--> Fundamental Study of CO2 Substitution in Methane Hydrates for Energy Production and Long Term Sequestration

AAPG Asia Pacific Region Geosciences Technology Workshop:
Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?

Datapages, Inc.Print this page

Fundamental Study of CO2 Substitution in Methane Hydrates for Energy Production and Long Term Sequestration

Abstract

Due to increasing energy demands, the potential for use of gas hydrates as a future energy source is very important. Natural gas hydrates can be environmentally friendly because their high hydrogen/carbon ratio means that they emit less CO2 during combustion than other heavier fossil fuels. Gas hydrates have a complex non-equilibrium nature with hydrate formers supplied from different phases, and each of these has a different composition and density. Hence, any change in chemical potential of the guest molecules due to change in concentration will lead to a new hydrate formation. Kinetic of hydrate formation has two physically well-defined stages. The first of these, nucleation, is the process in which the free energy benefit of the phase transition competes with the penalty of pushing aside initial phases to free up space for the hydrate crystals. This stage contains natural random elements related to molecular transport, When the size and shape of a hydrate particle reaches a size (critical size) in which the benefit of the phase transition exceed the push-work penalty reaches into the second stage of stable growth. Both of these processes and constrained by associated mass and heat transport. Formation of hydrate on a hydrate former/water interface rapidly leads to situations in which transport of mass through the hydrate film becomes rate limiting. Without hydrodynamic shear forces that break these films and open up for med mass supply of both hydrate former and water the onset of massive growth can be very slow. The time for onset of massive growth is typically called induction time. A fast mechanism for replacing in situ CH4 with CO2 involves the formation of a new CO2 hydrate from free water in the pores and injected CO2, including the mixing of surfactants and a small amount of Nitrogen (roughly 25 mole%) into CH4 hydrate. The process will lead to a conversion from CH4 hydrate to CO2 hydrate while releasing CH4 gas from the methane hydrate. Because adding surfactant to the injected CO2 will reduce hydrate formation at the interface it should be possible to control the combined CH4 production and CO2 storage. Different types of surfactants are possible. Low molecular surfactants constructed on the basis of physical solvents for CO2 are attractive because they enhance the interface thickness and dynamics without making very stable emulsions that could partly trap hydrate particles and clog pore space. But even alcohols will up-concentrate close to the water/non-polar interface. This is also the reason that methanol in small concentrations will actually act as a hydrate activator. It keeps the water/methane interface free of hydrate while at the same time reducing interface free energy and opening up for higher supersaturation below the methanol enriched water interface where hydrate formation will not close the interface transport between methane and water. The small amount of N2 should fill the small cavities of new CO2 hydrate formation thereby stabilizing the structure. The main goal of this study is to better understand the role of surfactants in kinetic of hydrate formation and dissociation.