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Kinetics of hydrate formation in the porous media is important for carbon storage and the feasibility assessment of developing natural gas hydrates. In this study, we used a novel and real-time monitoring apparatus which combined the hydrate formation system with the low-field nuclear magnetic resonance measurement system to study on the characteristics of methane hydrate formation in a partially saturated sandstone. Results show that hydrate coexists with water and methane in the sandstone at the end of methane hydrate formation by an excess-gas method. Magnetic resonance imaging shows that the spatial distribution of hydrate is affected by the initial distribution of water in the sandstone. Water content plays a role in controlling the termination of hydrate formation. Based on the transverse relaxation time distribution analysis, free gas exists mainly in the large pores, while methane molecules can enter the small pores by dissolution and diffusion, forming methane hydrate with water molecules in the absence of free gas. Methane hydrate is formed in the center of both large and small pores by the excess-gas method. The rate of methane hydrate formation is slower in the small pores than that in the large pores. The range of pore size gradually decreases with the hydrate formation.

Hydrate reservoirs generally have some unique characteristics such as hydrate dissociation and reformation, permeability change, which may result in quite different pressure-transient behavior from that in conventional reservoirs. In this work, we develop the pressure-transient analysis method for Class III hydrate reservoirs. By establishing a 2D cylindrical model, we then conduct a pressure buildup test and five flow regimes are identified according to the exhibited characteristics observed from the pressure-transient curves. Furthermore, sensitivity analysis is conducted to study the effects of initial reservoir pressure, initial reservoir temperature, permeability reduction exponent, BHP (bottom-hole pressure), and production time on pressure-transient behavior. We identify five flow regimes on the pressure-transient curves. Due to the dissociation of gas hydrate, the first regime is dominated by the non-uniform distribution of permeability, and the pressure derivative curve exhibits an upward straight line. We identify a “V-shaped” valley in the second regime with a peak in the third regime, which suggests the occurrence of hydrate reformation near the wellbore. Such characteristics exhibited in the three regimes are unique to hydrate reservoirs. In addition, the methodology proposed in this study is successfully applied to analyze the actual well testing data obtained from the Malik hydrate reservoir in Canada.

The water-gas relative permeability is an important parameter to characterize multiphase flow in sediments. To study the water-gas relative permeability of hydrate-bearing porous media, multiphase flow simulations were carried out at the pore scale using the lattice Boltzmann method. The effects of hydrate saturation and hydrate-growth habits on the water-gas relative permeability, which is scaled by the relative permeability considering the hydrate only, were evaluated in a two-dimensional porous medium. Results show that the increase of hydrate saturation causes the decrease of water-gas effective permeability as expected. However, the effect of hydrate saturation on the water-gas relative permeability is different from that of hydrate saturation on the water-gas effective permeability. The water-gas relative permeability increases with the increase of hydrate saturation in the pore-filling case. The water-gas relative permeability decreases with the increase of hydrate saturation in the grain-coating case. The wettability of solid phase has a different effect on the relative permeability of wetting phase and nonwetting phase. The Jamin effect (phase blocking) was observed and may exist in the production of gas from natural gas hydrate reservoirs. This seriously affects the multiphase flow characteristics. The changes of microscale fluid distribution effect the changes of water-gas relative permeability. The relationship between the water-gas relative permeability and the characterization parameters of microscale fluid distribution was analyzed. ISSN:0016-2361

