Numerical simulation of formation water salinity redistribution in fractured shale reservoirs during hydraulic fracturing

Using numerical simulation methods to predict the distribution of salinity in reservoirs during hydraulic fracturing is challenging due to the dynamic propagation of hydraulic fractures (HFs), random distribution of natural fractures (NFs), multiphase heat and mass transfer behavior, and various connotative mechanisms. In this study, to accurately predict changes in matrix salinity during hydraulic fracturing, a multiphase heat and mass transfer model of HFs, the matrix, and NFs was derived based on the principles of material and energy conservation, the electro-chemical potential, and the embedded discrete fracture model (EDFM). The model considers the dynamic propagation of HFs, multiphase flow, chemical osmotic pressure, capillary pressure imbibition, heat conduction, and NFs. The prediction model of solute salinity developed in this study is then verified using previous findings. Finally, the effects on the salinity distribution within the matrix by volume differences of injected fracturing fluid, salinities of the fracturing fluid, number of NFs, temperatures of fracturing fluid, and mechanisms were analyzed. The results show that the injection volume and temperature of the fracturing fluid, and the number of NFs are positively correlated with the decreasing range of salinity in the matrix and damage range of clay swelling, while the salinity of the fracturing fluid is negatively correlated with the decreasing range of salinity in the matrix and the damage range of clay swelling. During fracturing, the random distribution of NFs and local strong heterogeneity hinder prediction and cause strong heterogeneity in the distribution of salinity, pore pressure, and matrix temperature. At the same time, the pressure gradient, chemical osmotic pressure, and imbibition of capillary pressures under mixed wetting decrease salinity within the matrix and aggravate clay swelling damage. Of the three mechanisms affecting the salinity distribution within the matrix, the pressure gradient has the greatest contribution, followed by osmotic pressure and capillary pressure imbibition. The results of this study provide insights into the theoretical prediction of salinity distribution in the matrix after fracturing and hence the design of HFs.