The Productivity Models Of Natural Gas Hydrate Production By Depressurization

Natural gas hydrate(NGH)is considered as the most potential alternative energy source in China.Its efficient development is of great significance to China's building a powerful marine country and ensuring national energy security.There is a lack of reliable NGH productivity models and systematic productivity evaluation system at home and abroad,so it is urgent to systematically carry out NGH reservoir productivity evaluation,sensitivity factor analysis and related research on stimulation technology.In this paper,three productivity models of NGH depressurization production(NGH depressurization by vertical well,NGH depressurization by fractured vertical well and NGH depressurization by horizontal well and)were established by considering the compressibility of NGH reservoir,the permeability change during NGH decomposition.Boltzmann transform,conformal transform and Laplace transform are used to solve the three models.The main factors affecting the productivity of NGH depressurization production was studied and analied,the three depressurization production models were compared.The results show that the lower the bottom hole pressure and permeability decline index,the higher the gas production rate of NGH depressurization production,the higher the decomposition rate constant of NGH and absolute permeability of reservoir,the higher the gas production rate of NGH depressurization production;in the early stage of production,the gas production rate decreases with the increase of initial NGH saturation,and in the later stage of production,the gas production rate increases with the increase of initial NGH saturation.Section length has a great influence on productivity.Fracture width and fracture permeability have no significant impact on the productivity of depressurized NGH production,while fracture length has a significant impact on the productivity of depressurized production.Gas production rate increases with the increase of fracture length,the optimum fracture penetration ratio of the model is 0.7,that is,the optimum fracture length is 700 m.With the increase of horizontal well length,the gas production rate increases,and the optimum horizontal well penetration ratio of the model is 0.8,that is,the optimum horizontal well length is 800 m.The comparison of NGH productivity of vertical well,horizontal well and fractured vertical well shows that the fractured vertical well has the best production efficiency,followed by horizontal well and vertical well.The maximum gas production rate of fractured wells with fracture length of 400 m 2.89 times of that of vertical well.The maximum gas production rate of horizontal wells with horizontal length of 400 m is 2 times of that of vertical wells.Therefore,hydraulic fracturing technology and horizontal well technology can improve the productivity of NGH production by depressurization.