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The exploitation of natural gas hydrate, an unconventional natural gas, has drawn attention on a global scale due to increasing imbalance between energy supply and demand. Horizontal wells are considered to have a better production performance, while vertical wells have been mostly employed in previous field tests and laboratory research. Therefore, based on the representative properties of three kinds of hydrate reservoirs, a hydrate decomposition model is built to study the gas generation behaviors of various hydrate reservoirs with different horizontal well-spacing strategies in this work. The results show that for Class I hydrate reservoirs, placing the production well near the free gas layer can improve gas generation by 47%, and GWR (gas-water ratio) to 346.0 and maintain higher temperature in hydrate reservoir than that with placing the well in the hydrate reservoir. For Class Ⅱ hydrate reservoirs, placing the producing well away from the free water layer can cut down water production by 92%, while gas production could be also elevated, causing GWR increasing to 175.8. In Class Ⅲ reservoirs, the reduction of 30% in water production, and increment in GWR from 81.5 to 118.0 can be achieved by placing the well on the top of the hydrate layer, rather than that on the bottom. In addition, the presence of free gas layer, free water layer can promote the pressure-drop interface spread, provide sensible heat, and then accelerate hydrate decomposition further. [Display omitted] • Gas behavior in different reservoirs and horizontal well locations were studied. • Placing bottom-well patterns can increase gas production in Class I reservoir. • Placing middle-well patterns can get higher gas-water ratio in Class II reservoir. • Placing upper-well patterns can get higher gas yield rate in Class III reservoir. • Free air and water layers can promote pressure drop diffusion and provide heat. [ABSTRACT FROM AUTHOR]

• Numerical simulation of gas production by stepwise depressurization is presented. • The depressurization rate is optimized by genetic algorithm. • The configuration strategies of stepwise depressurization are optimized. • The exploitation efficiency has been improved. Depressurization is one of the most effective methods for hydrate exploitation. Optimization of the pressure-drop strategy for improving the exploitation efficiency requires immediate attention. In this study, a hydrate decomposition model was for the first time combined with a genetic algorithm. The pressure-drop rate and strategy were optimized based on the exploitation efficiency. The depressurization rate of the continuous slow depressurization method was optimized. Subsequently, the best pressure-drop rate for stepwise depressurization was hypothesized and validated. In addition, the stepwise strategy was optimized. For the two depressurization methods, increasing the pressure-drop rate increased the exploitation efficiency, while increasing the number of pressure-drop stages in the stepwise depressurization method adversely impacted the exploitation efficiency. Prolonged constant-pressure intervals reduced the efficiency when there were few pressure-drop stages; however, the opposite trend emerged for a high number of depressurization steps. This study provides guidance for optimizing the production efficiency of hydrate exploitation via depressurization. [ABSTRACT FROM AUTHOR]

Permeability in hydrate-bearing sediment critically governs fluid flow and determines hydrate nucleation, growth, and distribution, making it important to characterize the evolution of permeability with respect to water and gas during hydrate formation. This study uses CT scanning of krypton hydrate formation in silica sand using the excess gas method, together with a pore network model, to investigate variations in hydrate morphology and associated permeation. The results show that during hydrate formation, the growth habit mainly varies from grain-coating to patchy with an increase in hydrate saturation; however, at relatively low saturation, hydrate preferentially grows as a grain-cementing habit. Theoretical models of a capillary tube and of a Kozeny grain both predict that permeability in grain-coating hydrates will be higher than in patchy hydrates. We thus recognise a correlation of permeability and hydrate saturation for multiple growth habits that is of interest for gas production from hydrate reservoirs. Under lower water saturation, there is a decrease in relative permeability to water but an increase in relative permeability to gas, due to the reduction in pore shape factors. Conversely, an upward shift in relative permeability to water and a downward shift in relative permeability to gas is found with hydrate formation under higher water saturation, owing to the Jamin effect. Steeper curves of relative permeability to gas and water with increasing hydrate saturation resulted in a reduction of the co-cementation zone for water and gas, even if or though no gas migrates. The results suggest that, due to the low relative permeability of gas, higher water saturation sediments result in an excess water yield accompanied by low gas production that is not desirable for natural gas hydrate production. Therefore, improving the permeability and weakening the Jamin effect are critical for gas production in marine hydrate reservoirs, especially in low permeability sediments. • CT scanning of Kr hydrate growth is used as input to modeling of multi-phase flow properties in hydrate bearing sediments • Hydrate pore habit shifts from coating to cementing as hydrate saturation increases. • Under lower S w , hydrate formation inhibits water flow due to reduced shape factor. • Under higher S w , hydrate growth impedes gas migration in fine pores due to the Jamin effect. [ABSTRACT FROM AUTHOR]

