Mass transfer model of fracture-controlled matrix unit: Model derivation and experimental verification based on fractal theory and micro-CT scanning technology

Abstract

Imbibition oil recovery in low permeability fractured reservoirs has been acknowledged as an efficient method to enhance oil recovery. However, the effects of the complex pore structure and the hydraulic pressure in fracture on the fluids transport were not considered comprehensively in any model at pore scale in literature. This paper aims to study the microscopic mechanism of the imbibition process and define the concept of an imbibition unit using mathematical methods. Based on fractal theory, the improved effective porosity, permeability model, mass transfer model, and the fracture control range model of the matrix are derived, considering the effects of surface roughness, heterogeneity of the pore structure, irreducible water and the hydraulic pressure in fracture. The required parameters for model calculation are obtained by the three-dimensional reconstructed CT images, density test experiment and wetting test experiment. Good agreements are achieved between the result calculated by the proposed model and that of the experiment. The sensitivity analysis of the mass transfer model is then conducted and discussed. The results indicate that the cumulative oil recovery during the imbibition process is mainly controlled by the surface fractal dimension (Ds), the pore pressure gradient (Δp), the pore fractal dimension (Dλ), the hydraulic pressure in the fracture (Ph), and the tortuosity of the capillary (τ). And Δp, Dλ, and Ph shows a positive relationship with the cumulative oil recovery, while Ds and τ shows a negative influence.