The shale gas reservoirs are rich in organic and inorganic nano-sized pores. Generally, both adsorbed gas and free gas are considered to be exist in organic matrix pores, while there is no adsorbed gas in inorganic matrix pores. Therefore, gas transport mechanism is quite different in these two types of nanopores. Many researchers have provided the presence of water film on the inorganic pore surface due to its strong hydrophilic ability. However, most of the models for gas transport in shale inorganic nanopores ignore the effect of water distribution on gas transport, which lead to overestimating of gas transport capability.
In this paper, a new gas transport model for inorganic nanopores in shale gas reservoirs is proposed. First, considering the gas transport capability varies with pore size, the logarithmic normal distribution function is utilized to describe the nanopores distribution in shale inorganic porous media. Then, the influence of real gas effect, stress dependence and water distribution are all considered to derive the model. The validation results show that the proposed model and published experimental data can be well fitted. Finally, the effect of each factor on gas transport capability in shale inorganic nanopores is analyzed and discussed.
The results indicate that the gas transport capability will decrease with the increase of relative humidity. When the relative humidity increases to a critical value, the nanopores will be blocked with capillary water. During depressurization development process, the effective pore size will apparently reduce due to the influence of stress dependence, which cannot be ignored. Furthermore, when the shale inorganic matrix pores have the same mean pore size, the gas transport capability under various pore distribution probability is quite different, and the lower the peak frequency, the higher the transport capability. Meanwhile, under high temperature and low pressure conditions, methane transport capacity is significantly higher than ethane and carbon dioxide.
The research results of this paper can provide a reference for the analysis of nanoscale gas flow mechanism in shale matrix, and also provide a theoretical basis for more accurate production prediction of shale gas wells.
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