Search

Current researches related to natural gas hydrate mainly focus on its physical and chemical properties, as well as the approaches to the production (decomposition) of hydrate. Physical modeling of the flow process in hydrate deposits is critical to the study on the exploitation or decomposition of hydrate. However, investigation of the dynamic hydrate process by virtue of porous media like sand-packed tubes which are widely used in petroleum production research is rarely reported in literature. In this paper, physical simulation of methane hydrate generation process was conducted using river sand-packed tubes in the core displacement apparatus. During the simulation, the influences of parameters such as reservoir temperature, methane pressure and reservoir model properties on the process of hydrate generation were investigated. The following results are revealed. First, the use of ice-melted water as the immobile water in the reservoir model can significantly enhance the rate of methane hydrate generation. Second, the process driving force in porous media (i.e., extents to which the experimental pressure or temperature deviating those corresponding to the hydrate phase equilibrium) plays a key role in the generation of methane hydrate. Third, the induction period of methane hydrate generation almost does not change with temperature or pressure when the methane pressure is above 1.4 folds of the hydrate phase equilibrium pressure or the laboratory temperature is lower than the phase equilibrium temperature by 3 °C or more. Fourth, the parameters such as permeability, water saturation and wettability don't have much influence on the generation of methane hydrate.

Natural gas hydrate (NGH), as a clean energy, has a great development potential in the future for its immense reserves. The CO2 replacement method has dual effects in energy exploitation and greenhouse gas (GHG) sequestration, but its CH4 replacement rate is too low. When an inhibitor is added, CH4 hydrate and CO2 hydrate are different in terms of their thermodynamic stability. In this regard, a new method was proposed and experimentally proved for NGH production. This new method combines CO2 replacement method with thermodynamic hydrate inhibitor technology to accelerate the decomposition of CH4 hydrate. Three common thermodynamic CH4 hydrate inhibitors (i.e. methanol, sodium chloride and magnesium chloride) in two categories were compared by means of core displacement experiments. It is shown that, when methanol solution is used as the inhibitor, the decomposition rate of CH4 hydrate reaches 0.01194 mol/h, which is much higher than that of electrolyte solution (0.00086 mol/h and 0.00141 mol/h respectively). Therefore, methanol solution is used to accelerate hydrate decomposition. The decomposition ratio of CH4 hydrate was over 92% when methanol solution plug was injected at the early stage and CO2 was continuously injected at the late stage. Moreover, CO2 was sequestrated in the form of hydrate. Eventually, the generated CO2 hydrate accounted for 16–27% of the initial total CH4 hydrate.