Natural gas hydrate is recognized as an ideal substitute for traditional energy resources. Gas production from hydrate reservoirs is accompanied by the phase transition of solid hydrate, which inevitably results in changes to the microstructure of hydrate sediments. Therefore, the interaction mechanism between the microstructure and permeation characteristics of hydrate sediments must be investigated, especially permeability anisotropy. In this study, we assess the variations in permeability anisotropy during krypton hydrate formation using a pore network model and X-ray computed tomography. The results showed that absolute permeability in the vertical (k v) and horizontal (k h) directions both decreased throughout the formation process, but that the decline in k v was steeper, thus resulting in a sharp increase in the degree of permeability anisotropy (k h /k v). The relative permeability to gas was always higher in the horizontal direction than of that in the vertical direction, whereas the relative permeability to water was the opposite. Moreover, the difference in the relative permeability to water and gas between the two directions increased with the increasing hydrate saturation (S h). When extrapolating to the field-scale for hydrate field trials, we consider that horizontal wells provide a better option for gas production, especially for hydrate reservoirs with a higher hydrate saturation. • Prediction of permeability anisotropy during hydrate formation is presented. • Hydrate formation increases the degree of permeability anisotropy (k h /k v). • Difference in gas/water flow between two directions increases with high S h. • Horizontal wells are found to be a better option for gas hydrate production. [ABSTRACT FROM AUTHOR]

Multiphase seepage characteristics are vital factors for improving the recovery efficiency of natural gas hydrate (NGH) via stepwise depressurization. However, the difficulties to experimentally construct real and natural hydrate-bearing sediments in a laboratory hinder the study of the association between multiphase seepage and the development of NGH. In this study, an optimized simulation model based on the features of fine shaly silt sediment in the Shenhu area of the South China Sea is proposed to analyze the influence of seepage characteristics on hydrate exploitation induced by stepwise depressurization. Multiple perspectives, including the intrinsic permeability, gas and water relative permeability, porosity, and permeability anisotropy of the reservoir are regarded as sensitive parameters of the hydrate production. The exploitation economy of fine hydrate-bearing sediment was evaluated by introducing the Ste number, gas-water ratio (GWR), and sensitivity analysis. The results demonstrate that the contribution of sensible heat to hydrate exploitation in reservoirs with poor seepage characteristics is limited; the effects of the abovementioned parameters on the GWR are different; porosity and gas relative permeability are the most important factors that affect hydrate production. Thus, detailed geological examination should be performed to ensure the exploitation economy. This study provides a profound theoretical basis for the stepwise depressurization strategy design of NGH exploitation in the field. [Display omitted] • Numerical simulation of gas production by stepwise depressurization is presented. •Reservoir seepage on hydrate exploitation is analyzed from multiple perspectives. •The exploitation potential and economy of different reservoirs are analyzed. •Porosity, intrinsic and gas relative permeability are key factors of gas recovery. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Natural gas resources trapped in hydrate reserves worldwide are a new substitute for conventional energy. To safely and efficiently extract natural gas from hydrates, fields test and laboratory investigations were systematically conducted. Salinity is a significant factor impacting the exploitation of the marine hydrates. In this study, a two-dimensional cylindrical model was developed to determine the influences of salinity on gas recovery induced by depressurization and thermal stimulation. The results indicated that, with the same exploitation method, higher salinity accelerated the pressure-drop propagation rate and temperature increment spread, thereby promoting the hydrate dissociation rate and shortening the gas production process. Furthermore, thermal stimulation improved the average gas generation rate by almost five times that with depressurization at the same salinity. • Salinity accelerates the pressure and temperature propagation rates. • Salinity promotes the gas production rate and shortens the process. • Thermal stimulation improves the average gas generation rate more. [ABSTRACT FROM AUTHOR]

• Equilibrium data of CH 4 hydrate in chloride salt solutions are detected by the isochoric method. • The Chen-Guo model coupled with Hu-Lee-Sum correlation are used for hydrate prediction. • The reliability of the experimental data is validated by three thermodynamic consistency criteria. • The proposed model is successfully verified by the experimental data. The phase equilibria of hydrate are necessary to study the gas control/production from marine sediments. Considering the abundance of chloride salts in the ocean, the phase equilibrium conditions of CH 4 hydrate in 3.5 wt% NaCl, KCl, CaCl 2 and MgCl 2 solutions were experimentally investigated at the temperature and pressure ranges of 281.9 K~287.1 K and 7.33 MPa~14.02 MPa, respectively. Then the reliability of the obtained data was validated by three thermodynamic consistency criteria. In addition, a new calculation model was proposed based on the Chen-Guo model and the Hu-Lee-Sum (HLS) correlation for hydrate prediction. And the prediction accuracy of the proposed model was described by the absolute average deviation of temperature (AADT). Results show that the inhibition effect of MgCl 2 is stronger than that of other salts. Also, the experimental data obtained in this study can meet all the requirements of the criteria. Furthermore, the prediction results from the proposed model well agree with the experimental data. And the proposed model shows a good performance in predicting the phase equilibria of CH 4 hydrate in the systems containing single chloride salt and mixed chloride salts with AADT of 0.54 K and 0.82 K, respectively. [ABSTRACT FROM AUTHOR]