Negative Effects of Inorganic Salt Invasion on the Dissociation Kinetics of Silica-Confined Gas Hydrate via Thermal Stimulation

Abstract

Methane hydrate dissociation kinetics can be inhibited in NaCl solutions; however, this effect is reversed by promoting bubble formation that enhances dissociation. The negative and positive effects of inorganic salt injection on gas production from hydrate-bearing sediments are still controversial. Here, molecular dynamics simulations were performed to investigate the characteristics of NaCl solution invasion into hydrate-occupied nanopores and the effects on the confined hydrate dissociation kinetics. Two initial configurations comprising liquid and silica pore phases were studied with a low or high NaCl concentration, respectively. The results show that, under the simulation conditions, salt invasion decelerated hydrate dissociation within the silica pore as NaCl invasion into the pore is stepwise. Initially, few ions can diffuse into the pore phase, and gas nanobubbles form on the solid surface mainly via confinement and surface effects, independent of NaCl solution invasion. Subsequently, gradual salt diffusion immersed the residual hydrate in the salt solution and hindered hydrate decomposition until the dissociation finished. More ions could diffuse into the pore phase at the high NaCl concentrations with a low diffusion efficiency, leading to surface nanobubble growth toward the residual hydrate and somewhat accelerated hydrate dissociation. This severely hinders the escape of released methane from the pore. This study yields molecular-level insight into the origin of the negative effect of salt invasion on hydrate dissociation, which should be avoided during gas production from hydrate reservoirs with low permeabilities via salt injection combined with thermal stimulation.

Dynamic Dissociation Behaviors of sII Hydrates in Liquid Water by Heating: A Molecular Dynamics Simulation Approach

“…Previous studies have demonstrated that nanobubbles are the main form of released gas molecules and also play an essential role in the further dissociation of residual hydrate, 63 , 64 especially in confined pore spaces. 33 − 35 …”

“…These molecules then aggregate into a small gas cluster owing to the low solubility of small hydrocarbon molecules in water, and these clusters eventually evolve into nanobubbles (Figures and ). Previous studies have demonstrated that nanobubbles are the main form of released gas molecules and also play an essential role in the further dissociation of residual hydrate, , especially in confined pore spaces. − …”

“…Recently, increasing attention has been paid to the hydrate dissociation process within a confined phase using a multi-phase initial system in MD simulations. 33 − 35 The results have shown that this dynamic behavior is quite different from that in the pure liquid phase, especially for the mass transfer of methane molecules.…”

“…The surface of the hydrate clusters shows a series of rapid decay and transition states, and the hydrates dissociate in a layer-by-layer manner. − On this basis, several studies have investigated the effects of temperature, , pressure, salts, static electrical fields, and so forth on the dissociation process in the liquid phase using MD simulations. Recently, increasing attention has been paid to the hydrate dissociation process within a confined phase using a multi-phase initial system in MD simulations. − The results have shown that this dynamic behavior is quite different from that in the pure liquid phase, especially for the mass transfer of methane molecules.…”

Production Characteristics of the Methane Hydrate Sediment under Ionic Liquid Injection

“…Natural gas hydrate (NGH), a solid form of natural gas, is a naturally occurring substance consisted of guest molecules like as small molecular alkane substances, and host molecules shaping into hydrogen bond cages under specific conditions of lower temperature and higher pressure, , generally existing in the permafrost strata and the sea continental margin typically found at water depths exceeding 1000 m . The estimated range for natural gas hydrate resources is from 10 15 to 10 18 m 3 , which is roughly double than the total quantity of discovered fossil fuels. , At present, the commonly used technologies for NGH production mainly include depressurization, − inhibitor injection, − thermal stimulation, , and CO 2 replacement. − Among, the Geological Survey of Canada and Japan National Oil Company have used depressurization combined with thermal stimulation to carry out natural gas hydrate production test in the permafrost belt of Mallik continent and initially proved that depressurization combined with heating is a high-yield hydrate development method. In addition, the electromagnetic heating, solar heating, and steam heating were used for the trial production in the Qilian Mountain after the depressurization, which also proved that the thermal stimulation method was helpful to the natural gas hydrate exploitation to a certain extent.…”