Oil & Gas Geology ›› 2017, Vol. 38 ›› Issue (6): 1165-1171.doi: 10.11743/ogg20170617

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Pore-scale simulation of gas flow in low-permeability tight sandstone with gas slippage effect and Knudsen diffusion taken into consideration

Cao Tingkuan1,2, Liu Chengchuan2, Pu Tao2, Duan Yonggang3, Fang Quantang3, Sun Lianpu4   

  1. 1. Post-doctoral Research Station, SINOPEC Southwest Oil & Gas Company, Chengdu, Sichuan 610041, China;
    2. Research Institute of Exploration and Development, SINOPEC Southwest Oil & Gas Company, Chengdu, Sichuan 610041, China;
    3. College of Petroleum Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China;
    4. Sinc Geotech(Beijing) Science & Technology Co., Ltd., Beijing 100020, China
  • Received:2016-08-15 Revised:2017-10-29 Online:2017-12-28 Published:2018-01-08

Abstract: The strong heterogeneity and complicated pore structure of low-permeable tight sandstone with nano-and micro-scale pore-throats could not be accurately covered by using current core testing methods.Micro-scale flowing simulation based on digital core theories was then introduced to study the gas flowing in such stones.Samples taken from the Shaximiao Formation in Western Sichuan Basin were scanned with CT to build up three-dimensional (3D) digital cores.Methods such as pore-throat identification,pore topology simplification and throat adjustment,were also resorted to establish 3D pore network models with various throat distribution patterns.Considering the complexities of gas flow in such nano-and micro-scale pore-throats,we also established a multi-mechanism flow equation that takes Darcy flow,gas slippage effect and Knudsen diffusion into consideration,and tried to solve discretely the equation by combining it with the pore network models.Numerical simulations were then carried out to study the influence of various transport mechanisms upon gas migration under different pressures.Results indicate that smaller pore-throat size and lower pore pressure mean a stronger non-linear gas flow.Gas flow was observed to basically follow the Darcy's law when pore pressure higher than 10MPa and average throat radius larger than 0.1 μm.While in samples with average pore-throat radius less than 0.1 μm and pore pressure less than 10 MPa,gas flow was dominated by gas slippage and Knudsen diffusion,a phenomenon worthy of considering during gas production.

Key words: CT scanning, Darcy flow, Knudsen diffusion, gas slippage, 3D pore network model, low-permeability tight sandstone, western Sichuan Basin

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