Numerical Simulation Of Optimal Reservoir Stimulation Strategy On Natural Gas Hydrate Productivity

Natural gas hydrate has attracted more and more attention in the world as alternative clean energy in the future.Depressurization method is considered to be the most suitable development means for hydrate commercial exploitation because of its economic and mature advantages.However,traditional depressurization by vertical well processes disadvantages of limited range of bottom-hole pressure drop and poor formation conductivity.At present,the wellbore design represented by horizontal well and multi-lateral well and the hydraulic fracturing tecnnology for the hydrate reservoir with compressibility（such as permafrost）have become the research hot spots of hydrate depressurization production.In this study,the productivity model of natural gas hydrate developed by depressurization method was established to study the influence of reservoir stimulation strategy（including wellbore design and hydraulic fracturing）on the gas hydrate productivity.The productivity of conventional vertical well,horizontal well,radial well,fracturing vertical well and multiple fracturing horizontal wells were compared.The dynamic changes of temperature field,pressure field,hydrate saturation and water saturation in the process of mining were analyzed,and the sensitivity analysis of the factors affecting the productivity of radial well,fracturing vertical well and horizontal wells was carried out.The above development methods and stimulation measures were optimized by taking cumulative CH₄production in 1000 days as the performence index.The results shows that the average gas production rate of depressurization production in vertical wells is only 3480 m³·d^-1,which reveals a poor development effect.Horizontal wells have increased production by approximately 1.65 times over a 1,000-day production cycle compared to vertical wells.In this specific natural gas hydrate reservoir,when the radial well is vertically distributed in four rows and four radial holes with a length of 100 m are arranged in the same plane,the best mining effect can be achieved,which is 2.4 times of the depressurization production capacity of the vertical well.In the research of hydraulic fracturing productivity,fractures can not only effectively increase the range of bottomhole pressure drop and promote hydrate decomposition,but also provide a stable seepage channel for gas and water flow after hydrate decomposition,which has a significant effect on increasing hydrate productivity and improving hydrate development.The best fracture parameters for fracturing vertical well are the fracture length of 100 m and fracture conductivity of 30μm²·cm.In contrast,the optimal combination of multiple fracturing horizontal wells parameter is found to be fracture number of 5,fracture length of 100 m,and fracture conductivity of 50μm²·cm,which is 5.8 times of the conventional horizontal well production and 3.3 times of the vertical well fracturing.Taking cumulative gas production in1000 days as performance index,it is considered that multiple fracturing horizontal well has the most significant mining effect,followed by vertical well fracturing,radial well mining,horizontal well mining,and vertical well which processes the worst mining effect.