Отправлено: 07.11.18 14:53. Заголовок: Molecular sieving in low permeability reservoirs

Effect of pore sizes on composition distribution and enhance recovery from liquid shale—Molecular sieving in low permeability reservoirs

Masoud Alfi, Maria Barrufet, John Killough

ABSTRACT

Current paper proposes a model to account for different pore sizes in shale and its influence on hydrocarbon
distribution.

Such a partitioning scheme provides a multiporosity-like approach where the fluid composition in
different pore sizes varies due to size filtration and sieving effect. The proposed approach opens up new ways to
interpret anomalous gas-oil ratios in shale.

Low permeability shales are believed to form a considerable portion
of the recoverable reserves in North America. Such an enhanced potential comes with new challenges and
technical difficulties.

Recent advances in high precision analytical tools have revealed that pore size distribution
in shale reservoirs spans a wide range. Molecules in pores with different sizes may exhibit significantly different
thermodynamics behavior. Rock fluid interactions and space hindrance effects play an important role when pore
sizes become close to species’ molecular dimeters. The tight porous media in such situations can act as a semipermeable
membrane that selectively filters molecules based on their sizes. This effect can result in a composition
difference between pores with large and small diameters in shale reservoirs (size filtration or sieving
effect), with small pores mostly filled with smaller hydrocarbon molecules and large molecules residing in larger
pores.

To account for such a diverse behavior, this paper proposes a pore partitioning approach, which divides
shale media into two different porosity systems: large and small pores. We use a modified version of the PengRobinson
equation of state to model the equilibrium hydrocarbon distribution in large and small pores. Our
thermodynamics calculations show that as pore dimeter decreases, the concentration of larger hydrocarbon
molecules in those pores decreases because of size filtration. Considering the small pore size in shale and rockfluid
interactions, our current model was used to analyze the potential of different injection gases as improve
recovery agents.

Our results show that a higher pore wall affinity for CO2 (compared to CH4 and N2) helps it
reach deeper in small shale pores, making it a perfect fit for improve oil recovery from extremely tight media.
Results show that our model provides a powerful tool to evaluate the complicated rock-fluid dynamics in liquid
shales