Abstract
Many methods to produce hydrate reservoirs have been proposed in the last three decades. Thermal stimulation and injection of thermodynamic hydrate inhibitors are just two examples of methods which have seen reduced attention due to their high cost. However, different methods for producing hydrates are not evaluated thermodynamically prior to planning expensive experiments or pilot tests. This can be due to lack of a thermodynamic toolbox for the purpose. Another challenge is the lack of focus on the limitations of the hydrate phase transition itself. The interface between hydrate and liquid water is a kinetic bottle neck. Reducing pressure does not address this problem. An injection of CO2 will lead to the formation of a new CO2 hydrate. This hydrate formation is an efficient heat source for dissociating hydrate since heating breaks the hydrogen bonds, directly addressing the problem of nano scale kinetic limitation. Adding limited amounts of N2 increases the permeability of the injection gas. The addition of surfactant increases gas/water interface dynamics and promotes heterogeneous hydrate formation. In this work we demonstrate a residual thermodynamic scheme that allows thermodynamic analysis of different routes for hydrate formation and dissociation. We demonstrate that 20 moles per N2 added to the CO2 is thermodynamically feasible for generating a new hydrate into the pores. When N2 is added, the available hydrate formation enthalpy is reduced as compared to pure CO2, but is still considered sufficient. Up to 3 mole percent ethanol in the free pore water is also thermodynamically feasible. The addition of alcohol will not greatly disturb the ability to form new hydrate from the injection gas. Homogeneous hydrate formation from dissolved CH4 and/or CO2 is limited in amount and not important. However, the hydrate stability limits related to concentration of hydrate former in surrounding water are important. Mineral surfaces can act as hydrate promotors through direct adsorption, or adsorption in water that is structured by mineral surface charges. These aspects will be quantified in a follow-up paper, along with kinetic modelling based on thermodynamic modelling in this work.
Highlights
Natural gas hydrates are classes of composite structures in which water organizes to create cavities that enclathrate small non-polar molecules such as CH4, C2H6, C3H8, and i-C4H10
Mineral surfaces can act as hydrate promotors through direct adsorption, or adsorption in water which is structured by the mineral surface charges. These aspects will be quantified in a follow-up paper, along with kinetic modelling based on thermodynamic modelling in this work
In this work I have examined the effect of ethanol on thermodynamic properties, and the ability for CO2/N2 mixtures to form a new hydrate with free pore water when injected in CH4 hydrate filled sediments
Summary
Natural gas hydrates are classes of composite structures in which water organizes to create cavities that enclathrate small non-polar molecules such as CH4, C2H6, C3H8, and i-C4H10. Methane released through biogenic degradation of organic material in the upper crust is the most abundant source of known hydrates worldwide. These are almost pure methane hydrates and structure I hydrates. The smallest symmetrical unit of structure I hydrate is a cubic box containing 46 water molecules, 6 large cavities (24 water molecules), and 2 small cavities (20 water molecules) that can host molecules such as CH4. Molecules that enter these cavities of hydrogen-bounded water molecules are called guest molecules. For typical temperatures above the water freezing point, a constant unit box length of 12.01 Å is generally accurate enough
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