Revaluating coal permeability-gas pressure relation under various gas pressure differential conditions

International Journal of Coal Science & Technology

Research

Hongxu Wang¹  Chunguang Wang¹  Wenxin Li¹  Meng Zhou¹  Derek Elsworth²  Guanglei Cui³  Bingqian Li⁴  Jiyuan Zhang⁵ 

[1] College of Energy and Mining Engineering, Shandong University of Science and Technology, 266590, Qingdao, China;Energy and Mineral Engineering, Geosciences, G3 Center, and EMS Energy Institute, Pennsylvania State University, University Park, PA, USA;Key Laboratory of Ministry of Education On Safe Mining of Deep Metal Mines, Northeastern University, 110004, Shenyang, China;Pingdingshan Tianan Coal Mining Co., LTD, 467003, Pingdingshan, Henan, China;School of Petroleum Engineering, China University of Petroleum (East China), 266580, Qingdao, China;

关键词: Gas compressibility;    Coal permeability;    Pressure differential;    Mean gas pressure;   

DOI  :  10.1007/s40789-023-00609-1

 received in 2023-05-04, accepted in 2023-07-18,  发布年份 2023

来源: Springer

【 摘 要 】

Identifying changes in coal permeability with gas pressure and accurately codifying mean effective stresses in laboratory samples are crucial in predicting gas-flow behavior in coal reservoirs. Traditionally, coal permeability to gas is assessed using the steady-state method, where the equivalent gas pressure in the coal is indexed to the average of upstream and downstream pressures of the coal, while ignoring the nonlinear gas pressure gradient along the gas flow path. For the flow of a compressible gas, the traditional method consistently underestimates the length/volume-averaged pressure and overestimates mean effective stress. The higher the pressure differential within the sample, the greater the error between the true mean pressure for a compressible fluid and that assumed as the average between upstream and downstream pressures under typical reservoir conditions. A correction coefficient for the compressible fluid pressure asymptotes to approximately 1.3%, representing that the error in mean pressure and effective stress can be on the order of approximately 30%, particularly for highly pressure-sensitive permeabilities and compressibilities, further amplifying errors in evaluated reservoir properties. We utilized this volume-averaged pressure and effective stress to correct permeability and compressibility data reported in the literature. Both the corrected initial permeability and the corrected pore compressibility were found to be smaller than the uncorrected values, due to the underestimation of the true mean fluid pressure, resulting in an overestimation of reservoir permeability if not corrected. The correction coefficient for the initial permeability ranges from 0.6 to 0.1 (reservoir values are only approximately 40% to 90% of laboratory values), while the correction coefficient for pore compressibility remains at approximately 0.75 (reservoir values are only approximately 25% of laboratory value). Errors between the uncorrected and corrected parameters are quantified under various factors, such as confining pressure, gas sorption, and temperature. By analyzing the evolutions of the initial permeability and pore compressibility, the coupling mechanisms of mechanical compression, adsorption swelling, and thermal expansion on the pore structure of the coal can be interpreted. These findings can provide insights that are useful for assessing the sensitivity of coal permeability to gas pressure as truly representative of reservoir conditions.

【 授权许可】

CC BY   
© China Coal Research Institute 2023

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