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Understanding the characteristics of hydrate formation and dissociation in porous media is of great significance for natural gas hydrate exploitation and gas storage underground. In this study, methane hydrate formation and dissociation were conducted in a heterogeneous micromodel. Combined with the lattice Boltzmann method, the permeability evolution of the hydrate-bearing micromodel was analyzed during hydrate formation and dissociation. The random behavior of induction time is captured in porous media. The results show that methane hydrate formation only occurs at the gas-liquid interface and towards the gas phase when the gas phase and the excess liquid phase coexist in pores. Local hydrate dissociation occurs in the process of methane hydrate formation. The difference in changes of pore shape and size available for flow causes a difference in permeability evolution between hydrate formation and dissociation. The hydrate reformation phenomenon is observed at the pore scale. The results show that hydrate reformation only occurs in the aqueous solution in the original hydrate area. It is different from the hydrate formation with no previous hydrate history. The difference in permeability caused by the change in hydrate distribution is smaller than that caused by the change in hydrate saturation during hydrate reformation. • LBM is used to simulated water flow in a hydrate-bearing micromodel. • Stochastic phenomenon of hydrate nucleation is captured in porous media. • Formation process of methane hydrate at the gas-liquid interface is revealed. • Hydrate Reformation phenomenon is observed directly at the pore scale. • Effect of hydrate phase transitions on relative permeability is analyzed. [ABSTRACT FROM AUTHOR]

Class II hydrate deposits are characterized by a mobile water zone (WZ) underneath the hydrate-bearing layer (HBL) and are encountered in permafrost and deep-sea sediments. In this work, an efficient method of depressurization combining low-frequency electric heating under dual horizontal wells is proposed to exploit Class II hydrate deposits, in which two parallel horizontal wells are arranged in the HBL and the WZ. Based on the geological parameters in the Mallik deposit, the energy recovery behaviors are studied through a numerical simulation approach. Electric heating significantly improves hydrate dissociation and gas production compared with the depressurization method. However, gas production lags electric heating for a long time, and the energy efficiency ratio decreases with time in the later stage. To address these shortcomings, two additional electric heating schemes are designed and optimized. The results show that the additional wellbore heating at the beginning of production accelerates the dissociation of hydrates near the production well, thus greatly reducing the lag time. When electric heating is terminated after the 800th day, the cumulative gas production is reduced by 9.1%, but the energy efficiency ratio is improved as high as 48.71, which confirms the great potential of the proposed method. • The method of depressurization combining low-frequency electric heating under dual horizontal wells is proposed. • Gas production behavior and heat transfer characteristics are analyzed by numerical simulation. • The low-frequency electric heating significantly improve the hydrate dissociation and gas recovery. • The enforced heat convection from bottom water enhances heat utilization efficiency. [ABSTRACT FROM AUTHOR]

• A mathematical model of low-frequency electrical heating assisted depressurization (LF-EHAD) method is established. • The energy recovery behaviors from hydrate deposits with LF-EHAD are analyzed. • Gas production is significantly enhanced by electrical heating with a high energy efficiency ratio of 9.65. • The LF-EHAD method shows better production performance than hot water flooding. With the increasing global demand for clean energy, methane hydrate has been regarded as a potential alternative energy source in future due to its huge resources and high energy density. In this study, a mathematical model is established to evaluate the production performance of low-frequency electrical heating assisted depressurization (LF-EHAD) method for hydrate deposits recovery. Within the framework of this model, the integral finite difference method and Newton-Raphson iteration are employed for solution, which is then validated by the experiment data. Using this model, the energy recovery behaviors with LF-EHAD from Shenhu hydrate deposits are investigated and then compared with those performed by hot water flooding and depressurization. Simulation results show that hydrate dissociation and gas production are significantly enhanced as a result of sufficient heat supplied by the electrical heating, validating the potential of the LF-EHAD for hydrate deposits recovery. After 2000 days of production, the cumulative gas production of LF-EHAD and hot water injection are 2.37 times and 1.47 times of that conducted by the depressurization method. Moreover, the heat consumption used to increase the formation temperature in the hydrate dissociation region under LF-EHAD is much less than that of hot water flooding, which results in a higher energy efficiency ratio (EER) of 9.65 for LF-EHAD, while the EER of hot water flooding is 4.42. Overall, LF-EHAD shows better performance than hot water flooding with the same amount of heat input because of higher gas production and energy utilization efficiency, but lower water production. [ABSTRACT FROM AUTHOR]

