Salinity‐buffered methane hydrate formation and dissociation in gas‐rich systems

Abstract

Methane hydrate formation and dissociation are buffered by salinity in a closed system. During hydrate formation, salt excluded from hydrate increases salinity, drives the system to three-phase (gas, water, and hydrate phases) equilibrium, and limits further hydrate formation and dissociation. We developed a zero-dimensional local thermodynamic equilibrium-based model to explain this concept. We demonstrated this concept by forming and melting methane hydrate from a partially brine-saturated sand sample in a controlled laboratory experiment by holding pressure constant (6.94 MPa) and changing temperature stepwise. The modeled methane gas consumptions and hydrate saturations agreed well with the experimental measurements after hydrate nucleation. Hydrate dissociation occurred synchronously with temperature increase. The exception to this behavior is that substantial subcooling (6.4°C in this study) was observed for hydrate nucleation. X-ray computed tomography scanning images showed that core-scale hydrate distribution was heterogeneous. This implied core-scale water and salt transport induced by hydrate formation. Bulk resistivity increased sharply with initial hydrate formation and then decreased as the hydrate ripened. This study reproduced the salinity-buffered hydrate behavior interpreted for natural gas-rich hydrate systems by allowing methane gas to freely enter/leave the sample in response to volume changes associated with hydrate formation and dissociation. It provides insights into observations made at the core scale and log scale of salinity elevation to three-phase equilibrium in natural hydrate systems.

Mechanisms of Methane Hydrate Formation in Geological Systems

“…If the methane and water are well mixed and within the methane HSZ, they immediately form hydrate to the thermodynamic‐limited extent. This assumption is supported by the observation that (1) with enough methane supply in laboratory experiments, thermodynamic equilibrium was achieved within few hours to few days (You, Kneafsey, et al, ; Zatsepina & Buffett, ), which is fast compared with geological time scales, and (2) methane hydrate formation in geological systems is usually limited by the availability of methane (except in gas‐rich environments) or by the transport of methane to a hydrate solidification front (Davie & Buffett, ). We discuss kinetic models later. Methane is the only hydrate‐forming gas. There is no water in the free gas phase because the solubility of water in free gas is small (Duan et al, ; Liu & Flemings, ). Salt is present only in the liquid phase because salinity is assumed never to be high enough to precipitate salt. Fluid flow follows Darcy's law.…”

“…This could happen at local hydrate solidification front where methane is abundant, such as the surroundings of gas bubbles and gas conduits of vent systems. Migration of water to local hydrate‐bearing sediments has been interpreted in both field (Soloviev & Ginsburg, ) and laboratory sediments (e.g., Kneafsey et al, ; Rees, Kneafsey et al, ; You, Kneafsey, et al, ). However, few quantitative studies of water transport‐limited model are available.…”

Experimental Investigation of Gas Flow and Hydrate Formation Within the Hydrate Stability Zone

“…We performed each experiment in a vertical pressure vessel consisting of steel endcaps and an X‐ray transparent, aluminum cylinder surrounded by a cooling jacket (Figure ), similar to other hydrate formation cells (Kneafsey et al, ; Seol & Kneafsey, ; You, Kneafsey, et al, ). We packed the sediment samples in a Viton® sleeve (17.8 cm length; 5.1 cm internal diameter; 0.25 cm wall thickness) that was sealed on each end with steel endcaps.…”

Quantifying hydrate solidification front advancing using method of characteristics

“…Figure ). We assume the following: The sediment has a uniform porosity. The initial pressure, temperature, and salinity is homogeneous. There is local thermodynamic equilibrium: the kinetics of hydrate formation are assumed to be negligible because hydrate formation has been shown to be rapid by laboratory [ You et al ., ; Zatsepina and Buffett , ] and field studies [ Rehder et al ., ]. Methane gas fractional flow ( f g ) is equal to 1 (100% gas) at the gas inlet. Methane is the only component in the gas phase because the solubility of water in gas phase is small [ Duan et al ., ; Liu and Flemings , ]. There is no chemical diffusion. The system is isothermal (There is latent heat when hydrate is formed, but there is also a large thermal mass in the surrounding sediment, and thus the temperature changes will be minimal. ). Capillary effects are not present. Both liquid and gas phases remain connected. Only horizontal flow is present.…”

“…3. There is local thermodynamic equilibrium: the kinetics of hydrate formation are assumed to be negligible because hydrate formation has been shown to be rapid by laboratory [You et al, 2015;Zatsepina and Buffett, 2003] and field studies [Rehder et al, 2002]. 4.…”