Abstract
In the current numerical simulation studies, bottom water in Class II hydrate-bearing layers is represented by grids with high water saturation that significantly extends the calculation time if the volume of the bottom water is large or grid size is small. Moreover, the influence of the bottom water volume on the depressurization performance of Class II hydrate-bearing layers has not been fully investigated. In this study, the Fetkovich analytic aquifer model was coupled with a simulation model of a hydrate reservoir to accelerate the simulation of Class II hydrate-bearing layers. Then the simulation results and calculation time were compared between the coupled model and the model in which the bottom water layer is only represented by grids. Finally, the influence of the bottom water volume on the productivity of gas and water in the depressurization method was investigated and the variation of pressure, temperature, and hydrate saturation during the production process was analyzed. The results show that the coupled model can significantly reduce the simulation time of Class II hydrate-bearing layer while ensuring calculation accuracy. When the pore volume of the aquifer increases to 20 times that of the bottom water layer, the computation time of a single model in which the bottom water layer is represented by grids is 18.7 times that of the coupled model. Bottom water invasion slows down the depressurization, and therefore, the larger the aquifer, the lower the peak value of gas production, and the later it appears. However, the invading bottom water can provide heat for hydrate dissociation; therefore, the gas production rate of the hydrate-bearing layer with bottom water is higher than that of the hydrate-bearing layer without bottom water in the late development stage. Generally, the presence of bottom water reduces the cumulative gas production and increases the cumulative water production; therefore, the larger the aquifer, the more unfavorable the depressurization development of the hydrate-bearing layer.
Highlights
Natural gas hydrates (NGHs) are ice-like substances formed by gas and water that remain stable under low-temperature and high-pressure conditions (Chong et al, 2016; Aydin et al, 2016; Yang et al, 2019)
The coupled model built in this study shows good performance and the calculated water invasion rates are consistent with those obtained by the commercial software
At present, it is quite difficult to construct a physical model in which the upper layer is a hydrate layer and the lower layer is a water layer
Summary
Natural gas hydrates (NGHs) are ice-like substances formed by gas and water that remain stable under low-temperature and high-pressure conditions (Chong et al, 2016; Aydin et al, 2016; Yang et al, 2019). When the bottom water layer exists, the heat energy contained in the invasion water can promote hydrate dissociation, and the gas production for case 4 with the largest aquifer is the highest in the later stages of depressurization. For cases 2 and 3 (Figures 10B,C), the temperature of the area far away from the vertical well dropped to near the phase equilibrium temperature, but there was an evident cone-shaped high-temperature area at the bottom of the well This is mainly because bottom water coning on the vertical well can supplement the heat energy consumed by hydrate dissociation near the bottom of the well. For case 4, the temperature of the HBL was significantly higher than the phase equilibrium temperature This is mainly because it is difficult for the HBL to achieve rapid depressurization when the aquifer is large (Figure 10D); the hydrate dissociation rate is lowered, and the thermal energy of the HBL is not fully utilized. The invasion of bottom water is not conducive to the depressurization development of Class II HBLs
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