Abstract
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 μm to 1.29 μm,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.
NSTL