Stages in the Dynamics of Hydrate Formation and Consequences for Design of Experiments for Hydrate Formation in Sediments

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.

Hydrate Production Philosophy and Thermodynamic Calculations

“…These two aspects have both been modified by Kvamme [13,14], and Kvamme et.al. [15,28]. The result is new models for the mass transport terms in the Classical Nucleation Theory.…”

“…Equation (13) implies that there is no net heat transport between the two phases, Equation (14) is simply Newton's second law while (15) suggests that the average net mass transport between the two phases is zero.…”

“…The number of independent variables in Equations (13)-(15) is 8 and the number of conservation equations and conditions of equilibrium, Equations (13)- (15), is 6. Mathematically this system can be solved if two of the independent variables are defined, as also indicated in Table 1 below.…”

“…The modified version of CNT and the use of residual thermodynamics for all phases, including hydrate phase [17] provides a consistent concept that also includes enthalpies of hydrate formation and dissociation from the same concept [14,15,30].…”

“…Detailed models for water and methane chemical potentials in the two phases can be found in Kvamme [13,14] and Kvamme et.al. [15] and will not be repeated here. Numerical solutions for the distribution of the two components in gas and water for various combinations of temperature and pressure are given in Figure 2.…”