A comparative study of ion diffusion during water imbibition in shale, sandstone and volcanic rock

The recovered fracturing fluid is generally high in salinity (close to 200 kppm), which is related to the diffusion of salt ions from shale reservoir to the fracturing fluid. However, it is not clear about the diffusion capacity of salt ions in different types. In this paper, the shale, tight volcanic and sandstone are selected as comparative study and a series of tests are carried out for the porosity, permeability and mineral composition. The results show that the shale immersing in fracturing fluid will cause ions dissolution and diffusion, which will increase the salinity of the fracturing fluid. The solution salinity increases rapidly in the early stage and gradually slows down in the later stage. The salinity of the fracturing fluid has a linear relationship with the square root of time, so the slope of the curve can be used as a characteristic parameter to evaluate the ion diffusion rate. The process of dissolution and diffusion of salt ions will induce the expansion of micro-cracks, increasing the contact area between the fracturing fluid and shale and enhancing the solution salinity. The ion diffusion rate is positively related with the content of clay minerals and carbonate. The soluble ions include mainly ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}\mathrm{-}}$, ${\mathrm{Ca}}^{\mathrm{2}\mathrm{+}}$, ${\mathrm{Na}}^{\mathrm{+}}$ and ${\mathrm{K}}^{\mathrm{+}}$. The ${\mathrm{Na}}^{\mathrm{+}}$/ ${\mathrm{Cl}}^{\mathrm{-}}$ ratio is closely related to the content of clay minerals and carbonate minerals. It has a positive correlation with content of illite and chlorite, and a negative correlation with carbonate minerals, suggesting NaCl source from illite and chlorite. This study is significant for understanding the salinity characteristics of recovered fracturing fluid and evaluating the fracture network shape.