Quantitative characterization of nano-scale pores in shale reservoirs of Wufeng-Longmaxi formation based on image processing

In this paper, we conducted image processing on FE-SEM images of representative shale samples from WX-1 well in the north-east Chongqing area. The structural parameters and proportions of major pore types were determined and analyzed following a workflow consisting of nano-scale images acquisition, image processing, pore-network evaluation and comprehensive analysis. We found four types of major reservoir space in matrix including organic matter pores (OM-Ps), clay mineral related pores (CM-Ps), brittleness mineral related pores (BM-Ps) and micro-fractures (MFs) with different pore size and morphological characteristics. OM-Ps are dominated by micro-meso pores, while mineral related pores have a wider pore size range. Pores on the mineralogical edge are the predominated contributor to the connectivity of pore network in shale matrix with wide spatial distribution and relative larger size in meso-macro pore scale. Vertical variation in pore structure, morphology and proportions are measured by parameters of plane porosity, average pore size, form factor, extensibility and eccentricity, indicating that the high-quality shale reservoirs of Wufeng to bottom Longmaxi formation are developed with the highest values of OM-Ps proportion (49%-68%) and form factor. A workflow was summarized to make pore-network characterization for shale matrix based on image processing, which is capable of making characterization of different types of pores and determining the quantification of pore proportions. Quantitative characterization method based on image processing was summarized to provide researchers with new approaches to make pore structure quantification and reservoir quantitative evaluation on nano-scale pores