Methane Hydrate Formation in Thick Sandstones by Free Gas Flow

Abstract

We show with a two‐dimensional multiphase flow and multicomponent transport model that free gas flow is a viable mechanism to form concentrated methane hydrate in meter‐scale, dipping sandstones far above the base of the hydrate stability zone (BHSZ). In this model, gas preferentially flows updip along the top of sandstone due to buoyancy. This drives hydrate formation, increasing the local salinity to the stability limit and developing three‐phase (gas, liquid, and hydrate) equilibrium above the BHSZ. With time, the gas and the hydrate solidification front (HSF) advance together updip. Behind the HSF, hydrate continues to form as the elevated salinity diffuses away. High hydrate saturations reduce the sediment permeability significantly. As a result, as the gas and HSF move updip, they are also pushed perpendicularly from the top to the base of the sandstone. The hydrate system ultimately self‐seals itself due to reduced permeability across the entire thickness of the sandstone. Gas starts to retreat downdip and accumulates below the BHSZ. With this model high hydrate saturations form far above the BHSZ at high methane supply rates while hydrate is concentrated at the BHSZ or no hydrate forms at low methane supply rates. This study provides further insights into hydrate formation by free gas flow, which can be used to design the best strategies for economic and environmental production of methane from hydrate reservoirs.

Mechanisms of Methane Hydrate Formation in Geological Systems

“…The rate of methane hydrate formation is usually high enough to induce pore water salinity increase and local temperature perturbation, because free gas flow is a very efficient transport mechanism. A deposit with considerable amount of methane hydrate can be formed within thousands of years from free gas flow at a high velocity that may originate from leakage of a deeper gas reservoir (Haeckel et al, ; Torres et al, ; You & Flemings, ). Permeable pathways with low capillary entry pressure and high permeability, such as faults, highly fractured zones, or coarse‐grained layers, are required for free gas flow.…”

“…Therefore, hydrate deposits formed by free gas flow are usually localized along those permeable pathways, filling the pores of sand‐rich intervals or filling the fractures within massive muddy sediments. Local hydrate saturation formed from free gas flow can be more than 90% (e.g., You & Flemings, ).…”

“…This mechanism of free gas advance differs from the capillary pressure‐dependent solubility model presented in the previous section. In the capillary model, the thickness of free gas within the HSZ is limited by pore size distribution and for permeable reservoirs with large pores the distance that the gas front travels is typically small (e.g., less than several meters even with more than 90% hydrate saturation; You & Flemings, ). In contrast, in this salinity exclusion model, the gas advance into the HSZ is quite large.…”

“…Two‐dimensional models have also been developed that illustrate how the hydrate saturation is strongly influenced by both buoyancy‐driven free gas flow (Figure ; You & Flemings, ) and capillary pressure (Figure ). For example, when free gas is injected into a homogenous 12‐m‐thick sand, it first flows by buoyancy along the top of the sand.…”

Introduction to Special Issue on Gas Hydrate in Porous Media: Linking Laboratory and Field‐Scale Phenomena

“…Liu and Flemings () provide the critical study for hydrate formation on gas bubbles within the porous medium of the GHSZ in water‐limited settings or those dominated by pore‐filling brines that can inhibit gas hydrate formation and stability. A new modeling study in the Special Section (You & Flemings, ) expands on this earlier work to demonstrate that hydrate formation from a free gas phase can concentrate hydrate in coarse‐grained layers within the GHSZ. Thus, this single Special Section has numerical modeling studies showing that hydrate formation from both dissolved phase methane (VanderBeek & Rempel, ) and from gaseous methane (You & Flemings, ) can lead to high saturations of hydrate in coarse‐grained layers, underscoring the long‐recognized role of permeability in controlling hydrate distributions (e.g., Nimblett & Ruppel, ).…”