Study On Gas Hydrate Reservoir Reconstruction For Enhanced Gas Production In Shenhu Area Of South China Sea Based On 3D Heterogeneous Geological Model

As one of the clean energy resources with huge exploitation potential,the organic carbon content of natural gas hydrate(NGH)is about twice that of oil,natural gas and coal combined.The exploitation of NGH has attracted more and more attention of all countries in the world.Since 2007,China has carried out multiple NGH exploration work,delineated three NGH ore bodies with reserves of more than 100 billion cubic meters,and conducted two offshore NGH trial production projects in 2017 and 2020 respectively.In these two production tests,the stable gas recovery from the clayey silt reservoir,which is the most difficult to be exploited,has been achieved,and the stage leaping from "exploratory test production" to "experimental test production" has been successfully achieved.However,how to realize the long-term efficient and safe exploitation of NGH is still the key challenge to promote its commercial exploitation.The NGH reservoir in Shenhu Area,South China Sea occurs in the clayey silt or silty sediments with very low permeability(only a few m D),which seriously restricts the efficiency of NGH exploitation.Reservoir stimulation technology derived from traditional oil and gas production can effectively increase formation permeability and increase temperature and pressure conductivity and gas-liquid transport efficiency within the reservoir,which can also be used to increase the gas production from NGH production.Numerical simulation is an effective means to predict the effect of reservoir stimulation and optimize the stimulation scheme.In the process of numerical simulation of NGH exploitation,the reservoir is usually generalized to be homogeneous.This not only cannot describe the real heterogeneity characteristics of the reservoir,but also ignores the spatial differences of temperature field,pressure field,stress field and other parameters caused by NGH decomposition caused by heterogeneity,which will lead to the failure to accurately predict the gas production of NGH reservoir and the spatial evolution characteristics of various physical parameters,and to design the reservoir reconstruction scheme for the low permeability area.Therefore,building a spatial heterogeneous geological model of NGH bearing sediments,using numerical simulation method to predict hydrate decomposition behavior and targeted reservoir reconstruction scheme design,and provide technical support for the long-term and efficient exploitation of marine NGH resources in China.In this paper,the NGH reservoir in the Shenhu Area,South China Sea is taken as the research object.Combined with the logging interpretation data of each site of the GMGS1 expedition,a heterogeneous geological model of NGH bearing strata is firstly constructed,and a numerical simulation model for NGH exploitation at Site SH2 is established,and the solution is solved by the Tough + Hydrate Program.The gas and water production behavior of heterogeneous reservoir was evaluated,and the temporal and spatial evolution characteristics of physical property parameters caused by NGH decomposition were obtained.Finally,based on the spatial heterogeneity of NGH saturation,formation permeability and other parameters,the effect of high pressure jet grouting technology on NGH reservoir reconstruction is studied.The grouting method,the direction of the grouting hole and the spacing of the grouting hole were optimized to evaluate the stimulation effect.The simulation results show that:(1)Compared with the heterogeneous geological model,the gas production capacity of hydrate reservoir predicted by the previous homogeneous geological model is higher(about 20%);(2)The vertical grouting method,with injection holes arranged in the front of horizontal wells and spacing of 10 m,has the best stimulation effect and the gas production capacity is increased by about 49.39%;(3)The economical and reasonable design of reservoir stimulation scheme must rely on the accurate understanding of the spatial heterogeneity of NGH reservoir,which is the necessary prerequisite to realize the longterm and efficient exploitation of NGH.