Study On Natural Gas Hydrate Production By Depressurization Method

In this paper,based on the published geologic characteristic parameters of the Shenhu Area in South China sea,a numerical model for class 1 hydrate is established by CMG-STARS module.The effects of various petrophysical parameters and development factors on the production performance of hydrate reservoirs by depressurization when two different well types are involved,horizontal well and vertical well,are respectively analyzed and compared.The key factors are discovered by summarizing the basic production performance characteristics and the evolution of temperature,pressure as well as hydrate saturation.The productivity of horizontal well using depressurization method and the combined depressurization-thermal motivation method is compared.By means of the establishment of an aligned fracture model and a staggered fracture model,the influence of different fracturing methods on the development effect of combined depressurization-thermal motivation method in horizontal well is evaluated.Main results are listed as follows:(1)A numerical model for the production of class 1 hydrates by depressurization method in a single straight well is established using the STARS module of CMG software.According to the calculation results of the model,the cumulative production of gas within 60 days is 30.4×10~4m~3and the average proportion of hydrate decomposition gas is 82.8%.The relative error between these calculation results and the published data of the first NGH test in the Shenhu area is less than2%.(2)In the case of vertical well,the productivity of gas well increases with the increase of permeability,hydrate saturation,gas saturation as well as perforation percentage of hydrate layers,while it also increases with the decrease of bottomhole pressure.Permeability of the hydrate layer is the main factor among petrophysical parameters.When the permeability rises from 1m D to10m D,the cumulative production of gas within 60 days can change from 14.8×10~4m~3 to109.1×10~4m~3,which is an increase of more than 6 times.In terms of development factors,both perforation position and working system are the main factors.When only the free gas layer is perforated,the cumulative production of gas within 60 days is 2.0×10~4m~3,which is 93.4%lower than that of the basic situation.When the bottomhole pressure reducing from 11MPa to 3MPa,the cumulative production of gas within 60 days increases by 24 times from 1.3×10~4m~3 to 30.4×10~4m~3.(3)In the case of horizontal well,permeability of the hydrate layer is the most influential petrophysical parameter.When the permeability increases from 1m D to 10m D,the cumulative production of gas within 60 days indicates a prominent increase from 39.1×10~4m~3 to 267.2×10~4m~3.Among development factors,the location of horizontal well and working systems are the most crucial factors.The formation near the perforated part of wellbore is the main place where hydrates decompose.The cumulative production of the gas well placed 2m below the BSR is f 3.1×10~4m~3,which is 93%lower than that of the well at the BSR.When the bottomhole pressure decrease from11MPa to 3MPa,the cumulative production of gas increases by 7.8 times from 5.0×10~4m~3 to43.8×10~4m~3.For hydrate reservoirs which possess poor physical properties and need to be exploited with a small pressure difference,the productivity of horizontal well is triple that of vertical well.Compared to vertical well,horizontal well is more suitable for the development of Type-I hydrate reservoirs by depressurization method.(4)For the combined depressurization-thermal motivation method,the injected hot steam supplies energy to the formation and reforms the temperature field of the reservoir,by which the hydrates can continuously decompose.The cumulative production of gas is 162.5×10~4m~3,which is 73.1%higher than that of the reservoir developed only by depressurization method.The fracturing and assisted heat injection technology can effectively increase the productivity.In the aligned fracture model,a large amount of heat can be injected into the formation at the beginning of the simulation to further increase the productivity.The cumulative production of gas within 60days is 144.0×10~4m~3,which is 36.8%higher compared with that in the staggered fracture model.