effect of depressurization-induced sediment morphology changes on hydrate-memory effect and gas conversion was obtained under different backpressures. The results indicated that the memory effect tended to remain stable in the STSP, but changed with depressurization dissociation. The memory effect after depressurization dissociation gradually weakened with decreasing backpressure. Notably, an anomalous memory effects was observed, wherein the memory effect was not weakened, but significantly enhanced after depressurization dissociation at 0.1 MPa. Furthermore, depressurization may not only change the memory effect but could also increase the stable interval of the memory effect and the supercooling degree. The average gas conversion ratio of stable interval after dissociation at 0.1 MPa decreased by nearly 8%.

Utilization of water-gas flow on natural gas hydrate recovery with different depressurization modes

Water-gas two-phase flow always exists in the natural gas hydrate production process. It has been confirmed that a suitable water-gas flow rate ratio can efficiently induce hydrate decomposition above hydrate phase equilibrium. Meanwhile, the water-gas flow may help to prevent the ice generation and hydrate reformation that need to be solved in the depressurization process. However, little is known about the synthetic effect mechanism of different depressurization modes and water-gas flow on hydrate decomposition. In this study, the piecewise depressurization, constant depressurization rate, constant gas recovery rate assisted with water-gas flow was used to decompose methane hydrate, respectively. The hydrate decomposition behaviors were investigated via magnetic resonance imaging. The results indicated that the hydrate decomposition characteristics showed spatial dependence, and the decomposition front was moved to the center from the edges along the interface of water and hydrate. Moreover, the higher water-gas flow rate ratio and faster depressurization rate led to a lower energy input (gas and water injection volume) and higher energy recovery rate (hydrate decomposition rate). Compared to sudden depressurization, the ice generation was efficiently avoided and the hydrate decomposition rate was remarkably improved for the all three decomposition modes. By comparison, the combination of piecewise depressurization incorporated with water-gas flow (mode I) was the best mode to recover gas from hydrate reservoirs in our experimental scale.

Guest molecular exchange and stability of carbon dioxide hydrates under the influence of methane gas

The molecular exchange is a frontline technology in the process of gas recovery from natural gas hydrate deposits. In such a case, the caged methane (CH4) molecules are exchanged with carbon dioxide (CO2). The process efficiency depends on several factors, including pressure (p) and temperature (T) of the injected gas. There are quite a few conflicting views on the guest-guest exchangeability; mainly, it is unclear if the process is by molecular diffusion or by the reformation of hydrates. This study reports the replacement of CO2 by CH4 in primeval CO2 hydrates, below ice-melting temperature. Intuitively such a process is least preferred because CO2- hydrates are thermodynamically more stable than CH4- hydrates. We varied the injected gas (CH4) pressure in the range of 0.7–8.5 MPa. The co-existence of Raman signatures specific to caged CO2 & CH4 molecules supports the molecular replacement in virgin CO2 hydrates. Further, the diffusion of CH4 in both 51262 & 512 cages of sI hydrates offers a path for the mixed hydrate formation. Subsequently, the hydrate dissociation pattern also supports the rapid growth of mixed hydrates.

Numerical study of depressurization and hot water injection for gas hydrate production in China's first offshore test site

In 2017, China successfully conducted its first offshore gas hydrate production test by using vertical well in the Shenhu area, achieving a total gas production of 3.09 × 105 m3. However, there is still a long way to commercial production. In this study, to evaluate and improve the gas production potential of China's first hydrate production site, depressurization, and the combination of depressurization and hot water injection, were applied to natural gas hydrate (NGH) reservoir in China's first offshore hydrate production site to analyze 10 years' hydrate production features. The results indicated that the decomposition front of hydrate when using depressurization in two horizontal production wells had obvious non-uniform characteristics, and there were rapid decomposition regions and hydrate reformation regions. Among them, the hydrate reformation region may have adverse effects on hydrate decomposition. The combination of depressurization and hot water injection could improve total gas production and alleviate hydrate reformation. However, when the temperature of hot water injected increased from 35 °C to 80 °C, the energy efficiency decreased from 10.22 to 1.53, as a lot of energy was consumed to heat the sediments in the reservoir. When the hot water injection time was reduced from 3650 days to 1825 days, the energy efficiency could be effectively increased from 10.22 to 21.78 without significant reduction of the gas production. This work contributed to the present understanding of production behavior of depressurization and the combination of depressurization and hot water injection, and helped to evaluate the influence of hot water injected on gas production from NGH reservoir.

