| Summary: | 博士 === 國立成功大學 === 資源工程學系 === 106 === The future energy policy of Taiwan is going to heavily rely on the clean energy, including renewable and low-carbon energies, to meet the target of mitigating CO2 emission. In addition to developing the renewable energy like solar and wind resources, Taiwan will increase the natural gas consumption to obtain enough electrical power with low-carbon emission. The vast resources of gas hydrates recognized in southwestern offshore Taiwan makes a great opportunity for Taiwan to increase energy independence resources in the future.
Gas hydrates are crystalline energy resource, which are formed when methane and water mixtures are subjected to high pressure and low temperature conditions. Gas hydrates can be found in subsurface geological environments of deep-sea sediments and permafrost regions, where the presence of in-situ hydrates had been confirmed by many exploration activities around the world. Class-1 hydrate deposit is recognized as the largest potential to be developed in the future because there is a free gas zone under the hydrate storage area. Of all the production methods in gas hydrate production, depressurization is the most promising method to economically produce gas from hydrate deposits. In depressurization, the dissociation efficiency will be affected by the pressure drawdown disturbance. However, when the pore pressure of hydrate deposits is depressurized for gas production, the rock matrix will surfer more stresses and the formation deformation might be occurred.
Numerical simulation is a common method used in the petroleum industry to evaluate the petroleum resource before development. In gas hydrate numerical simulation, STARS simulator, which was developed by CMG Ltd., had been proven to be capable to simulate the hydrate dissociation behavior and gas production from hydrate. In the literature studies, hydrate was set as a heavily viscos oil-phase to simulate its immobility in STARS. However, gas hydrate is solid-phase in practice. The purpose of this study was to analyze the difference between oil-phase and solid-phase designed hydrate models in Class-1 hydrate deposit. The geomechanical effects on both models were also well discussed in this study.
A vertical well perforated at the top of the gas zone in a Class-1 hydrate deposit was designed based on the literature study. The production behavior of oil-phase and solid-phase designed hydrate were well discussed and the solid-phase designed hydrate was calibrated to obtain an accurate result. After that, the geomechanical module was introduced and the calculated subsidence between different phase design in gas hydrate were compared. The results show that, the oil-phase designed hydrate model has a better description on fluid flow behavior than the solid-phase designed hydrate model. As the solid-phase designed hydrate was applied in the simulation, the relative permeability should be calibrated. In geomechanical effects, the solid-phase design prefers for considering the geomechanical characteristics of gas hydrate. If the geomechanical mechanism is considered in the simulation model, solid-phase designed hydrate is suggested.
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