Loss of Hydrostatic Pressure due to Lost Return

In case of totally lost return, the annulus must be fully filled with fluid, normally water, as fast as we can. Water filled in annulus causes loss of hydrostatic pressure in the wellbore. This post demonstrates how to determine hydrostatic pressure reduction due to fully filling water into annulus.

There are two main concepts, annular capacity and hydrostatic pressure, applied to determine loss of hydrostatic pressure.

Please following concepts below.

Number of feet of water in annulus

Ft of water added = water added in bbl ÷ annular capacity in bbl/ft

Bottomhole (BHP) pressure reduction

In order to calculate bottom hole pressure reduction, we assume the column of water in annulus is true vertical depth.

BHP decrease in psi = (current mud weight in ppg – weight of water in ppg) x 0.052 x (ft of water added)

Equivalent Mud Weight at TD

EMW in ppg = current mud weight in ppg – (BHP decrease in psi ÷ 0.052 ÷ TVD ft of hole)

Example: Determine bottom hole pressure loss and equivalent mud weight at TD due to filling up water into annulus.

Mud weight = 13.0 ppg
Water added = 140 bbl required to fill annulus
Weight of water = 8.6 ppg **
Annular capacity = 0.1422 bbl/ft
Hole TVD = 6,000 ft

** If you fill lighter mud in hole instead of water, please adjust water weight to your mud weight.

Number of feet of water in annulus

Feet of water in annulus = 140 bbl ÷ 0.1422 bbl/ft
Feet = 984.5 ft

Bottomhole (BHP) pressure reduction

BHP reduction = (13.0 ppg – 8.6 ppg) x 0.052 x 984.5 ft
BHP reduction = 225.3 psi

Equivalent mud weight at TD

EMW in ppg = 13.0 – (225.3 psi ÷ (0.052 x 6,000 ft))
EMW = 12.3 ppg

Please find the Excel sheet for calculating how much pressure loss due to lost return