Desorption of CH4/CO2 from kerogen during explosive fracturing

Abstract

Herein, the desorption behaviors of CO2/CH4 onto kerogen atomistic representation at higher temperatures and pressures and its inspirations of dividing stages on deflagration fracturing was clarified quantitatively via molecular simulation and mathematical model, as well as the effects of temperature and pressure. Molecular probe detection results suggested that accessible free volume of kerogen macromolecule was generated among different aliphatic chains or between the aliphatic chains and aromatic clusters, where the former type provides the primary free volume within the kerogen macromolecule. The quantitative division of desorption stages suggested four desorption stages via the established three critical pressures, start pressure (Pst), transition pressure (Ptr), and sensitive pressure (Pse), dividing the desorption properties into four stages: low efficiency desorption stage (P > Pst), slow desorption stage (Ptr < P < Pst), rapid desorption stage (Pse < P < Ptr), and sensitive desorption stage (P < Pse) respectively. All these three critical pressures firstly increase slowly and then significantly with the increasing temperature, indicating that high temperature was favorable for the early arrival of the rapid desorption stage and sensitive desorption stage. Both the pressure reductions for the former two stages keep stable for the temperature < 458 K. However, for the temperature > 458 K, the required pressure reduction for rapid desorption stage increases, suggesting that high temperature was also favorable for enhancement of shale gas production. Thus, the reservoir pressure was rather high after the deflagration fracturing process, resulting in the long duration of slow desorption stage and low efficiency desorption stage. The pressure reduction rate should be extended as long as possible to ensure the complete desorption of CH4, which could make an extension of the rapid desorption stage. The outcome of this paper was of broad interest for CO2 enhanced shale gas recovery and deflagration fracturing, as well as carbon capture and utilization engineering.