Chloride is how you catch a salt stringer or a saltwater flow before it wrecks the rheology. The test is a silver-nitrate titration and the maths is short — but the strength of your AgNO₃ decides what the number means.
Why chloride is tracked every tour
Rising chloride in the filtrate means salt is entering the system — from a drilled salt section, a saline formation-water influx, or make-up water. Salt flocculates clay, spikes yield point and can crash fluid-loss control. Trending chloride against depth turns a surprise into an early warning.
How the titration works
A measured volume of filtrate is acidified and titrated with silver nitrate (AgNO₃) against a potassium-chromate indicator. Silver reacts with chloride to form white silver chloride; once the chloride is consumed, the next drop of silver forms red silver chromate and the colour turns. The millilitres of AgNO₃ to the endpoint is your reading. API RP 13B-1 is the method.
The equation
For a silver-nitrate solution where 1 mL neutralises 1 mg of chloride (0.0282 N):
Cl− (mg/L) = (mL AgNO3 × 1000) ÷ mL of sample
Convert chloride to sodium-chloride salinity:
NaCl (mg/L) ≈ Cl− × 1.65
For high-chloride brines a stronger 0.282 N solution is used (1 mL = 10 mg Cl); the factor in the numerator changes accordingly.
A jump in chloride with no planned salt section is the tell-tale of a saltwater influx — treat it as a well-monitoring signal, not just a mud number, and cross-check against pit gain and flow.
Common questions
Why multiply chloride by 1.65 to get salt?
Because sodium chloride is only about 60.7% chloride by mass. Dividing 1 by 0.607 gives roughly 1.65, so multiplying the chloride concentration by 1.65 converts it to equivalent NaCl.
Which AgNO₃ strength should I use?
Use the weaker 0.0282 N solution for low to moderate chlorides and the stronger 0.282 N for high-salinity brines so the titration volume stays in a readable range. Just keep the numerator factor consistent with the strength you used.
