Influence of rock fabric on salt ion diffusion behavior in upper cretaceous lacustrine shale from Songliao Basin

Abstract

Some shale reservoirs have high salt ion concentration in flow-back fluid, about which the mechanism is still unclear. To learn the rock fabric of lacustrine shale and its influence on the salt ion diffusion behavior, the investigation of rock component, pore structure, spontaneous imbibition and salt ion diffusion were conducted. Rock component includes inorganic minerals and organic matter. Pore structure includes overburden porosity-permeability, scanning electron microscope (SEM), nitrogen adsorption (NA), mercury intrusion porosimetry (MIP). Comparing the pore size distribution between NA and MIP, lots of pores are connected by small throat, and the pore volume controlled by throat can be 18 times of throat volume. Based on fractal dimensions analysis, which was defined by using the fractal Frenkel-Halsey-Hill (FHH) method based on NA, lacustrine shale has a smoother pore surface and a complex pore structure. The fractal dimensions are mainly controlled by specific surface area. Imbibition liquid is a medium for salt ion diffusing out of matrix and the salt ion diffuse nearly linearly with time in log scale after imbibition becomes stable. The slope of fitting line by using late diffusion data is defined as salt ion diffusion rate. The salt ion diffusion rate is mainly controlled by clay content, TOC, pore volume and specific surface area. The interaction between clay and liquid has the potential to release lots of salt ion, and a higher specific surface area provide more area for liquid interaction with minerals, and a higher pore volume provide more channel for salt ion diffusing out of sample. Our research is conducive to understanding the rock fabric of lacustrine shale in Qingshankou Formation and its influence on salt ion diffusion behavior.