Study on the influence of organic pores on shale gas flow ability

Abstract

In shale gas reservoirs, gas is stored in organic pores and inorganic pores with nm-scale pore size. Conventional single tube apparent permeability model is based on homogeneity assumption and neglects the influences of organic pores spatial distribution pattern, pore size distribution on gas flow ability. In this study, three dimensional shale digital core is reconstructed by Markov Chain Monte Carlo (MCMC) algorithm and medial axis in the pore space is extracted. According to geological study on the organic pores distribution pattern and pore size distribution, different sizes of organic pores and inorganic pores are assigned to corresponding spatial location of pores and throats. Three different types of shale pore network are built: shale pore network model with dense continuous layers of organic pores; shale pore network model with sparse continuous layers of organic pores; shale pore network model with discontinuous organic pores. Gas transport mechanisms of adsorption and desorption, surface diffusion, viscous flow, Knudsen diffusion are considered in organic pores. Gas transport mechanisms of viscous flow, Knudsen diffusion are considered in inorganic pores. Gas property calculation takes the influences of real gas effects and phase behavior change into account. Analysis results indicate that when the organic pores size distribution is smaller than inorganic pores size distribution, the permeability of shale pore network model with dense continuous layers of organic pores is larger than that of the other two types of shale pore network model. The permeability of shale pore network model with sparse continuous layers of organic pores is smallest. When the organic pores size distribution is bigger than inorganic pores size distribution, the permeability of shale pore network model with dense continuous layers of organic pores is also larger than that of the other two types of shale pore network model. The permeability of shale pore network model with discontinuous organic pores is smallest. Our study results are significant for understanding gas flow ability in shale gas reservoirs.