Experimental Research On Combined Replacement And Depressurization Method For Natural Gas Hydrates Exploitation

As modern society develops rapidly, energy requirements grow dramatically. Energy sustainable supply becomes a key part of strategic issues for every country. Natural gas hydrates considered as subsequent energy for 21st century, have gained extensive attention. According to incomplete statistics, the amount of carbon bound in NGH is twice that to be found in known fossil fuels on Earth. Researches on efficient NGH exploitation have been conducted worldwide. Meanwhile, carbon emission reduction is one of the issues on environment protection and CO2 is closely related to global climate change. Researchers have attempted to sequestrate CO2 in seabed in the form of hydrates.Based on the review of kinetical and thernodynamical feasibility of CO2 replacement reaction, replacement constrains, experiment and simulation study on replacement percent and rate and replacement mechanism, we designed a cryogenic high pressure experiment platform. We simulated the porious media in sediments, formed NGHs and studied repalcement properties considering affecting factors such as NGH saturations, phae change of CO2, temperature, pressure and phase equilibrium diagram. On this basis, we studied repalcement mechanism combing Avrami and shrinking core model. It is found replacement procedure could be diveded into two stages:surface reaction and inner layer reaction, and the constraint of reaction to be the barriers for CO2 to diffuse in inner layer reaction. We also analysed the detailed repalcement procedure under differient operating points. Based on the study of replacement properties and replacement mechanism, in order to enhance replacement percent and rate, we bring out a new method combing depressurization with CO2 replacement. This method aimed at partially melting hydrates to provide path for CO2 diffusion. We designed a detailed operating steps, compared the replacement percent of runs with replacement method and new combined method and concluded that new method could inprove replacement percent and rate effectively. The new method provided a new thought for enhancing replacement method. In addition, in this study we have conducted a number of replacement experiments enriching the database of exploiting NGH by replacement method.Replacement reaction in Zone A(liquid CO2) and C(NGH unstable zone) is more effective than Zone B(gaseous CO2 and NGH stable zone). Temperature conditions under ice point will restrict the replacement process. Two key factors are methane hydrates area for replacement reaction and diffusibility of CO2 in three zones. In Zone A, replacement percent increases as pressure and MH saturation increases. In Zone C, replacement percent decreases as pressure and MH saturation increases. In Zone B, replacement percent increases as temperature and MHs saturation increase, while such tendency is not obvious when temperature is below ice point. Pressure signals change of replacement procedure in three Zones was discussed to investigate the possible CO2-CH4 exchange kinetics. Results showed that the CH4-CO2 replacement reaction can be divided into two stages. The first stage can be described as the surface reaction stage; the reaction in this stage is rapid and the results fit the Avrami model. In the second stage, the rate of the reaction is slow due to hydrates blocking the diffusion pathways and the results fit the shrinking core model. Experimental results with and without depressurization were compared to reveal the effects of depressurization on the final replacement percent. The combination method had a higher replacement percent than the traditional replacement reaction. It is supposed that methane hydrates melt once, which provides path for carbon dioxide penetrate into inner layer of hydrates.