Mathematical modeling for occurrence state of hydrocarbon sin micro/nano-scale pores

Key words:  micro/nano-pores  shale  interaction potential energy  occurrence state  adsorption behavior

Abstract:

A mathematical model for the occurrence state of hydrocarbons in micro/nano-scale pores was derived, in which a simplified local density function was introduced considering the interaction between hydrocarbon molecules and pore surface walls. Simultaneously, the behavior of critical shift of fluids in micro/nano-pores was also considered. The occurrence states of hydrocarbon fluid with single component, two components and three components were simulated respectively using the model. The effects of pore size, component fraction and fluid composition were investigated. The simulation results indicate that, due to the effect of pore proximity, the fluids in micro/nano-pores will exist in two states:bulk phase and adsorbed phase. The density of the adsorbed phase around the pore wall is higher than that of the bulk phase in the center of the pore. For hydrocarbon fluid with multi-components, it is found that the thickness of its adsorption film is greater than that of the single light component, but less than that of the single heavy component. As the pore size increases, the thickness of the adsorption film is increased, which appears in an exponential relationship. Furthermore, as the average carbon number of the hydrocarbon fluid increases, the interaction energy between fluid and pore walls is increased, thus the thickness of the adsorption film is increased.