Abstract
Natural gas hydrates in sediments can never reach thermodynamic equilibrium. Every section of any hydrate-filled reservoir is unique and resides in a stationary balance that depends on many factors. Fluxes of hydrocarbons from below support formation of new hydrate, and inflow of water through fracture systems leads to hydrate dissociation. Mineral/fluid/hydrate interaction and geochemistry are some of the many other factors that determine local hydrate saturation in the pores. Even when using real sediments from coring it is impossible to reproduce in the laboratory a natural gas hydrate reservoir which has developed over geological time-scales. In this work we discuss the various stages of hydrate formation, with a focus on dynamic rate limiting processes which can lead to trapped pockets of gas and trapped liquid water inside hydrate. Heterogeneous hydrate nucleation on the interface between liquid water and the phase containing the hydrate former rapidly leads to mass transport limiting films of hydrate. These hydrate films can delay the onset of massive, and visible, hydrate growth by several hours. Heat transport in systems of liquid water and hydrate is orders of magnitude faster than mass transport. We demonstrate that a simple mass transport model is able to predict induction times for selective available experimental data for CO2 hydrate formation and CH4 hydrate formation. Another route to hydrate nucleation is towards mineral surfaces. CH4 cannot adsorb directly but can get trapped in water structures as a secondary adsorption. H2S has a significant dipole moment and can adsorb directly on mineral surfaces. The quadropole-moment in CO2 also plays a significant role in adsorption on minerals. Hydrate that nucleates toward minerals cannot stick to the mineral surfaces so the role of these nucleation sites is to produce hydrate cores for further growth elsewhere in the system. Various ways to overcome these obstacles and create realistic hydrate saturation in laboratory sediment are also discussed.
Highlights
Natural gas hydrates are crystalline compounds which are mainly stabilized by hydrogen bonds that forms cavities which enclathrate small hydrocarbons
At equilibrium the free energy changes of the hydrate formation from liquid water and hydrate formers coming from gas, liquid or fluid state is zero
Within the focus of this work we examine a couple of values for Dliq in (16) and plot the time needed to reach 1 mm hydrate film thickness
Summary
Natural gas hydrates are crystalline compounds which are mainly stabilized by hydrogen bonds that forms cavities which enclathrate small hydrocarbons. One drawback of the pressure reduction method is that the heat necessary for dissociation of the hydrate still needs to be supplied. In addition to the stability limit considerations, hydrate dissociation in sediment is limited by the transport processes across a thin interface (1.2 nm) between the hydrate and surrounding phases. These phase transition dynamics are implicitly coupled to the dynamics of the flow and all phase transitions in every pore. Injected CO2 will form new hydrate with free pore water Released heat from this hydrate formation, and other factors, will dissociate the in situ hydrate. Reducing trapped gas pockets and trapped liquid water to a minimum will be an important step towards creating hydrates in sediments that in important ways can be compared to natural gas hydrates in nature
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