Distribution and reformation characteristics of gas hydrate during hydrate dissociation by thermal stimulation and depressurization methods

Pore-scale distribution and reformation characteristics of gas hydrate in porous sediments can provide invaluable information on macroscale production behaviors. In this work, the X-ray computed tomography (X-ray -CT) has been conducted to detect distribution characteristics of the hydrate-bearing sample during hydrate formation and dissociation. Experimental results indicate that, during hydrate formation, mass and heat transfer can lead to the transformation of grain-attaching (grain-cementing and grain-contacting) hydrate to pore-filling hydrate, as well as the heterogeneous distribution of gas hydrate in pores. During hydrate dissociation, whether the thermal stimulation stage or the depressurization stage, the hydrate decomposition initiates from the ablation of the hydrate-gas interface, and the grain-cementing hydrate remains intact until the hydrate cracks into particles and collapses. In addition, during the thermal stimulation stage, the migration of “memory water” under the equilibrium hydrate formation condition leads to the hydrate reformation, and the hydrate reformation promotes the homogeneous distribution of gas hydrate in pores. During the depressurization stage, gas hydrate is reformed below the hydrate “dissociation front” because of the endothermic process of hydrate dissociation and the pressure-driven fluid flow. The reformed grain-cementing hydrate provides fluid flow channels instead of plugging the pores and throats. However, the shut-in time after the depressurization-induced gas production should not be too long to prevent the pore plugging by the further growth of the reformed hydrate. Additionally, the depressurization process finally leads to the grain migration and may reduce the sediment strength under the loose grain filling condition.

The enhancement effect of water-gas two-phase flow on depressurization process: Important for gas hydrate production

Depressurization is one of the most efficient methods in the production testing of natural gas hydrates. However, problems such as hydrate reformation, ice generation and insufficient dissociation driving force in the later period of depressurization adversely affect the gas production. It has been confirmed that controlling the flowrate ratio of a water-gas two-phase flow can help enhance the hydrate dissociation. However, the effect of the water-gas flowrate ratio on the hydrate dissociation behaviors during depressurization is unclear. In this study, three dissociation modes involving a combination of water-gas flow and depressurization were examined: mode 1 (concurrent start of depressurization and water-gas flow), mode 2 (depressurization is first used to dissociate the hydrate, and the water-gas flow is initiated after 15 min), and mode 3 (depressurization is first used, and the water-gas flow is initiated after 30 min). The feasibility of water-gas flow to accelerate hydrate dissociation and mitigate ice generation was confirmed during the depressurization process. In all the modes, the higher water-gas flowrate ratio and lower dissociation pressure significantly increased the energy recovery rate and decreased the energy input. Additionally, the water-gas flow, especially that with a higher flowrate ratio, effectively accelerated the elimination of the dark-zone (mixture of ice and hydrate) by providing continuous heat transfer. Mode 1 corresponded to the highest energy recovery rate, lowest energy input and most rapid disappearance of the dark-zone under the same experimental conditions. Therefore, mode 1 was regarded as the most efficient mode to dissociate hydrate in an actual hydrate production.

Hydrate-induced clogging of sand-control screen and its implication on hydrate production operation

