Abstract
Accurate prediction of unconventional gas production requires deep understanding of the permeability of complex rock samples. Several predictive expressions of permeability, which include either simplifications of the porous media structure or the flow mechanisms, have been proposed recently. The main objective of this research is to quantify the impact of solid matrix complexity on both intrinsic and apparent permeability. To this end, numerous two-dimensional random porous media structures are constructed using the quartet structure generation set algorithm. Parametric and statistical analysis reveals the importance of the specific surface area of pores, tortuosity, heterogeneity and degree of anisotropy. Special focus is given to the directional dependency of the permeability on isotropic and anisotropic geometries, considering the great impact of anisotropy on the laboratory evaluation of permeability data and the anisotropic nature of shale rocks. Simulation results, for the same value of porosity, clearly indicate the drastic improvement of permeability due to the reduction of specific surface area of pores and their height to width ratio. This suggests that rock matrix complexity has significant impact on permeability and should not be neglected while forming permeability formulations for porous media. Finally, the results of the apparent permeability, obtained by solving the gas kinetic equation, are taken into consideration to demonstrate the enhancement ratio, slip factor and their correlation with the aforementioned parameters. Semi-analytical expressions for intrinsic and apparent permeability, considering continuum and slip flow respectively, are derived. The proposed formulations, suitable for both isotropic and anisotropic structures, have the advantage of not entailing any numerical or experimental data as input.
Highlights
Shale gas has attracted significant global interest due to its successful production in the United States, favoured by the recent development of economic extraction technologies
The structural properties of porous media significantly affect the intrinsic permeability, and the slip factor appearing in the apparent permeability, in different ways as analysed
This assertion is based on the numerical dataset produced from flow simulations of 400 geometries, generated using the Quartet Structure Generation Set (QSGS) method
Summary
Shale gas has attracted significant global interest due to its successful production in the United States, favoured by the recent development of economic extraction technologies. To make global exploitation of shale gas reserves possible, accurate monitoring, prediction and optimization of the production is indispensable This is achieved through reservoirscale fluid flow simulations in the order of kilometres, which require input parameters, such as porosity and permeability, that characterize the rock formation. Many researchers have focused on deriving analytical formulations to describe the apparent permeability of reservoir rocks, most of them based on the straight tube simplification (Civan, 2010; Javadpour, 2009; Lin et al, 2017) Another modelling approach uses the fractal dimension and tortuosity to account for the roughness of pore surfaces and sinuous flow paths respectively (Darabi et al, 2012; Zhang et al, 2015).
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