A Production Forecasting Model And Optimization Of Completion Parameters For Shale Gas Reservoirs Based On The Micro/Nano-scale Flow

The recoverable reserves of shale gas in China is about 21.8 trillion cubic meters,which ranks first in the world.Currently,our country has achieved a breakthrough in shale gas exploitation.In 2016,our country has reached a production capacity of 75 trillion cubic meters per year.However,shale gas reservoirs contain multi-scale pore medium,diverse gas occurrence types and highly developed natural fractures.Thus the shale gas transport mechanisms are complex.It is difficult to forecast shale gas production after hydraulic fracturing.The theoretical basis for the design of the well completion parameters is still lacking.Therefore,it is of great importance to develop a set of production forecasting model and optimization method of the well completion parameters based on the micro/nano-scale flow characteristics.In the paper,firstly,a real gas transport model which is applicable to a wide range of Knudsen number is developed based on the Extened Navier-Stokes Equations.The proposed model is compared with the experimental data and Monte-Carlo simulation results to demonstrate its reliability.The influences of real gas effect,pore size and pressure on gas transport are analyzed.Secondly,the organic/ inorganic cell permeability model is developed based on the real gas transport model with the fractal theory.The shale gas transport mechanism is revealed.Then the incomplete layer method is used to obtain the core-scale(hundrends of micrometers)permeanbility model.The sensitivity of the apparent permeability to the fractal dimension,pore shape,porosity,TOC and pore size distribution of matrix is investigated.The solution process of calculating the permeability of the shale core in which there exists the fracal matrix pore size distribution is developed.Thirdly,we combine the continuum medium theory with the discrete fracture model and consider the heterogeneous pore size distribution of shale matrix to develop a fractal production forecasting model for shale gas reservoirs based on the fractal permeability model.The results of the proposed model are compared with the actual field data to validate the model.The effect of natural fracture parameters,hydraulic fracture parameters,pore size distribution of matrix and adsorption phenomena on culmulative gas production is investigated.Finally,the production forecasing model is used to design the completion parameters of the horizontal well in QY shale gas reservoir in Sichuan Province on the basis of the production forecasting model.The orthogonal test is used to analyze the effect of fracture conductivity,fracture stage number,fracture half-length,stage spacing and cluster spacing on the culmulative gas production.The results show that the fracture stage number is the factor that affects the gas production most significantly.The conducticity has the minor effect.Then the well completion parameters are optimized to achieve the maximum production.The optimized range of the parameters is obtained.The optimization method of the well completion parameters is developed based on the micro/nano-scale gas flow.Summing up,this paper has developed the micro/nano-scale shale gas transport and production forecasiting model which is applicable to the shale gas reservoirs in China.A set of optimization method of the well completion parameters is also proposed.This research provides a theoretical basis for the shale gas production forecasting and well completion optimization design.