| Natural gas hydrate is deemed to be a new energy with large commercial exploitation value in the 21st century because of its advantages of abundance resource, high combustion value and low pollution. At present, depressurization, thermal stimulation, and adding inhibitors are the traditional methods of natural gas hydrate exploration. However, all of the three methods are easily to cause geological instability in hydrate zone, underwater landslides and other natural disasters. Exploiting methane hydrate by replacement with CO2, as a kind of new technology, can not only exploit the natural gas storage on hydrate, but also can bury the greenhouse gas CO2 on the sea bottom in the form of hydrate at the same time, and the method also can reduce the risk of geological disasters. Therefore, researching the method of exploiting methane hydrate by replacement with CO2 has important scientific and practical significance.In this paper, we regarded the process of CO2 replacement of methane hydrate ina capillary high-pressure optical cell by Raman in situ observation as the research object, analyzing its replacement mechanism and studying the effect of temperature, pressure and other factors on replacement efficiency. First of all, we established the quantitative relationship between Raman spectra and the concentration of dissolved methane in the pure water solution and the concentration of dissolved CO2 in the pure water solution, and carried on hydrate growth-solution equilibrium experiment of the mixed gas of CO2 and methane gas. We analyzed the changing trend of methane and CO2 in the hydrate, liquid phase at different conditions of temperature and pressure, completing the experiment of methane hydrate by replacement with CO2 in the micro capillary tube and the analysis of the mechanism and influence factors of replacement in the end. We also carried on the experiment of methane hydrate by replacement with CO2 in sediments in the macro reactor, analyzed the effect of temperature, pressure conditions on the replacement efficiency, and observed the vertical distribution characteristics of CH4 and CO2 in the sediments by Raman, which could provide good technical support for the reasonable exploration and development of gas hydrate reasonable.Through the above research, we not only had the innovation in technology, but also obtained the following conclusion:1. The solubility of a certain gas in water is a regular value at a certain temperature, pressure and salinity conditions, and a good linear relationship was found between the solubility and the peak area ratio(PAR) measured by Raman spectrum at the same temperature, pressure of salinity.This paper established a quantitative relationship between Raman spectra and the concentration of dissolved CO2 gas and dissolved methane in the pure water solution. In this experiment we found that equation of the quantitative relationship between methane concentration and spectrum characteristics under 41.85～321.1bar isX(CH4)=42.469 [A(CH4)aq/A(H2O)] (R2= 0.9552) and equation of the quantitative relationship between CO2 concentration and spectrum characteristics under 15～447.83bar is CO2=179.2 [A(CO2)aq/A(H2O)] (R2= 0.9486).By comparison, the experimental pure water data is consistent with the experimental data of previous results, and the maximum deviation is less than+8%, proving the experimental datas with high precision.2. The pore size of normal seafloor sediment was from a few microns to hundreds of microns, so micron regular capillary could provide a better simulation for capillary pore on the fine sediment. Eequilibrium conditions of hydrate growth and dissolution conditions were measured at different temperature (274.15K-293.15K) and pressure (100-40bar) in the experiment.The test results show that the temperature is the main factor on the hydrate growth and dissolution in pore fluid, methane dissolved in water increases with the rise of temperature, the influence of pressure is relatively smaller for the dissolution equilibrium of hydrate, and the increase of pressure made dissolved methane change to hydrate in a certain extent.3. In the methane and CO2 mixed gas hydrate growth-dissolution equilibrium experiment, CO2 concentration in the solution is obtained under different temperature conditions. In the experiment, we measured methane and carbon dioxide concentrations in the hydrate phase and liquid phase at 275.15K,278.15K,283.15K,288.15K and 293.15K, and found that as the decrease of temperature, methane and CO2 content in hydrates increased,while CH4 and CO2 concentration in the solution decreased.This is becausedissolved methane and carbon dioxide transformednew hydrate in the cooling processing,resulting in the decrease of these two concentration.The lower the temperature, the smaller the residual mCH4A (dissolved methane) and mCO2A (dissolved CO2). In the hydrate crystals, when the temperature is decreasing,there is much CO2 contained in the new hydrate, while methane proportion in gas hydrates gradually reduced. With the decrease of the temperature, the partitionation coefficient K of methane (CH4) and carbon dioxide (CO2) showed a rising trend. Temperature has a great effect on the partitionation coefficient. The reason why the KCH4value is much higher than KCO2 is that mCO2 A in aqueous solution is much larger than that of mCH4 Aat the same temperature and pressure condition.4. We also analyse the hydrate crystal structure of this mixed gas hydrate sample.With the decrease of temperature, the content of CO2 in gas hydrates increases gradually, and at the same time, the content of CH4 in the cage in 51262 reduced, showingthat at low er temperature, CO2 substituting CH4 occupied most of the big cages in hydrate cavities.With the decreasing of temperature, the content of methane in the small cages 512 reduced while hydration number increased gradually. We guess that in the actual process of methane hydrate by replacement with CO2,in order to maintain the stability of crystal structure. CO2 may not be completely drive all the methane out of the crystal, but as the temperature decreased, more and more CH4is replaced by CO2.Under the condition of low temperature,methane hydrate by replacement with CO2 has a higher efficiency.5. In the CO2 replacement experiment,the replacement can be divided into the following process:a. CO2 molecular diffused to the CH4 hydrate surface; b. with the CH4 hydrate decomposition, the cage structure of CH4 hydrate wasdestructed, CH4 molecules escaped;c. when the CO2-CH4 mixed gas hydrate formed, CO2 molecules mainly occupied the big cages in the crystal structure, and CH4 molecules went into the big and small cages; d. CO2 molecular diffused to the deep of hydrate, CH4 molecules diffused from the inside of the hydrate tothe gas phase, and the replacement reaction continued.6. In this paper, we carried on the experiment of methane hydrate by replacement with CO2 in the in the self-built test platform, and analysied the influence factors.The study found that:the system temperature is rising in the beginning stages affected by circumstances; it willcontain constant after reaching the setting temperature. When the replacement experiment begins, the pressure risesinfluced by the increased gas phase temperature and methane hydratedecomposion. When the system is stable, pressure in the reactor showed a slight downward trend.7. In early stage of replacement reaction, the speed of the increase of the content of CH4 in gas phase and the decrease of CO2 was fast, and then the speed gradually decreased. Methane hydrate CO2 displacement efficiency increased with the rising of replacement temperature, the higher the temperature was, the higher the degree of impact on displacement efficiency was. The replacement rate increased by 6.17% when the temperature was 5℃, while the replacement rate increased by 3.65% when the temperature was 3.5℃. Displacement efficiency decreased with the increase of displacement pressure, when the CO2 injection pressure is less than the equilibrium pressure at the replacement temperature, the influence of pressure changing on the replacement rate was more obvious; when the injection pressure of CO2 is close to or higher than that of methane hydrate phase equilibrium pressure, the influence degree of pressure on displacement efficiency was relatively weakened. Replacement rate increased by 10.58% at the displacement pressure was 2.0 MPa, while the replacement rate increased by only 4.42% at the pressure of 3.5MPa. The higher the initial hydrate saturation was, the more amount of CH4 was replaced, and the lower the replacement efficiency, the less CO2 was consumed.8. The processing of methane hydrate replacedby CO2 could be divided into the early fast reaction stage and the late sustained reaction stage. Replacement reaction at the early stage hydrate compose is affected by the driving force of decomposition/synthesis and hydrate memory. Replacement reaction rate at late stage depended on the diffusion rate of CO2 to the deep hydrate.9. Using the Raman spectrum to determinate the vertical distribution for CO2 and CH4 of hydrate in the sediment after replacement, we got the consistent result as the data obtained from gas chromatograph.As the depth of sediment layer increasing, the mol content ofC02 gradually decreases, and the mol content ofCH4 increases gradually.CO2 gas diffusion in liquid was faster than that in the sediments, and replacement rate is larger; the higher the temperature was, the greater the replacement rate was, and the faster the CO2 diffusion in sediment depth was. |
PDF Full Text Request