Quantitative characterization of pore size and structural features in ultra-low permeability reservoirs based on X-ray computed tomography

The pore structure of ultra-low permeability sandstone (0.1 mD < K < 1 mD) is complex and has high heterogeneity, which leads to great challenges in reservoir characterization and effective evaluation. To study the effect of the heterogeneity of ultra-low permeability sandstone on permeability, a digital core model was established using X-ray computer tomography (CT) technology. Then a three-dimensional pore network model (PNM) in core samples was created using the "maximum ball method" algorithm. Finally, the geometric topology features of ultra-low permeability reservoirs were quantitatively characterized. According to the physical properties and test results of samples, the porosity distribution of targeted area ranged between 5.92% and 10.08%, and the permeability distribution range was 0.145 mD-0.75 mD, which was typical for an ultra-low permeability reservoir. The distribution of pore throat radii ranged from 0.27 mu m to 1.29 mu m, the shape factor was 0.04-0.06, the disconnected pores of the four samples made up between 16.89% and 58.71% of all pores, and the fractal dimension fluctuated between 1.05 and 1.5. Plane porosity also showed certain fractal characteristics. Correlation analysis showed that the disconnected isolated pores and the micron-scale pore throat radii of the reservoir sandstone were the main factors resulting in ultra-low permeability. The above-mentioned quantitative characterization parameters can be subsequently used to conduct microscopic seepage simulation studies of such reservoirs.