Hydrostatic: 2005 #24 (similar)

Sand at 9,000 ft. Pr = 4,000 psi. Two fluids. Which A-B-C option below is 500 psi underbalanced?

(A) 3,000 ft 0.1 ppf N2 cushion, 6,000 ft 10 ppg, 80-psi surface pressure.
(B) 2,000 ft 0.1 ppf N2 cushion, 7,000 10 ppg, 160-psi surface pressure.
(C) 2,500 ft 0.1 ppf N2 cushion, 6,500 10 ppf, 400-psi surface pressure.

Use the hydrostatic pressure equation (2 DRL 1). Calculating pressures p = 0.052(D)MW) + gradient(D) + surface pressure - reservoir pressure for:

(A) 300 + 3,120 + 80 - 4,000 = -500 psi
(B) 200 + 3,640 + 160 - 4,000 = 0 psi
(C) 250 + 3,380 + 400 - 4,000 = 30 psi

Expect this sort of problem with any fluid, depths, or pressures. Just remember to line them up in an orderly fashion; speed is of the essence.