Evaluation of Multi-Gas Transport Behavior in Shales Through a Universal Model

Shale gas production results from multi-gas transport mechanisms over a wide range of pore sizes. Meanwhile, it depends on a real gas effect, a sorption layer effect and an effective stress sensitivity. Therefore, a comprehensive consideration of these factors is significant for accurately evaluating real shale gas flow capacity. Laboratory measurements and modelling techniques are combined in this work. First, pore structure of shale is deeply analyzed. Second, a weighted contribution of each mechanism to the total gas flux is established. Finally, a unified model is proposed considering a real gas effect, a sorption layer effect and an effective stress sensitivity. Steady state flow experiments in shale cores are conducted to validate this model. Also, methane adsorption experiments and effective stress experiments are applied to quantitatively find the effects of adsorption and poromechanical behavior on gas flow, which are used in the model parameters. Then this model is used to evaluate gas transfer capability of a shale gas reservoir over multiple pore sizes. It is shown that the dominant gas transport mechanism is different in different scale of pores/fractures. Moreover, the results show that the contribution of each flow mechanism changes obviously with a different pressure as the pore hydraulic radius ranges from 10-100nm. Then the effects of gas-solid properties, pore structure and temperature-pressure conditions on shale gas flow capacity with the pore hydraulic radius of ranging from 10-100nm are deeply analyzed, respectively. Finally, a specific methodology on evaluating multiscale gas flow capacity in shale is proposed considering gas-solid properties, a volume proportion of a pore diameter from 50-200nm and reservoir temperature-pressure conditions. This work systematically investigates shale gas flow behavior through a unified model and provides an alternative workflow for evaluating capacity of multiscale shale gas flow.