| Based on the research and development of shale gas reservoirs in recent years acquired by review and summary of a large number of shale gas reservoir documents,the research subject is established.Currently,the determination of adsorption type and diffusion type is mainly based on previous experience,selecting Langmuir adsorption model and Knudsen diffusion model,although the three flow characteristics-adsorption / desorption,slippage,diffusion have been taken into consideration.the innovation of this paper are the percolation model considering BET adsorption and comparison about influence of different diffusion type and adsorption type on pressure distribution.The percolation model and theory research have important significance to the efficient exploition of shale gas.Based on the comprehension of basic characteristics about shale gas reservoir and three flow characteristics-adsorption / desorption,slippage,diffusion,the mathematical model and numerical model of shale gas reservoir is established in sequence,thus the migration and output of shale gas in reservoir are deeply recognized.Through the simplification of procedure and model,solving the pressure distribution of shale gas reservoir,the corresponding function relationship of pressure and radius is determined.The pressure distribution considering different types of adsorption and diffusion is solved by adjusting parameters,the pressure drop curve is plotted to compare the influence of different types of adsorption and diffusion on pressure distribution.The main research work are as follows:(1)The exploration and development status of shale gas reservoir is acquired through scanning the concerned documents of recent 5 years,the encountered problems in development and the latest theories and techniques are summarized to obtain development tendency.(2)Through the summary and comparison of principles and affecting factors between thermogenic and biogenic origin,introduction of mixed origin,the conclusion that thermogenic origin is major besides composition and total of shale gas vary according to environmental factors is drawn.The systematic summary and analysis over characteristics of porosity and permeability,occurrence mode,special evaluation index is achieved by scanning large number of documents,laying the theoretical foundation for subsequent research.(3)The special migration mechanism of shale gas is systematically surveyed,select BET adsorption model based on 3 types of diffusion,Knudsen number has important significanceon the determination of diffusion type,the major reflection of slippage is the increase of apparent permeability,the above conclusions provide theoretical foundation for the establishment of percolation model.(4)According to the research above,with hypothesis,basement and crack systems distinguished,aqueous phase and gas phase distinguished,the mathematical model and numerical model considering BET adsorption model is established in sequence.(5)Simply the percolation model,establish flow controling equations,and determine the corresponding function relationship of pressure and radius.According to the idea of variable-controlling approach,the influence of different types of adsorption and diffusion on pressure distribution can be compared by altering desorption amount and Knudsen number,the influence can be compared more intuitively by plotting pressure drop curves through solved equations programming with dichotomy.To the shale gas reservoir which fits in BET adsorption model,selecting Langmuir adsorption model causes lower pressure than the real,the pressure drop rates controlled by three types of diffusion,from fast to slow,are rate of Knudsen diffusion,transition diffusion,Fick diffusion.To the pressure distribution,the influence of diffusion is smaller than that of adsorption. |
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