Methane and carbon dioxide exchange production studies from exposed natural gas hydrate
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Two laboratory experimental setups have been designed in collaboration with the Reservoir Physics Group at the Department of Physics and Technology. The equipments have been completed and tested. The first experiment was conducted using the four electrode resistivity measurement method on porous media. The equipment has been used to study the influence of resistance with core samples saturated with salinity concentration at 1 kHz, 1200 psig pressure and temperatures down to 3oC during stages of hydrate formation. The experiment was conducted to measure the resistance in Bentheim sandstones in order to correlate resistivity in porous media as function of different filling (gas, aqueous mixture, hydrate). The results showed that resistivity decreases before hydrate formation and then increased to a higher value after hydrate formation. The resistivity increment was observed when the system was cooled down for hydrate formation. The temperature was also observed to decrease with lower conductivity of the brine water in the pores. Finally, the resistivity of the system was higher than corresponding value before hydrate formation when free gas was present. The reason for this was interpreted to be that solid hydrates are filling the pores of the matrix formed by sand grains and change their cementation condition. This is expected to have a great impact on the resistivity of the sample. The production of CH4 from its hydrates in porous media during CO2 sequestration was investigated in a second experiment. The results showed that when CH4 hydrate was exposed to CO2 an exchange of gas molecules occurred spontaneously. The reason for this is that, CO2-hydrate is the thermodynamically favored hydrate in terms of free energy under the conditions of temperature and pressure used in these experiments. In view of the huge amounts of hydrate worldwide there is a corresponding great potential of these CH4 hydrates to store CO2 on long terms while at the releasing natural gas. This win win situation for environment and energy supply makes this process attractive. To complement the experimental work, Phase Field Theory models have been used to study the dynamics of CH4 exchange in CH4 hydrate with liquid CO2 at temperatures 273.15 – 284.17K and pressures in the range from 100 - 113.24 bars. The results from the quantitative analysis showed CH4 in the hydrate gradually moved to the liquid CO2 phase while CO2 in the liquid phase penetrated into the hydrate. MatLab was used as a post processor for the sampled data for detailed analysis of the decomposition process of the CH4 hydrate during the exchange. This was needed in order to distinguish the mass transfer behavior of the CH4 and CO2 liquid but was also a useful tool in organizing the data for more detailed illustrations the microscopic aspects of the exchange process in the hydrate. The observed decomposition process of CH4 was found to proceed faster than the reformation of CO2 due to CH4 presence in both the small and large cages, whereas the guest molecule exchange of CH4 with CO2 could occur only in the large cages. Based on the simulation data presented, the results indicated that while the driving force is difference in chemical potential for the two components between the liquid phase and the hydrate phase for the exchange process is essentially dominated by mass transport limitation.
UtgiverThe University of Bergen
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