AN ESTIMATION ON THE PERMEABILITY OF TIGHT SANDSTONE BASED ON PORE MICROSTRUCTURE AND FRACTAL ANALYSIS

Using fractal dimension to calculate the permeability of sandstone is still a challenging technique. A more rigorous and systematic approach is tested in this study. Ten Mid-Permian coal bed associated sandstone samples of Shanxi Group in the North China Basin are used to test the method. The sandstone has been modified by diagenetic processes and the original pore space was mostly destroyed. 98% of the current pore space is now diagenetic in origin. Instead of treating all the secondary pore space as one type, the host of pore space are identified into three different categories by origin: scattered authigenic kaolinite; mechanically deformed phyllite rock fragment; and dissolution holes in feldspars. Each category has characteristic pore geometry and should give different fractal dimensions. The permeability of the rock should also be evaluated as a weighted sum given by the three categories.

The pore geometry of each category is observed under SEM. Parameters in the same type of pore are quantified over all samples under magnification from 200 times to 20000 times. In total, 150 SEM images were taken and 42 images which show good pore geometry were selected for measurement and calculation. ImageJ developed by NIH is used to measure and calculate the number of pores, pore area and pore perimeter in SEM images. Fractal dimension is calculated by the 2D perimeter-area method. Porosity and permeability obtained from individual image are weighted by critical parameters, and are averaged to be the representative porosity and permeability of each pore category. Then the value of porosity and permeability of a rock are calculated according to the abundance of host minerals evaluated from the thin section of a rock sample. Calculate results are checked by experimental measurements obtained from nitrogen gas porosimetry. The calculated values are constantly, but proportionally higher than the measured values. Factors that caused the discrepancy are investigated and future work is outlined to refine the measurement and calculation procedures.