Quantitative description of shale pore structure using image analysis and determination of controls on shape, size and orientation complexity

Abstract

Pore structure characteristics are important aspects of shale reservoirs. Shale samples were collected from the Taiyuan and Shanxi Formations from the WX-1 well in the Qinshui basin, Shanxi, China. Pore morphology was analyzed using argon ion polishing scanning electron microscopy (Ar-SEM) and focused ion beam scanning electron microscopy (FIB-SEM). Subsequently, the structural pore features in the images were quantitatively characterized using the pores (Particles) and cracks analysis system (PCAS) method. The results show that the sample pores are primarily organic pores, intergranular pores, and intragranular pores, and the pore sizes are concentrated at less than 100 nm. In addition, the following rules are determined: first, with the pixel accuracy (PA) increase on the same pore, the pore edge becomes smooth, the degree of pore orientations (DPO) improves, and the apparent pore structure complexity (APSC) decreases. In addition, when the pore edge roughness increases and the DPO decreases, this effect becomes larger to change PA. Second, when the pore shape tends to be “round,” the edge becomes smooth, and APSC decreases; when the pore tends to be “narrow,” the edge becomes rough, and the APSC increases. Third, with the increase of depth, the pore size and DPO first increase and then decrease, and the APSC decreases gradually; with the increase of maturity, the pore size first decreases and then increases. Conversely, the APSC first increases and then decreases when the turning point occurs in the high-mature to over-mature stages of the transition, and the DPO improves.