What mud density (ppg) is required to prevent flow from a permeable formation at 12200 ft with pore pressure 8247 psig?

• A. 10
• B. 13
• C. 12
• D. 15

Answer: (B)

Balancing formation pore pressure with the mud’s hydrostatic pressure at that depth is what this is testing. The mud’s hydrostatic pressure at depth is P = 0.052 × MW × depth, with MW in pounds per gallon and depth in feet. To prevent flow, set this equal to the pore pressure and solve for MW: MW = Ppore / (0.052 × depth).

Plug in depth = 12200 ft and Ppore = 8247 psi: 0.052 × 12200 = 634.4, so MW ≈ 8247 / 634.4 ≈ 13 ppg. Therefore, about 13 ppg mud is needed to balance the pore pressure at that depth, stopping flow. Heavier mud would overbalance; lighter mud would allow flow.