Validity Standards for Digital Rock Imaging and Analysis

In digital rock study of pore distribution and seepage mechanism using Micro-/Nano-CT, FIB-SEM, and so on, the samples are generally tiny with very limited scanning area, whose representativeness has been questioned. Microscale researches have been done on large number of samples with different lithology, different mineral composition, and different permeability. However, there is still no specific technical standards to refer to as which kind of scanning method to choose, how high the resolution to use, how big the sample to cut, and whether the targeted porous medium component is representative. This study used different methods to scan shale, sandstone, and carbonate rock samples, and the validity of scanning methods, resolution, sample size and targeted component is analyzed. Results show that, the resolution should be determined according to the major distribution range of pores. Porous medium components with diameters less than 10 voxels cannot be effectively described in shape or in actual size. In terms of sample size, the length of the sample cube should be no less than 500 voxels to ensure a valid sample space, regardless of the scanning method and the rock lithology. Within a defined sample space, only the distribution of porous medium component with the diameter smaller than 1/5 cube length demonstrates certain regularity. These validity criteria should be applicable to digital rock analysis of other reservoir rock samples with similar lithology.