Semi‐quantitative Analysis of Factors Affecting Gas Hydrate Formation Conditions and Its Fractions

Abstract

Gas hydrate formation conditions and its fraction out of pore space are two important issues. Based on the theoretical thermo‐dynamic model of gas hydrate formation and accumulation, the semi‐quantitative analysis on various intrinsic factors (such as temperature, pressure, gas composition, pore‐water salinity, sediment pore size) is conducted to characterize their effects on gas hydrate formation conditions and its fraction out of pore space. The analytical results indicate that gas composition plays a significant role in affecting gas hydrate formation conditions of temperature and pressure; comparatively the effect of the mixture of methane and propane is the largest and that of the mixture of methane and hydrogen sulphide is secondary, and effects of the mixture of methane and ethane and the mixture of methane and carbon dioxide follow in turn with respect to pure methane. Pore‐water salinity also greatly affects gas hydrate formation conditions. Sediment pore size within a certain range (1 × 10–6 ~ 4 × 10–8m) has a small effect on gas hydrate formation conditions; beyond this range when sediment pore size becomes smaller and smaller, its effect gets more and more evident on inhibiting gas hydrate formation and especially when sediment pore size is smaller than 1×10–9m, it seems impossible for pure methane and pure water to form gas hydrate; contrarily when sediment pore size is bigger than 1 × 10–3m, it nearly has no effect on gas hydrate formation conditions. Gas hydrate fraction out of pore space is little affected by pore size and pore‐water salinity, but is strongly dependent on gas supply.

Theoretical Prediction of the Occurrence of Gas Hydrate Stability Zones: A Case Study of the Mohe Basin, Northeast China

“…Moreover, it was verified via the simulation of hydrate formation and decomposition that under the same temperature and pressure conditions, heavier gas molecules are more prone to turn into hydrates with a higher thermal stability than pure methane. 3 , 4 , 50 This agrees with the simulation and calculation results in this study. It was found that pure methane HSZs cannot form under a geothermal gradient of 2.3 °C/100 m and a permafrost thickness of 86 m. However, a HSZ with a thickness of 735 m can be found at a depth of 115–850 m when wet gas is present with a dryness coefficient of 0.88.…”

Modeling on Gas Hydrate Formation Conditions in the Qinghai‐Tibet Plateau Permafrost

“…Actually differences in gas composition inevitably exist between the Qaidam basin, the Tarim basin and the QHPP; thus the gas composition of their referred gas fields can not be fully representative of the study area. More over, the hydrocarbon gas composition is not only an important factor affecting gas hydrate formation conditions besides temperature and pressure, but it is also a decisive element for gas hydrate content [25] .…”

Characteristics of Gas Hydrate Stability Zone in The Muli Permafrost, Qinghai: Results Compared Between Modeling and Drilling

“…have various effect on gas hydrate stability zone. Quantitative-semi quantitative modeling results show that gas composition, especially the entry of propane has a maximal effect on gas hydrate stability zone [19] .…”

“…If the thermal gradient does not change, the depth of permafrost base is closely related to the top surface temperature of permafrost and they have a negative correlation. Theoretical calculation [19] shows that gas hydrate formation conditions are more sensitive to temperature than pressure (the depth of permafrost base); for 1 • C increment in temperature, the increment in pressure required for gas hydrate formation will be about 700 kPa (approximately 7 atmospheric pressures or 70 m of permafrost thickness in hydrostatic pressure). Namely when thermal gradient within the permafrost keeps constant the interval of gas hydrate occurrence is determined by permafrost thickness.…”