Upscaling characterizing pore connectivity, morphology and orientation of shale from nano-scale to micro-scale

Abstract

The fluid transport in tight rocks are dominantly controlled by the preferential migration pathways. In this paper, the geometric and topological characteristics of multiscale pore systems of shale were delicately characterized by X-ray computed tomography (CT) and focused ion beam-scanning electron microscope (FIB-SEM). The preferential migration pathways of the three-dimensional pore network of shale were recognized at the micro-to nano-scales. According to the constructed pore network model based on the maximum sphere algorithm, the pore geometric parameters such as pore size, pore coordination number, pore throat size and length were calculated using Micro-CT, Nano-CT and FIB-SEM. Besides, sphericity, azimuthal angle and polar angle of pores were counted to characterize pore morphology and pore orientation. Different from the commonly reported results at the macroscopic scale, this paper proposed to use pore orientation to represent the preferential migration orientation at the pore scale. Results show that the general pore size range of shale is 0.054–50 μm, and the dominant pore size ranges observed using Micro-CT, Nano-CT and FIB-SEM are 2.759∼5 μm, 200∼500 nm and 54∼200 nm, respectively. Pore connectivity is best at FIB-SEM observation scale, middle at Nano-CT observation scale and worst at Micro-CT observation scale. The connected pore volume percentage using Micro-CT, Nano-CT and FIB-SEM is 10.2%, 50.8%, and 90.5%, respectively. Pores are divided into blade pores with the sphericity of being <0.5, rod pores with the sphericity of being 0.5–0.8, and spherical pores with the sphericity of being >0.8. Though the spherical pores and rod pores are dominant in number, the blade pores have larger pore volumes, larger pore diameters and better pore connectivity, and are conducive to fluid transportation. The blade pores are corresponding to the slit-shaped pores and microfractures. The statistics of pore polar angle show that the preferred pore orientations are close to the parallel bedding plane, which is the dominant channel directions of fluid migration.