Blockage of sand-control media is one of the main obstacles that affect gas production efficiency from gas hydrate-bearing sediments. Hydrate reformation is a potential plugging inducer of sand-control media. In this study, we conduct a series of experiments using steel-wired screen mesh to examine the hydrate-induced clogging of sand-control screen. The screen mesh sample was installed into a closed-circuit circulating system under gas-water two-phase flow condition to simulate bottomhole multiphase production processes. The results indicate that hydrate formation within the screen would causes permeability loss of the screen up to 98%. The pseudo-permeability of the screen sample shows dual-gradient decreasing characteristics during hydrate clogging. Therefore, we speculate that hydrate accumulating and hydrate particle bridging are the two main mechanisms causing screen plugging. The hydrate accumulating sub-process is mainly controlled by the degree of subcooling, while the hydrate particle-bridging sub-process is affected by fluid flow rate. Furthermore, the “ J -shape” coiling of pressure-temperature relationship can be used as an indicator in diagnosing possible bottomhole screen plug during hydrate exploitation. Artificial interference of downhole temperature is strongly recommended to mitigate screen plugging induced by hydrate reformation. • Hydrate clogging causes permeability loss of sand-control screen up to 98%. • Hydrate accumulation causes dual-gradient decrease in the permeability of sand-control screen. • The “ J -shape” coiling of P - T curve is a indicator to diagnose downhole hydrate-induced screen clogging. • Artificial temperature interference is recommended to mitigate hydrate induced screen clogging.

"Ester"-A new ring-shear-apparatus for hydrate-bearing sediments.

We developed a new thermostated ring-shear-apparatus for investigation on hydrate- or ice-bearing sediments. A fluid inlet at the bottom of the static part of the cell and a fluid outlet at the top of the rotating half-cell allow exchanging and pressurizing the pore fluid in the sample cell to a certain value below the pressure providing the normal load that is applied hydraulically via a seal disk using a syringe pump. The volume change in the sample can be derived from the volume received or injected by the pump. The system allows the use of different methods for the formation and controlled decomposition of hydrate before, after, and during a shear test. The ring-shear-cell is designed for a maximum hydraulic pressure of 30 MPa. A commercial hollow shaft servo actuator applies the torque to the rotating half of the sample cell, and a rotary encoder provides information for determining the shear displacement. Stress path investigations in shear rate controlled experiments with large strain are possible with shear rates up to 12.6 mm/s and torques up to 1440 N m. The system design allows for complex experiments studying the behavior of a shear plane in hydrate- and/or ice-bearing sediments, including the decomposition and reformation of hydrate and/or ice under varying pressure and temperature conditions. It is a useful tool to provide experimental data to support research and engineering in solving problems related to permafrost and hydrate-bearing formations. The system performance is demonstrated by examples of tests on hydrate- and ice-bearing sand samples.

The determining factor of hydrate dissociation rate in the sediments with different water saturations

In nature, gas hydrate is mainly distributed in permafrost region and deep ocean sediments. For these two types of hydrate reservoirs, water contents in the sediments were different. The water content has a large influence on the characteristics of heat transfer and fluid flow in the sediments, thereby affecting the hydrate dissociation behavior. In this paper, the experiments of hydrate dissociation by depressurization in sediments with different water saturations were conducted. The influences of water saturation on the pressure, the temperature, the gas production, the water production, and the hydrate dissociation rate were analyzed. The experimental results showed that, in the depressurization stage (DS), the rate of pressure dropping and the amount of hydrate reformation increased with the raise of the water saturation in the sediments. In the constant pressure stage (CPS), although the heat conduction rate of the sediments increased with the increase of the water saturation, the mass transport rate of the gas released from hydrate dissociation was limited due to the increase of water saturation. As a result, the hydrate dissociation rate decreased with the increase of the water saturation in the sediments. According to these experimental results, during the hydrate dissociation in the sediments with different water saturations, the hydrate dissociation rate is determined by the mass transport rate of the gas released from the hydrate dissociation. For the hydrate reservoirs with the high-water saturation, the hydrate dissociation rate is largely limited by the diffusion rate of gas released from hydrate dissociation. Hence, the key factor to improve the hydrate dissociation rate in the sediments with high water saturation is to increase the diffusion rate of gas in the sediments.

