Numerical Simulation Study On Horizontal Well Depressurization Production Of Natural Gas Hydrate

With the world’s energy crisis extending,natural gas hydrate is regarded as the most ideal continuous energy for coal and oil in the 21 st century,and its development has become a research hotspot.Although the resource forecast and actual exploitation of natural gas hydrate are uncertain,in contrast,the natural gas resource contained in natural gas hydrate is much higher than that of traditional natural gas.More than 90% of the natural gas hydrates are found in the sediment layers in the deep sea area,and it is extremely difficult to discover.Although many countries in the world have actively conducted many successful test productions,the efficiency of depressurization test production in horizontal wells is much higher than that in vertical wells.These trial productions are expensive and far from meeting the needs of commercial discovery.Therefore,a lot of preliminary work is still needed before the commercial exploitation of natural gas hydrate can be realized.Numerical simulation is one of the important means to study the gas hydrate decomposition.Using numerical simulation to conduct more indepth research on the phase balance,heat and mass transfer and its extraction mechanism in the process of natural gas hydrate extraction,in order to design a better extraction plan and avoid unnecessary losses,is a very meaningful early stage work.In this study,a mathematical model of natural gas hydrate production was established with an infinitely open outer boundary in a three-dimensional cylindrical coordinate system.Then a finite difference method was used to solve it and an autonomous simulation program was developed.The effects of the permeability,the porosity,and the saturation on pressure distribution and the sensitivity of the production well pressure,the production well temperature,the kinetic reaction constant,the permeability,the porosity,and the saturation to gas production were investigated by using an autonomous simulation program.Using autonomous simulation program studied the first trial production in the Shenhu area and analyzed the reasons for the deviation between the current numerical simulation results and the actual test production data.At the same time,this study revised the production model accordingly,and then carried out the plan design of the well layout density;The numerical simulation study of the second test production process further verifies the model and simulation program,and evaluates the rationality of the production plan of horizontal wells arranged in the upper part of the mixed layer through numerical simulation.Concluded as follow:(1)With the increase of the initial permeability value and initial porosity value,the pressure transfer in the hydrate reservoir is faster,but the influence of permeability is relatively large;The slower the pressure transfer;the pressure of the production well,the temperature of the production well and the permeability are the main parameters that affect the gas production in the process of natural gas hydrate production.(2)The first trial production in the Shenhu sea area belongs to the argillaceous silttype reservoir,which has strong water sensitivity.The fresh water generated by the decomposition of hydrate causes the expansion of the clay and reduces the permeability,which is ignored by many scholars when building models,resulting in errors;in the process of test production,the pressure of the production well is not fixed due to natural and human factors,so the pressure of the production well must be used as a dynamic input parameter.Based on this,the permeability model was revised,and the time-varying pressure of the production well was dynamically input,and the simulated gas production was very close to the first test production data in the Shenhu sea area,which made the numerical simulation of depressurization production closer to the actual situation;In the decomposition,there are decomposition inner edge and decomposition outer edge,which can be used as one of the design basis of vertical well layout density.(3)The numerical simulation results have a high degree of agreement with the gas production data of the second trial production in the Shenhu area.In the early stage of horizontal wells depressurization production of natural gas hydrate,the gas production mainly comes from the free gas in the reservoir.However,the gas from hydrate decomposition is the main source of gas production in the late stage of production.So the horizontal wells are arranged in the upper part of the mixed layer during long-term depressurization of natural gas hydrate production.