STUDY ON GAS ADSORPTION AND TRANSPORT BEHAVIOR IN SHALE ORGANIC NANOPORE

Abstract

The characteristics of shale gas reservoir are expressed in terms of complex pore structure and multiple gas occurrence pattern. Adsorbed gas and bulk gas coexist in nano-scale organic pores. The influence of organic pore shape on confined gas adsorption and flow behavior is not clear, which consequently causes the inaccuracy to understand gas transport mechanisms in shale gas reservoir. To solve this problem, different shapes of organic pore (circular pore, slit pore, triangle pore, square pore) are constructed in this study and Grand canonical Monte Carlo method is first applied to simulate gas adsorption process in different shapes of organic pore across different pore pressure. We found that gas adsorption behavior in different shapes of pores conforms well with the Langmuir adsorption pattern. The absolute adsorption, excess adsorption and gas adsorption parameter change with pore size and pore pressure is analyzed and the influence of pore shape on gas adsorption is studied on this basis. The mathematical model of adsorbed gas surface diffusion and free gas slip flow in different shapes of organic pore is established based on the understanding of thermodynamic gas adsorption pattern in different shapes of organic pore. The respective contribution of adsorbed gas flow and free gas flow on total gas permeability is studied in organic pores with different pore shape and pore size combining molecular simulation results with established gas transport models. The results indicate that the slit pore exhibits the largest value of maximum gas concentration, Langmuir pressure and the weakest adsorbed gas surface diffusion capacity compared with the other three pore shapes. The adsorbed gas surface diffusion influence on gas permeability can be neglected at pore radius larger than 5 nm for different pore shapes. This study reveals the influencing mechanism of organic pore shape on gas adsorption and flow ability during practical shale gas reservoir production.