Has the drilling engineer at Invictus evaluated if there is...

Has the drilling engineer at Invictus evaluated if there is scope for decreasing the mud weight below 9.5 ppg in a rigorous way?

For example, at 3400m MD the expected TVD is 3363m (11033.5 ft TVD)

The mud pressure at that depth with 9.5 ppg (0.494 psi/ft) mud in the hole is 5450 psia.

How much mud overbalance is included in the design? 200 psi? However, when Invictus drilled M-2 they did not know exactly how mud overbalance was being applied.

If it is 200 psi, it will mean the pore pressure has to be less than 5250 psia.

In addition, there is an unknown brine pressure gradient because Invictus do not yet have an accurate formation brine density or brine resistivity (related to total brine solids content). The difference between gas pressure and brine pressure is the capillary pressure, Pc.

Pc = Pg – Pw, could be higher than normal even without over-pressure because the porosity is lower at that depth. Pc could be about 200 psi or 300 psi, maybe more?

If we assume that the Pc is 250 psi, it will mean 5250 psia – 250 psi = 5000 psia due to the brine gradient.

5000 psi corresponds to a brine gradient of 0.453 psi/ft at 11033.5 ft TVD.

So, when you look at different scenarios is there really scope for reducing the mud weight below 9.5 ppg?

In my opinion reducing the mud weight is risky. If you reduce the mud weight to 9.4 ppg (5393 psi) from 9.5 ppg, the mud overbalance would be 143 psi instead of 200 psi, but if the brine gradient is 0.46 psi/ft, that would mean (5393 – 5325) only 68 psi overbalance, and if the brine gradient is 0.465 psi/ft, (5393 – 5381 psi) the well would be almost “on balance” – a very risky situation.