A comprehensive characterization of North China tight sandstone using micro-CT, SEM imaging, and mercury intrusion

Abstract

A clear understanding of pore structure of tight oil reservoirs is essential for reservoir evaluation and enhanced oil recovery. This paper presents a multiscale characterization method using a combination of pressure-controlled porosimetry (PCP), micro-computed tomography (micro-CT), and scanning electron microscopy (SEM). Four tight sandstone samples from Chang 7 Formation in the Ordos Basin were collected for petrophysical characterization. Pore-throat size distributions (PTSDs) for these samples were measured via PCP. A high-resolution micro-CT scan (1 μm/pixel) was used to acquire 3D volumetric images of small core plugs to evaluate pore connectivity of these samples. Additionally, high-resolution digital images were obtained through SEM to identify different pore types. SEM analysis shows that pores in tight sandstones could be classified into four types, i.e., residual interparticle pores, grain dissolution pores, clay pores, and micro-fractures. Residual interparticle pores are often coated by fibrous illite and chlorite. Grain dissolution pores are mainly deduced from the dissolution of grain minerals, among them the feldspar dissolution pore is the primary type. According to the PCP experiments, these samples exhibit multiscale pore structures with a wide range of PTSD from 9.2 nm to 500 μm dominated by nanopores. Average mercury intrusion saturation and permeability contribution value of the dominating nanopores are 63.61% and 80%, respectively. Given the unresolved nanopores, CT images were segmented into three phases, including pore space, grain phase, and clay minerals. The results of connectivity analysis demonstrate that macroscopic pores are mostly connected by clay phases, implying that nanopores provide the critical flow paths. This novel multiscale characterization approach provides us a better understanding of complex pore structures of tight sandstones.