Numerical Simulation On The Influence Of Migration And Accumulation Conditions On Marine Gas Hydrate Accumulation Process

With the depletion of traditional oil and gas resources,natural gas hydrate,as a clean and efficient future energy,has been paid more and more attention all over the world.Natural gas hydrate is formed under low temperature and high pressure,mainly occurs in terrestrial permafrost and deep sea sediments in nature.Gas hydrates are widely distributed under the sea,but the existing drilling results show that the distribution of gas hydrate in the plane and vertical direction is obviously uneven.The hydrate accumulation factors represented by"transport condition of fluid"and"accumulation condition of deposition"control the formation and accumulation process of submarine gas hydrate.It is of great theoretical and practical significance to clarify the formation mechanism and accumulation law of submarine hydrate under different migration and accumulation conditions for finding the occurrence location of high reserves and exploitable hydrate reservoirs.In this paper,the typical gas hydrate prospect area Baiyun Sag in the northern slope of the South China Sea is taken as the research area.Focusing on the key issue of reservoir-forming process of seabed gas hydrate under different migration and accumulation conditions,the effects of reservoir porosity and permeability conditions,formation dip conditions,seafloor undulation and structural channel types on submarine sediments are studied by using the technical methods of theoretical analysis and numerical simulation.This paper analyzes the controlling factors of hydrate differential distribution in the study area,puts forward the migration and accumulation conditions for high saturation hydrate enrichment,and forecasts the dominant hydrate occurrence area based on the actual data.The research results can provide theoretical reference for the exploration and exploitation of natural gas hydrate in the South China Sea.The main conclusions of this study are as follows:(1)The heterogeneity of reservoir porosity and permeability will affect the temperature and pressure stability of local hydrate,reservoir space and gas migration in the stable zone,and then control the formation and distribution of hydrate.Combined with the characteristics of local high abundance of foraminifera and uneven distribution of fine-grained turbidites in Baiyun Sag,the effects of different porosity and permeability conditions(homogeneous,layered heterogeneous and spatial heterogeneous)on hydrate accumulation are simulated and analyzed.The results show that the heterogeneity of reservoir porosity and permeability can promote the lateral migration of methane,and some gas will appear"bypass"phenomenon.In the region with high permeability,the fluid migration rate and methane gas saturation is relatively high,which promotes the formation of high saturation hydrate.Heterogeneous porosity increases the contact area between methane and pore water,and promotes the formation of hydrate.Compared with the homogeneous model,the lateral migration distance of methane in the spatially heterogeneous model is increased by about 28%,and the cumulative generation of hydrate is increased by about 16.5%.Especially,when the hydrate reservoir is interbedded with sand and mud,there will be obvious stratification phenomenon in the hydrate formation.The hydrate saturation in the high permeability layer is higher,and the lateral distribution range will be increased to a certain extent.(2)The dip angle of the formation and the fluctuation of the seafloor topography will affect the lateral migration of methane in the marine sediments and the distribution of the hydrate stability zone,and then control the lateral distribution and thickness of the hydrate reservoir.Combined with the characteristics of multi-dip strata and seafloor fluctuation of the slope migration Canyon sedimentary system in Baiyun Sag,the migration of gas bearing fluid and the formation and enrichment process of hydrate in the strata with different dip angles(0°,5°,10°and 15°)are simulated and analyzed.The results show that the saturation,thickness and lateral distribution of hydrate in the upper part of inclined formation are larger than those in the lower part due to better gas supply conditions and stable temperature and pressure conditions.The larger formation dip angle will lead to the greater difference of hydrate saturation and distribution on both sides of the dip,which will enhance the heterogeneity of hydrate reservoir.When the dip angle is constant,the heterogeneity is more obvious with the increase of permeability and methane leakage rate.In this model,when the dip Angle is 15°,compared with the horizontal formation,the methane migration in the upper inclined strata increases by about 23.5%,the methane migration in the lower inclined strata decreases by about 18.1%,and the cumulative hydrate generation decreases by about7.0%.The inhibitory effect of the dip Angle on the formation of hydrate in the inclined lower part is stronger than the promoting effect on the formation of hydrate in the inclined upper part.At the same time,the formation and distribution of hydrate in different undulating terrains are preliminarily studied.When the undercut structure occurs on the seabed,the bottom boundary of the hydrate stability zone will move downward,and some hydrates are distributed along the lateral wall of the undercut structure.The greater the depth of the undercut on the seabed,the greater the inhibition on the lateral migration of gas and the formation of hydrate will be.When the uplift structure appears on the seafloor surface,the bottom boundary of the hydrate stability zone will be uplifted,and the gas in the shallow layer will mainly migrate along the side wall of the uplift,forming the hydrate reservoir nearly parallel to the seafloor surface of the uplift.In particular,when the two sides of the uplift structure have different sedimentation and erosion,the side with stronger sedimentation is more suitable for gas lateral migration and hydrate formation.(3)Typical structural channels(gas chimney,mud diapir and fault)are good channels for gas bearing fluid migration.The fluid migration characteristics in different structural channels are different,and the spatial distribution characteristics of hydrate reservoirs are also different.Combined with the characteristics of submarine faults,gas chimneys,mud diapirs and other structural channels in Baiyun Sag,the influence of different channels on the formation,accumulation and spatial distribution of hydrate is simulated and analyzed by characterizing different structural channels.The results show that in the upper part of the gas chimney,it is easy to form horizontally distributed layered high saturation hydrate.In the mud diapir structure,the internal overpressure potential makes the high-temperature fluid and methane gas migrate upward together,forming a dome-like hydrate reservoir and the plane shape of the hydrate in some layers is annular.The regional large-scale fault has a large cutting thickness,which may connect the bottom source rock and shallow gas the hydrate stable zone and promote the upward transportation of deep gas.Shallow small faults have different influences on gas migration and hydrate accumulation due to their different distribution positions and cut through horizons.The low angle faults on the flank will promote gas lateral migration.When shallow high angle faults connect the vertical gas channel and hydrate stability zone,they will promote the formation of high saturation hydrate.When they connect the vertical gas channel and the sea floor,they will increase the risk of gas leakage to the seafloor.(4)Based on the actual site data of a block in Baiyun Sag,a three-dimensional geological model is constructed.The location of hydrate occurrence area and its hydrate saturation are simulated and predicted by comprehensively considering the spatial-temporal coupling configuration relationship of different reservoir forming factors,and the contribution of different genetic gases to hydrate reservoir is quantitatively analyzed.Based on the analysis of the reasons for the different distribution of hydrate in Baiyun Sag,the formation and enrichment process of hydrate in the study area are simulated,and the spatial distribution and saturation of hydrate in the study area were described,so as to predict the dominant occurrence area of hydrate.The simulation results show that the hydrate production in the study area is estimated to be 2.90×10¹²kg.The gas source type is the mixture of biogenic gas(38.6%)and thermogenic gas(61.4%),and thermogenic gas is the main source.When high-quality reservoir conditions(relatively coarse-grained sediment area)and good fluid migration conditions(gas chimney,fault and other structural channels)can be well coupled in space,it is easy to form hydrate reservoirs with high saturation.Finally,it is determined that the submarine fan at the end of the migration canyon,the uplift area at the ridge of the submarine canyon and the upper part of some large-scale faults connected source rock,which also have relatively high porosity and permeability conditions,belong to the potential zone of high saturation hydrate reservoir in the study area.