Abstract
The formation of marine gas hydrates is controlled by gas migration and accumulation from lower sediments and by the conditions of the hydrate stability zone. Permeability and porosity are important factors to evaluate the gas migration capacity and reservoir sealing capacity, and to determine the distribution of hydrates in the stable region. Based on currently available geological data from field measurements in the Shenhu area of Baiyun Sag in the northern South China Sea, numerical simulations were conducted to estimate the influence of heterogeneities in porosity and permeability on the processes of hydrate formation and accumulation. The simulation results show that: (1) The heterogeneity of the hydrate stability zone will affect the methane migration within it and influence the formation and accumulation of hydrates. This is one of the reasons for the formation of heterogeneous hydrates. (2) When the reservoir is layered heterogeneously, stratified differences in gas lateral migration and hydrate formation will occur in the sediment, and the horizontal distribution range of the hydrate in a high porosity and permeability reservoir is wider. (3) To determine the dominant enrichment area of hydrate in a reservoir, we should consider both vertical and lateral conditions of the sedimentary layer, and the spatial coupling configuration relationships among the hydrate stability region, reservoir space and gas migration and drainage conditions should be considered comprehensively. The results are helpful to further understand the rules of hydrate accumulation in the Shenhu area on the northern slope of the South China Sea, and provide some references for future hydrate exploration and the estimation of reserves.
Highlights
Natural gas hydrate is an ice-like crystalline mineral, in which hydrocarbon and non-hydrocarbon gases are held within rigid cages of water molecules [1]
Scholars have carried out a lot of research on the formation conditions of hydrates in sea areas, some of which focuses on the analysis of the controlling factors of hydrate formation at the micro-scale [7,8,9,10], and other research on the macroscopic geological conditions, such as the influence of tectonic activities, sedimentary environment, reservoir temperature and pressure conditions on the saturation, burial depth and distribution of hydrate reservoirs [11,12]
Some studies found that the formation and accumulation of gas hydrates in the Shenhu area may be controlled by the sediment grain [17,18]
Summary
Natural gas hydrate is an ice-like crystalline mineral, in which hydrocarbon and non-hydrocarbon gases are held within rigid cages of water molecules [1]. Scholars have carried out a lot of research on the formation conditions of hydrates in sea areas, some of which focuses on the analysis of the controlling factors of hydrate formation at the micro-scale [7,8,9,10], and other research on the macroscopic geological conditions, such as the influence of tectonic activities, sedimentary environment, reservoir temperature and pressure conditions on the saturation, burial depth and distribution of hydrate reservoirs [11,12]. With the aim to evaluate the effects of heterogeneities in permeability and porosity on gas hydrate formation and accumulation, and to fully address gas hydrate distribution in heterogeneous sediments, three-dimensional numerical simulations are conducted based on data from the Shenhu sea area in the northern South China Sea. Compared with the previous two-dimensional model, the three-dimensional model can better reflect the actual stratigraphic situation and better analyze the influence of lateral heterogeneity of strata at the same depth on hydrate formation. After comparison of different modeling results, conclusions are drawn
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