Thermal Decomposition of Methane Hydrates combined with CO2 sequestration
Formation of methane hydrates followed by recovery of methane gas from the hydrate phase was investigated using thermal stimulation and combined thermal stimulation with CO2 injection in a laboratory reactor. A large-scale laboratory reactor of internal volume 59.3 liters was used to duplicate the permafrost conditions in which hydrates form naturally. The range of initial hydrate saturation values studied was 10 % to 50 % by pore volume and the range of heating rates studied was 0 W to 100 W. In the case of thermal stimulation combined with CO2 exchange, the range of CO2 injection rates studied was 0 ml/min to 580 ml/min. The test matrix was designed to investigate the effect of initial hydrate saturation and heating rate on the gas recovery and the thermal efficiency of the process. The CO2 injection was combined with thermal stimulation to study the enhancement in the gas recovery efficiency and to study the amount of CO2 sequestered. The concept of CO2 injection is similar to the fluid injection techniques used in enhanced oil recovery for expelling the desired product from the pores of the matrix through the production wells.
Currently, the field tests make use of a hot fluid injection (such as steam) for raising the temperature of the hydrate bearing sediment, however these methods suffer from the loss of heat during transit from ground to the hydrate sediment. This work made use of a concept of down-hole combustion in which, part of the methane gas released from dissociation of methane hydrates itself will be used as a fuel for in situ combustion. An electrical point heat source was used for simulating combustion in this work. The major purpose of the work with CO2 injection is to match the flow rates of CO2 gas released during the in situ combustion process. For example, generation of heat at the rate of 100 W by burning methane will produce the CO2 at the rate of 155 ml/min. A testing matrix was developed and executed to study the effect of methane recovery and thermal efficiency over a range of initial hydrate saturations (30 % & 50 %), heating rates during dissociation (20, 50 & 100 W) and CO2 injection rates (110 ml/min & 580 ml/min). Results from those tests demonstrate that, at a constant initial hydrate saturation; total recovery of methane increases with an increase in the heating rate and, the thermal efficiency decreases with an increase in heating rate. At a constant heating rate, total recovery of methane as well as thermal efficiency increased with an increase in initial hydrate saturation. For tests that combined CO2 injection and thermal stimulation during dissociation, at a constant value of initial saturation and constant heating rate; an increase in CO2 injection rate increased the total recovery of the methane. CO2 injection tests also gave higher recovery values in less time than their corresponding counterparts in the pure thermal stimulation tests. As a representative example, at 50 % saturation and 100 W heating, injection of 580 ml/min CO2 gave nearly 69 moles of methane in 18 hours as compared to 62 moles in 45 hours for a zero CO2 injection test.
The motivation for this work comes from the need to utilize unconventional natural gas resources. Natural gas extraction from the hydrates allows sequestration of carbon dioxide helping reduce the carbon footprints and contribute to reducing overall greenhouse gas emissions. Currently, the conventional gas extraction and recovery methods are cheaper than the gas hydrate recovery methods and it is expected that this will change as, more efficient methods are being developed in gas hydrate field. It is required to focus the research on a lab scale experiments to provide economically feasible guidelines for large-scale production tests.