Water phase salinity is a factor showing the activity level of salt in oil based mud. In order to control the water phase salinity, salt is added into the drilling fluid. The salt added into the system will be dissolved by water in the mud; therefore, the chloride content will increase.
By increasing the chloride concentration (adding salt), the activity level in the mud will decrease. Salt is added in order to create an activity level which is equal to or less than formation water. Therefore, the water phase in the mud will not move into formation and cause a clay swelling issue. Practically, calcium chloride (CaCl2) or sodium chloride (NaCl) is the chemical to be used.
When salt must be added into the mud system?
Typically, while drilling with oil based mud, cuttings are generally dry, hard, and easy to break into pieces. However, if the cuttings come together in big pieces and are wet, it may increase salt content in the drilling fluid. The reason is that water in the mud moves into formations and swells the clay particles in the formation. Swelled clay causes wet and mushy cuttings.
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Ryen Caenn, 2011. Composition and Properties of Drilling and Completion Fluids, Sixth Edition. 6 Edition. Gulf Professional Publishing.