Natural gas hydrate, as an unconventional resource, has been attracting increasing attention. Understanding the characteristics of methane hydrate formation and dissociation in porous media is important for developing gas hydrate-bearing reservoirs. This work discusses the use of low-field nuclear magnetic resonance (LF-NMR) technology to investigate the formation and dissociation of methane hydrate in the sandstone. In this work, an experimental assembly wherein methane hydrate can form and dissociate, is used to conduct LF-NMR measurements. LF-NMR, as a noninvasive measurement technology, combines the transverse relaxation time (T2) measurement with the magnetic resonance imaging (MRI). T2 measurements can explore the characteristics of methane hydrate formation and dissociation in core samples from a pore-scale perspective. MRI can display the spatial distribution of water from a core-scale perspective. The excess-gas method and the excess-water method are successively applied to form methane hydrate, and depressurization is applied to dissociate methane hydrate in the laboratory. The characteristics of methane hydrate formation and dissociation is studied in the sandstone. Experimental results show that the signal intensity of short T2 and long T2 decreases simultaneously in the process of the methane hydrate formation using the excess-gas method, indicating that methane hydrate is formed in both small and large pores. When using the excess-water method, the signal intensity of long T2 decreases, and the signal intensity of short T2 increases in the process of the methane hydrate formation, indicating that methane hydrate is mainly formed in large pores. Methane hydrate is dissociated simultaneously in both small and large pores when using the depressurization method. Water content in small pores gradually increases. Capillary pressure causes some water to remain in the core samples following dissociation. Water content in large pores decreases initially and then increases during depressurization. In the early stages of depressurization, more water leaves large pores than is generated by hydrate dissociation. In the later stages of depressurization, less water leaves the large pores than is generated by hydrate dissociation. This study may inspire the new understanding on distribution of fluid in sediments during the process of accumulation and exploitation of natural gas hydrates.

Class 1 hydrate deposits are characterized by a free gas layer (FGL) underneath the hydrate-bearing layer (HBL) and are considered as the most potential target for commercial exploitation. To improve the contribution of hydrate decomposition to gas production, an efficient development method of low-frequency electric heating assisted depressurization under five-point well pattern is proposed for Class 1 hydrate deposits, which involves the implementation of electric heating after a certain period of depressurization. Based on the geological data of Messoyakha gas field, the numerical simulation model is established. Then the energy recovery behaviors under single depressurization and the proposed method are investigated through numerical simulation approach. Results indicate that the gas production rate of Class 1 hydrate deposits is extremely high at the initial stage of depressurization, but decreases sharply due to insufficient reservoir energy. After the implementation of electric heating, hydrate decomposition and gas production are significantly improved, and a considerable energy efficiency ratio is obtained. Under the same heat input, the linear decreasing voltage mode exhibits better production performance than the constant voltage mode, and can avoid the continuous high temperature near the wellbore to damage the electrode elements, which has greater potential for the exploitation of hydrate deposits. At the end of 10 years of production, a cylindrical hydrate decomposition zone with a radius of about 40 m is formed around each production well. The simulation results confirm the feasibility of the proposed method and provide new insights for enhancing gas recovery in Class 1 hydrate deposits.

Natural gas hydrate is of wide distribution and great potential as clean energy. To improve the production performance, the production characteristics of class III gas hydrate are studied by numerical simulation method when initial gas saturation is below the irreducible gas saturation. Based on the gas production behavior, a quantitative method is developed using both the production data and deposit properties to analyze the production process. A new index is introduced to evaluate the energy utilization efficiency of production stages. Then the influencing factors are analyzed. The results indicate that production can be divided into four stages, including slow changing stage, rapid increasing stage, rapid decreasing stage and stable decreasing stage. The boundaries between stages are clearly defined. Compared with other production stages, the first stage has lower energy utilization efficiency. The ratio drop of energy consumed by this stage can enhance the accumulative gas production. The gas flow ability and drawdown pressure impact the production stage and production performance. Optimization of related factors can improve the production performance. Hot fluid injection and fracturing should be considered when reservoir energy is low or gas flow ability is weak.

The permeability variation caused by hydrate dissociation will affect the further exploitation of hydrate resources during the development of natural gas hydrate. To study the effect of hydrate on seepage characteristics in hydrate-bearing sediments, the micro-flow simulation was carried out by the lattice Boltzmann method. The effect of factors related to the pore-scale distribution of mineral particle and hydrate on permeability variation was analyzed in the two-dimensional porous media. Results show that the mineral particles arrangement has no effect on permeability variation for the two-dimensional homogeneous porous media, the permeability variation is sensitive to the hydrate-formation habit and hydrate-distribution morphology in the pore center. The concept of “control seepage channel” is proposed through comparative analysis of velocity field distribution. It controls the change characteristics of permeability. Based on the control seepage channel size and the control seepage channel tortuosity, a new model is presented to measure the permeability in porous media with gas hydrate. Considering two hydrate-formation habits: grain-coating and pore-filling, the relationships between relative permeability and hydrate saturation are proposed to describe permeability variation. The relationship of pore-filling case agrees well with the experimental results. It shows that hydrate grows mainly in the pore center under the experimental conditions.

