Comprehensive Investigation of the Petrophysical and Two-Phase Flow Properties of the Tight Sandstone in Yanchang Formation, Ordos Basin, China: Insights from Computed Tomography Imaging and Pore Scale Modelling

Abstract

Accurate determination of petrophysical and fluid transporting properties of rocks is essential for many engineering applications. In this paper, microcomputed tomography (CT) imaging technique is adopted to image the microstructure of tight sandstones drilled from Chang-7 member in Yanchang Formation. The pore geometry, pore-throat size distribution, pore connectivity, and tortuosity of the pore-throat structure are quantitatively characterized by extracting the pore network model (PNM). Direct numerical simulation (DNS) approach is applied on the segmented CT images to investigate the anisotropic permeability of tight sandstones. In addition, the unstructured mesh model of the pore space is reconstructed, and the pore scale immiscible two-phase flow is simulated by computational fluid dynamics (CFD) using the volume of fluid (VOF) model. The results indicate that the pore-throat system of tight sandstone reservoirs is mainly composed of discontinuous intergranular pores and grain margin microfractures with poor connectivity and diverse pore morphology. The two-phase flow simulations indicate that the strong heterogeneity of pore-throat structure can lead to obvious fingering phenomenon of the injected fluid, which reduces the sweep efficiency and the oil recovery as well. The residual fluids are mainly trapped in tiny throats and dead-end pores. It is found that the digital rock analysis based on CT imaging can be a cost-effective and time-saving alternative to routine core analysis of tight sandstone reservoirs.