Effect of methane hydrate dissociation and reformation on the permeability of clayey sediments

Natural gas hydrates in the South China Sea generally exists in clayey sediments, whose permeability is significantly affected by the swelling-shrinkage characteristics of clay that are caused by hydrate dissociation and reformation, and these characteristics influence the efficiency of gas production. In this study, montmorillonite with the most significant swelling characteristics and kaolinite with weak swelling characteristics in South China Sea sediments were investigated experimentally, respectively. The gas permeability was experimentally investigated for varying hydrate saturations during hydrate dissociation and reformation. The results showed the following: The clayey samples after hydrate dissociation had lower permeability than those without dissociation due to the clay particle swelling and the existence of bound water, but the swelling and hydrophilicity of kaolin are lower than for montmorillonite, which is confirmed by observation of SEM photography. Furtherly, the hydrate reformation improved the gas permeability of clayey samples to a certain extent except for those kaolin samples with initial ice saturations of 5% and 10% because of the increased pore space due to dehydration shrinkage of clay particle and the formation of typical aggregate structures, which is in contrast to the results for sand. Furthermore, the damage value of permeability after hydrate dissociation and the ratio of the permeability after reformation of hydrate to the initial permeability in both the montmorillonite and kaolin were obtained in this study.

Investigation into methane hydrate reformation in water-dominated bubbly flow

Hydrate reformation may lead to production line blockage in the development of natural gas hydrate reservoirs. However, few studies have focused on the flow characteristics and plugging risks during the hydrate reformation process. Therefore, experiments on hydrate reformation were carried out in a high-pressure flow loop. The hydrate induction time and formation subcooling approached for the first hydrate formation and reformation. The pressure and temperature of the first formation and reformation onset fell in a subcooling band (2.0 ± 0.5 °C). Furthermore, the flow stability of hydrate slurry for the reformation process was relatively poor compared with the first formation. Hydrate particles aggregated more violently during the reformation process when the initial flow rate was 1160 kg∙h−1. Moreover, the hydrate memory effect at the microlevel could be confirmed from two aspects, including an increasing number of methane microbubbles (MMBs) after hydrate dissociation and a shorter time required for the decrease in the number of MMBs during the reformation process. Then, the flow pattern evolutions were summarized for different experimental conditions, and the minimum flow rate of hydrate slurry with the stable flow ability could be predicted using the classical correlation. Finally, a prediction model was developed for predicting the pressure drop in hydrate slurry flow, which considered the hydraulic, particle-aggregation, and hydrate–liquid friction effects. The findings of this work provided an insight into the behavior of methane hydrate reformation in water-dominated bubbly flow, which is an advancing research topic in the field of development of natural gas hydrate reservoirs.

Effect of multiphase flow on natural gas hydrate production in marine sediment

Natural gas hydrates (NGHs), regarded as an alternative future energy source. Currently, tests for hydrate exploitation from marine sediment have been performed in the Nankai Trough of Japan and the Shenhu area of the South China Sea. Hydrate exploitation is influenced by water-gas flow in the sediment, and considering the huge seawater reserves in hydrate accumulation areas, an experiment of seawater-gas flow was performed to dissociate hydrate. The effects of seawater-gas flow rates and initial hydrate saturation on methane hydrate (MH) production were analyzed. The results showed that seawater-gas flow efficiently promotes hydrate dissociation and inhibits hydrate reformation. Moreover, there was a faster heat and mass transfer with increasing seawater flow rates and decreasing gas flow rates, which enhanced the average MH dissociation rate. In addition, the variation time of the flow channel increased with higher initial hydrate saturation. Additionally, seawater-gas flow promotes MH dissociation stronger than deionized water-gas flow.

The influence of heat perturbation on natural gas hydrate sediment in deepwater drilling and cementing process

ABSTRACT The deepwater drilling and cementing techniques achieved significant development in recent years. However, the engineering response mechanism of natural gas hydrate sediment (NGHS) during drilling and cementing process is far from clear, which challenges the safe development of offshore petroleum resources. To reveal the influence of heat perturbation on NGHS of hydration exotherm of cement slurry and drilling fluid circular temperature increment in the process of subsequent drilling operation in shallow seabed, a mathematical model of hydrate dissociation is developed to simulate the influence by using the numerical simulation approach in this paper, and taking a natural gas hydrate reservoir at Liwan slope in the northern South China Sea as an example. The mathematical model comprehensively takes into account heat conduction, heat consumption of hydrate dissociation, hydrate dissociation dynamics, fluid seepage around the wellbore, and their coupling reaction. The analysis shows that the increase of heat transmitted from drilling and cementing process to NGHS facilitates the hydrate dissociation, leading to the growth of the sediment pore pressure in the near wellbore, the fluctuation range of pore pressure is rapidly diffused and significantly larger than the wellbore radius. The gas from the hydrate dissociation will permeate to the wellbore-distant area and reform hydrate with water. The gas will not to totally reformed hydrate in the short term, partially gathering around the wellbore, and the maximum gas saturation in the sediment is between 10% and 12%. The hydrate reformation area will be several times than the wellbore radius. But the maximum hydrate saturation will not exceed a certain value. The transition area of hydrate dissociation area to hydrate reformation area is quite narrow, and the range of high pore pressure is identical to that of the hydrate reformation. The hydrate dissociation will lead to NGHS instability, so the influence of heat perturbation on NGHS should be paid attention to in deepwater drilling and cementing process.

