Methane gas hydrate may become a significant source of methane gas in the global energy mix for the next decades. The widespread distribution of methane gas hydrate, primarily in subsea sediments on continental margins, makes the crystalline compound attractive for countries with shorelines that seek self‐sustainable energy. Fundamental understanding of pore‐level methane gas hydrate distribution and dissociation pattern in reservoirs is important to anticipate the methane production rate and overall efficiency. Specifically, the local salinity gradients occurring during methane gas hydrate dissociation, and its impact on local dissociation characteristics, must be understood as the aqueous phase in most reservoirs is saline. We experimentally evaluate the salinity effect on methane gas hydrate dissociation using high‐pressure silicon‐wafer micromodels with realistic sandstone grain characteristics. Methane gas hydrate was formed for a range of brine salinities (0–5 wt% NaCl), and we report variations in dissociation patterns during depressurization and thermal stimulation as a function of brine salinity. A strong correlation between initial methane gas hydrate distribution and dissociation characteristic, and subsequent release and mobilization of methane gas, was observed. Local water salinities affected the methane gas hydrate structure leading to distinct dissociation patterns of self‐preservation due to water freshening.

This paper reports the formation and dissociation pattern of hydrate crystals with varying compositions of CH4 and CO2 in porous media. Direct visualization was carried out using a high-pressure, water-wet, silicon wafer-based micromodel with a pore network resembling sandstone rock. Hydrate crystals were formed under reservoir conditions (P = 45–65 bar and T = 1.7–3.5 °C) from either a two-phase system consisting of liquid water and a CH4–CO2 gas mixture or a three-phase system consisting of liquid water, CH4-rich gas, and CO2-rich liquid. A stepwise pressure reduction method was later applied to visualize multiple dissociation events occurring between the equilibrium pressures of pure CH4 hydrates and pure CO2 hydrates. The results showed that liberated gas from the initial dissociation became trapped and immobilized by surrounding undissociated hydrate crystals when the initial hydrate saturation was high. Mixing of liberated gas with liquid water led to rapid reformation of hydrates during the stepwise pressure reduction; the reformed hydrate crystals dissociated at a lower pressure close to the equilibrium pressure of pure CO2 hydrates. The results demonstrate the possibility of producing gas liberated from local hydrate dissociation while simultaneously reforming hydrates in other parts of the sediments. This is relevant for the proposed production method where CO2 injection in CH4 hydrate reservoirs is followed by pressure depletion to enhance the CH4 gas recovery.

We present an experimental protocol for fast determination of hydrate stability in porous media for a range of pressure and temperature (P, T) conditions. Using a lab-on-a-chip approach, we gain direct optical access to dynamic pore-scale hydrate formation and dissociation events to study the hydrate phase equilibria in sediments. Optical pore-scale observations of phase behavior reproduce the theoretical hydrate stability line with methane gas and distilled water, and demonstrate the accuracy of the new method. The procedure is applicable for any kind of hydrate transitions in sediments, and may be used to map gas hydrate stability zones in nature.

In this study, we demonstrate the effectiveness of combined production technique involving depressurization and thermal stimulation for gas production from CH4 gas hydrates in subzero temperature range between -3℃ to 0℃. CH4 gas hydrate phase transitions during formation, depressurization, re-formation, self -preservation, thermal stimulation stage were visualized using a high-pressure, water-wet, silicon-wafer micromodel with pore network of actual sandstone rock.A set of eight experiments were performed in which CH4 gas hydrate was formed at a constant pressure between 60 -85 bar and constant temperature between 0 °C -4°C. CH4 gas hydrate was then dissociated at constant system temperature between -3 °C to -2 °C by pressure depletion to study the effect of hydrate and fluid saturation on dissociation rate, self-preservation, and risk of ice formation. The dissociation rate and behaviour were heavily affected by the total hydrate saturation and initial hydrate distribution in the pore space. Additionally, the amount of produced CH4 gas was limited below 0 °C due to the rapid formation of ice from the liquid water that was liberated from the initial hydrate dissociation. The liberated CH4 gas was therefore immobilized and trapped by the formed ice and could not be produced without thermal stimulation. Thermal stimulation removed the blockage of pore space caused by ice and secondary hydrate formation and enhanced gas production. Visual observation showed self-preserved hydrates in metastable state dissociated before ice below subzero temperature providing experimental evidence of recently discovered methane leaking from gas hydrate deposits due to global warming. The results highlight the influence of heterogeneity in hydrate distribution and total saturation on the hydrate dissociation behaviour below 0 ℃ temperature. Micromodel observation provides direct insights into hydrate dissociation, self-preservation, fluid migration, gas coalescence, ice and secondary hydrate formation at pore scale below subzero temperature.

Summary This study reviews how production of methane from hydrates can be triggered by dissociation of the hydrate structure. Techniques leading to dissociation of hydrates are summarized by pressure depletion, thermal stimulation, and injection of inhibitors. Depressurization is considered to be the most-cost-effective method and is easily implemented in gas reservoirs with overlying hydrate layers. Examples and status of pressure-depletion tests on field scale will be reviewed. In hydrate reservoirs not adjacent to gas zones, the success of pressure depletion is dependent on sufficient permeability to allow for pressure perturbations to reach within the hydrate reservoir and to allow for flow of dissociated gas. This effect has been investigated in this paper by performing controlled pressure depletions on hydrate-filled sandstone cores. Dissociation pressures at given temperatures have been quantified as well as recovery of methane as a function of pressure decrements lower than dissociation pressure. Hydrate dissociation was found to take place over a range of pressure values because of salinity changes in the water phase. A 2D porous silicon-wafer micromodel has been used to gain insight into the mechanisms of hydrate dissociation. Direct visualization of hydrate melting induced by both depressurization and heating is reported from pores replicating authentic sandstone pores. Thermal stimulation led to a more-uniform hydrate melting compared with pressure depletion, and depressurization was most effective when the hydrate was in direct contact with gas bubbles.