Numerical Simulations Of Production Trial And Experimental Research On Replacement Method Of Gas Hydrate In Qilian Mountain Permafrost

Exploration and development of unconventional energy is imminent with the depletion of conventional fossil fuel and the energy requirements growth. Gas hydrate, known as the most commercial and perspective new cleaning energy, is widely developed in permafrost regions as well as along the continental margins in the oceans for its high energy density, large scale and distribution, shallow reservoir and so on. This thesis presents the research on numerical and experimental extraction of gas hydrate taking Qilian Mountain Muri Permafrost（QMMP） as an example.QMMP gas hydrate is commonly as thin layers, flakes and blocks filled in factures within siltstone, mudstone and oil shale and less appeared in sandstone pores. In 2011 depressurization and thermal stimulation methods were tried to develop gas hydrate in QMMP for the first time. Although the production trail was successful, the cumulative gas output and energy efficiency is rather low referring to 95 m3 natural gas produced in 101 h. Thus, further research is necessary in order to explain the low extraction rate and how to make better production in this area.The formation mechanism and reservoir characteristics of QMMP gas hydrate should be analyzed as the first step to choose optimal production method and parameters. The formation mechanism analysis shows that gas generated from source rock by tectonic subsidence and uplift and then migrated and accumulated along with fault and tight formation and finally stored as gas hydrate in permafrost. In more details, the deep pyrolysis gases migrated up through fault and accumulated in fracture zones and rock fractures with combined plugging effect of low porous and permeable sandstone and oil shale as well as high pressure and large torque F1 and F2 fault. Considered which and the complex gas composition and formation temperature and pressure, gas hydrate reservoir locates in the overlaps of gas accumulation zone and gas stability zone inferred around 100 – 400 m. According to this formation mechanism, the QMMP gas hydrate reservoir is strongly affected by fault and fracture, but generally shallow buried, thin and discontinuous. Therefore, extraction position and low reservoir pressure should be taken into account during production in this area.To obtain the saturation of QMMP gas hydrate and analyze the effect of depressurization and thermal stimulation method, Tough Hydrate V 1.0 software was applied to simulate the first gas hydrate production in QMMP. The depressurization simulation results show that the saturation of QMMP hydrate is low and can be regarded as 2.5 % according to the gas hydrate decomposition rate model. The simulation results also conclude that the influence range of heating by thermal stimulation production is significantly smaller than that by depressurization production. Besides, the decomposition rate of gas hydrate was effected marginally by thermal stimulation method in the first field production, which is perhaps due to the short heating period and small heating scale. As the saturation of QMMP gas hydrate is relatively low, it would be pricy and expensive to develop this area using thermal stimulation method because the whole reservoir should be heated to certain temperature to extract small amount of gas hydrate. Thus, CO₂–CH₄ replacement method for gas hydrate in Muri permafrost was presented in this paper.Gas production from hydrate in fracture media by gas replacement involved gas-liquid migration rules and phase equilibrium conditions of CO₂/N₂/CH₄, which were researched in this paper. Dual pore system was introduced into the equation of gas-liquid migration in fracture media containing gas hydrate. According to analysis, the main factors which affected the gasliquid migration were the saturations. In the fracture media model which has one single fracture, the flow characteristics of the whole model was controlled by fracture. So we could mainly consider the fracture system in replacement method, in order to inject CO₂ easily, the saturation of water in fracture should lower than 0.273, when the flow pattern of gas-liquid will be stratified flow. The phase equilibrium conditions of CO₂/CH₄ and CO₂/N₂/CH₄ were analyzed to discuss how replacement pressure, gas hydrate saturation and gas saturation affected the replacement results, so we could select the parameters of replacement method. For QMMP gas hydrate, which has low reservoir pressure and gas hydrate saturation, we could choose the low pressure CO₂ or CO₂/N₂（CO₂ is more） in gas replacement method.Then the experiment research on the gas production from CH₄ hydrate in fracture media with replacement method was run to discuss the effects from fracture width, hydrate form and replacement gas for replacement results. The different thickness of water layers in rock fracture were used to compound CH₄ hydrates which were for doing replacement experiments. When the water layer was thick, the consumption of water was high in the process of hydrate formation, and the replacement ratio was low in the process of replacement. The consumption ratio of water was 55% when the water layer was 3mm, and the final replacement ratio was 8.8%, which could be improved by 80% by using the mixture of CO₂/N₂（3:1）. In the initial stage of production, the production rate of CH₄ and the consumption rate of replacement gas were all high, and the replacement ratio in 24 h was 5⁸%. With the development of time, the replacement rate gradually lowered, which may be caused by new formed mixture hydrate or CO₂ hydrate blocking the mass transfer efficiency between replacement gas and CH₄ hydrate outside of CH₄ hydrate. In addition to the fracture width and replacement gas, hydrate form affected largely the replacement results. A comparative study of experiment 4 and the others experiments showed, when the CH₄ hydrate was well layered distribution, the final replacement ratio will be greatly improved, and its influence was stronger than the influence of fracture width and replacement gas.Based on the above analysis and experiments, the influence factors of replacement results in fracture media were gas hydrate form, the saturation of water, replacement pressure and replacement gas. Gas hydrate form affected the contact area of gas and hydrate, which further affected the replacement rate and the final replacement ratio; the saturation of water not only influenced the replacement contact area, but also impacted the gas-liquid migration in fracture media; the gas component in gas and hydrate when the system reached the equilibrium was related to replacement pressure and replacement gas, which will influence the theoretical replacement ratio, and when choosing replacement pressure and replacement gas, formation pressure, saturation of gas hydrate and gas should be considered.In this paper, the detailed formation mechanism of QMMP gas hydrate was discussed, which was verified by experiments and simulations, and may be used for looking for gas hydrate in other permafrost. The equations of gas-liquid migration in fracture media containing gas hydrate were established for the first time, and how the gas permeability and liquid permeability were controlled by saturations were analyzed in detail by theoretical calculation. With the experiment research on the gas production from CH₄ hydrate in fracture media with replacement method, the main influence factors for replacement results in fracture media were clearly put forward, which was convenient for further research on replacement mechanism.