Economic benefit of methane hydrate reformation management in transport pipeline by reducing thermodynamic hydrate inhibitor injection

Hydrate reformation is an inherent risk in a methane hydrate production system because dissociated water and methane flow through transport pipelines simultaneously. In this study, comprehensive experiments using high-pressure autoclaves are performed to investigate the effect of reducing the concentration of thermodynamic hydrate inhibitor on hydrate reformation characteristics. In order to simulate the production of methane hydrates, these are formed and dissociated in the preliminary stage; thereafter, the hydrates are reformed from dissociated water. Although dissociated water and fresh water are found to have similar formation rates, they have a major difference in relative torque, especially in the early stage of hydrate formation. The instant rise of the relative torque in the dissociated water is observed to be up to 2.5; thereafter, it remains approximately 3.0 until hydrate conversion reaches 28 vol%. This trend is different from that in fresh water, where a steady increase in relative torque with sudden spikes is observed. These results clearly show the high risk of hydrate blockage in the transport pipelines of a methane hydrate production system; hence, the injection of hydrate inhibitor is necessary. In this study, monoethylene glycol (MEG) is the selected thermodynamic hydrate inhibitor; its concentration is maintained below a specific value to completely avoid hydrate formation. Despite the occurrence of hydrate reformation, a stable relative torque is observed in a 20 wt% MEG concentration, where the conversion of water to hydrate approximately reaches 40 vol%. For a 10 wt% MEG concentration, a sudden increase in relative torque of up to 2.0 is observed in the early stage of hydrate formation; this suggests that the foregoing is less effective in sustaining flowability. Compared to fresh water, the addition of a 20 wt% MEG to dissociated water results in a more stable torque. Once the performance of the MEG concentration reduction is confirmed, the MEG regeneration process is designed accordingly by using both multiphase flow and process simulation models. A life cycle cost (LCC) analysis is conducted by estimating the capital expenditures (CAPEX) and operating expenditures (OPEX) of the MEG regeneration process. Considering both CAPEX and OPEX, the LCC for complete inhibition (35 wt%) is 90.15 MUSD (million United States dollars); however, for the 20 and 10 wt% MEG concentrations, the LCC values are reduced to 86.79 and 84.58 MUSD, respectively. The foregoing suggests the potential economic benefit of methane hydrate reformation management to avoid blockage.

Investigation of the hydrate reservoir production under different depressurization modes

Large reserves of natural gas hydrates exist, and the depressurization method has the greatest potential for gas hydrate reservoir recovery. Currently, the most commonly adopted depressurization simulation method is a constant bottom-hole pressure production scheme. This study proposes a new depressurization mode with decreasing bottom-hole pressure. The production characteristic was numerically investigated using this method. The results show the following: (1) As the depressurization exponent (n) decreases, the development effect improves, and production indexes including cumulative gas production/dissociation and gas-water ratio increase. However, the reservoir energy consumption is higher and the hydrate reformation is more severe. (2) Compared to the proposed depressurization mode, the hydrate production index of the constant bottom-hole pressure production (n = 0) is better. However, the hydrate reservoir energy consumption is higher and the hydrate reformation is more severe using constant bottom-hole pressure production. (3) To achieve a balance between production and reservoir energy consumption during depressurization production, the bottom-hole pressure should be controlled by selecting a suitable depressurization exponent between nmin and nmax, which can be determined through numerical simulations.