| With the change of global energy pattern,as a kind of clean energy,the total organic carbon reserves of natural gas hydrate are two times higher than the average of crude oil,gas and coal resources,which has been widely concerned and recognized by all parties in the world.China has carried out several hydrate exploration projects since 2007,discovering three gas hydrate deposits of more than 100 billion kilograms.In 2017 and2020,China successfully carried out two Marine gas hydrate production projects,from the "exploratory production" to the "experimental production",and achieved remarkable results,realizing the steady-state gas production of the muddy silt sand reservoir.However,it is still an important issue to accelerate the commercialization process in order to achieve sustainable,effective and safe exploitation of gas hydrate.In 2013 and 2017,Japan conducted two trials of gas hydrate production in the Nankai Trough respectively.The data obtained by drilling and logging interpretation showed that the spatial distribution of physical parameters such as porosity,permeability and saturation of the hydrate reservoir in the trough was heterogeneous.Therefore,it is very important for the accuracy of numerical simulation results to establish a three-dimensional heterogeneous model that can accurately reflect the internal structure and parameter distribution of gas hydrate reservoirs.In many hydrate exploitation projects in China and Japan,the traditional depressurization method of vertical well or horizontal well was adopted.The analysis found that traditional production methods have several drawbacks: a single vertical well has less contact area with the formation,limited temperature and pressure transfer area,and lower productivity.In addition,horizontal wells have larger bore sizes and higher production costs.Therefore,it is an inevitable trend to search for efficient exploitation methods for heterogeneous hydrate reservoirs.Complex structural wells,represented by multilateral well,have a high potential for application in the development of hydration.Multilateral wells increase the contact area between the well and the hydrate reservoir,increasing branching,thereby improving the pressure and temperature conductivity of the reservoir compared with traditional horizontal and straight wells.In addition,in areas where hydrate distribution is extremely heterogeneous,horizontal multilateral wells can be located at high hydrate saturation to improve oil recovery and realize vertical mining.Based on the well log interpretation data,core data and seismic exploration data of Nankai Trough in Japan,a 3D heterogeneous geological model describing hydrate reservoir structure and physical properties has been constructed by Kriging interpolation method.The numerical simulation model is solved by TOUGH+Hydrate program.Compared with the homogeneous model,the reliability of the heterogeneous model is verified.Finally,the multilateral wells of horizontal wells set in the hydrate saturation area to predict the hydrate reservoir gas production in different mining schemes.The main conclusions are as follows:(1)Kriging interpolation can be used to construct a three-dimensional heterogeneous geological model with limited site data.(2)Compared with the homogeneous model,the heterogeneous model of the Nankai Trough in Japan can describe the physical parameters of the reservoir more accurately,and can better reflect the evolution law of the physical parameters and temperature and pressure field in the real reservoir during mining.(3)Based on the three-dimensional heterogeneous geological model,the gas production rate of hydrate reservoir can be increased by up to 20%～50% by laying horizontal multilateral wells in the area with high hydrate saturation.(4)There is a positive correlation between the length of the branch well and the gas production rate,and the gas production efficiency is the highest when the branch dip angle is 45°. |
