The predominant solute in most formation water is sodium chloride (NaCl). Its concentration determines formation water density and R<sub>w</sub>. When only R<sub>w</sub> is available, we can use NaCl concentration to determine density. | The predominant solute in most formation water is sodium chloride (NaCl). Its concentration determines formation water density and R<sub>w</sub>. When only R<sub>w</sub> is available, we can use NaCl concentration to determine density. |