A method to improve the performance of hot water flooding in a hydrate bearing layer (HBL) is proposed. The development approach involves a horizontal production well completed by sliding sleeves and one vertical injection well. The horizontal section of the production well is first opened to yield a high production rate. As the gas production reduces to a certain degree, staged production from the toe to the heel is conducted by opening and closing the sliding sleeves. Subsequently, particle swarm optimization is coupled with a simulator to optimize the primary adjustable parameters. Furthermore, the proposed approach is compared with an un-staged approach and Sasaki's approach. The results indicate that the optimal injected water temperature is 51.3 °C considering both energy efficiency and recovery. The optimal distance between the horizontal wellbore and the top of the HBL is approximately 1/3 of the reservoir thickness, and the unperforated interval of the injection well should cover the upper 2/3 of the HBL. Based on the same injected heat, the proposed approach can improve cumulative gas production by 25.6%, 14.4%, and 39.1% compared with the un-staged approach, Sasaki's approaches 1 and 2, respectively. • A novel staged production method in HBLs is proposed. • A coupled model of the PSO and STARS module is built. • The primary adjustable parameters are optimized. • The effectiveness of the proposed method is proved. [ABSTRACT FROM AUTHOR]

The absence of impermeable overburden and underburden layers makes it difficult to develop the unconfined hydrate deposits. Using numerical simulation method, the hydrate dissociation and gas production behaviors are fully investigated during depressurization in Shenhu Area based on the available geological data. Results show that depressurization cannot result in significant pressure drop due to the presence of permeable boundaries, which leads to low gas production rate but excessive water production. To overcome these disadvantages, the method of forming artificial impermeable barriers by injecting gels in the permeable overburden and underburden is proposed in this work. Moreover, the effect of the radius of artificial impermeable barriers on gas production is analyzed numerically. Simulation results show that the artificial impermeable barriers prevent large amounts of seawater from entering the production well during the depressurization process, which significantly enhances the depressurization effect, and thus promotes hydrate dissociation and gas production. By forming the 90-m artificial impermeable barriers, the hydrate dissociation percent is greatly enhanced from 8.9% to 45.4%, the cumulative CH 4 production volume is increased from 4.46 × 106 ST m3 to 1.06 × 107 ST m3, while the average water production rate is remarkably decreased from 1412 m3/d to 360 m3/d. • Gas production from unconfined hydrate deposits is investigated with numerical simulation method. • The method of forming artificial impermeable barriers by injecting gels is proposed. • Depressurization is significantly enhanced after forming artificial impermeable barriers. • Hydrate dissociation percent is greatly enhanced from 8.9% to 45.4%. [ABSTRACT FROM AUTHOR]

Productivity prediction and energy evaluation can reduce the economic risk of hydrate development. Meanwhile, the study of conventional resources provides useful reference and guidance. Therefore, this paper aims to establish Inflow Performance Relationship (IPR) formulas for the multiphase, non-isothermal flow in Class III methane hydrate deposits. The production process is divided into ascent and decline stage based on production characteristics. Fetkovich's formula and Vogel's formula are selected respectively for these stages. To revise these formulas, new index and pressure value are introduced to reflect the complexity and variability of hydrate production. New index called pseudo-pressure describes the compound effect of multi-driven forces. New value of minimum production pressure can avoid the adverse impact of ice block. Coefficients in these formulas are quantitatively characterized by selected key factors. The coefficient in Fetkovich's formula is characterized by layer thickness and gas flowablity. The coefficient in Vogel's formula is characterized by hydrate saturation, layer thickness and salinity. The verified results indicate that the average errors of the revised Fetkovich's formula is around 8% and under 11% for the revised Vogel's formula. This means these revised IPR formulas can provide guidance for the productivity prediction and evaluation of Class III methane hydrate deposits. • Inflow Performance Relationship (IPR) method is established and verified for Class III methane hydrate deposits. • The two-stage method is proposed to revise existed IPR formulas. • New index describes the compound effect of drawdown pressure and hydrate dissociation. • New value of minimum production pressure can avoid the adverse impact of ice block. • The energy efficiency is quantitatively characterized by selected key parameters. [ABSTRACT FROM AUTHOR]