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.

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.

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.

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 CH4 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.

The oil recovery is improved by CO2 huff and puff in the ultra-high watercut reservoir, but gas channeling is very easy to occur. In this paper, the mechanism of increasing driving pressure and inhibiting gas channeling by CO2 foam puff-and-huff in ultra-high watercut reservoir was studied by nuclear magnetic resonance (NMR). The experimental equipment is a visualization experiment system using nuclear magnetic resonance for oil and gas displacement (the magnetic field intensity: 0.3±0.05T). The influence of CO2/water ratio, permeability and displacement velocity on displacement pressure was carried out. The water saturation was determined on the basis of T2 spectrum, and the water distribution image during the experiments were obtained using imaging software. The ratio of displacement pressure difference between the CO2 foam flooding and water flooding at the same injection velocity is defined as resistance factor. The correlation between the resistance factor of CO2 foam and water saturation was established. The experimental results show that with the increase of injected CO2 foam volume, the water saturation decreased gradually, but the resistance factor changed little. After the injection volume reaches 0.5PV, the water saturation is reduced to about 0.62, the resistance factor increases rapidly, thus the water saturation defined as rising pressure water saturation. After the injection volume reaches 5.0PV, the water saturation is reduced to about 0.4, the resistance factor becomes stable. It is shown that the lower the permeability is, the greater the rising pressure water saturation, the smaller the resistance factor. When the CO2/water ratio is low, the foam is at low-quality state, the resistance factor is mainly controlled by the gas flow velocity; when the CO2/water ratio is high, the foam is at high-quality state, the resistance factor is mainly controlled by the water flow velocity; overall, the resistance factor firstly increased and then decreased as the CO2/water ratio increased. The corresponding CO2/water ratio is the optimal value when the resistance factor is maximum. The results helps to further analyze the mechanism of CO2 foam plugging and gas channeling inhibition.

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.

Natural gas hydrate is widely distributed in the permafrost and marine deposits, and is regarded as an energy resource with great potential. The low-frequency electric heating assisted depressurization (LF-EHAD) has been proven to be an efficient method for exploiting hydrate sediments, which involves complex multi-physics processes, i.e. current conduction, multiphase flow, chemical reaction and heat transfer. The physical properties vary greatly in different hydrate sediments, which may profoundly affect the hydrate decomposition in the LF-EHAD process.In order to evaluate the influence of hydrate-bearing sediment properties on the gas production behavior and energy utilization efficiency of the LF-EHAD method, a geological model was first established based on the data of hydrate sediments in the Shenhu Area. Then, the influence of permeability, porosity, thermal conductivity, specific heat capacity, hydrate saturation and hydrate-bearing layer (HBL) thickness on gas production behavior is comprehensively analyzed by numerical simulation method. Finally, the energy efficiency ratio under different sediment properties is compared.Results indicate that higher gas production is obtained in the high-permeability hydrate sediments during depressurization. However, after the electric heating is implemented, the gas production first increases and then tends to be insensitive as the permeability decreases. With the increasing of porosity, the gas production during depressurization decreases due to the low effective permeability; while in the electric heating stage, this effect is reversed. High thermal conductivity is beneficial to enhance the heat conduction, thus promoting the hydrate decomposition. During depressurization, the gas production is enhanced with the increase of specific heat capacity. However, more heat is consumed to increase the reservoir temperature during electric heating, thereby reducing the gas production. High hydrate saturation is not conducive to depressurization because of the low effective permeability. After electric heating, the gas production increases significantly. High HBL thickness results in a higher gas production during depressurization, while in the electric heating stage, the gas production first increases and then remains unchanged with the increase of thickness, due to the limited heat supply. The comparison results of energy efficiency suggest that electric heating is more advantageous for hydrate sediments with low permeability, high porosity, high thermal conductivity, low specific heat capacity, high hydrate saturation and high HBL thickness. The findings in this work can provide a useful reference for evaluating the application of the LF-EHAD method in gas hydrate sediments.