Page 280 - Drilling Technology in Nontechnical Language

Chapter 11 – WELL CONTROL 271

This density is consistent with an influx of gas. An oil influx would

have a greater density (0.3 to 0.4 psi/ft), and a saltwater influx would be
even more (around 0.47 psi/ft). Therefore, the difference in annulus and

drillpipe pressures, together with the volume of the influx, allows the

probable type of influx to be calculated. It is important in particular to

identify whether the influx is gas or not because this affects the well kill
operation. Gas is much harder to handle; as it moves up the annulus, its
pressure will drop. As the pressure drops, the volume will increase as

predicted by Boyle’s law. Boyle’s law states that for a fixed mass of gas
at a fixed temperature, pressure is inversely proportional to volume. If the

volume is halved, the pressure will double. If this volume expansion is not
allowed to happen, pressures on the well will increase and may exceed
rock strength somewhere.
Next, the pressure in the kicking formation can be calculated.
Calculations of hydrostatic pressure were covered in chapter 1. As the fluid

inside the drillstring should not be contaminated with influx, the drillpipe
pressure (P ) is used to calculate the bottomhole pressure, BHP, which
dp
equals formation pressure.

BHP = Hydrostatic pressure + P = (0.5 psi/ft × 8,000 ft)
dp
+ 500 psi = 4,500 psi

To kill the well and restore primary control, a heavier mud must be
circulated into the well. To give a hydrostatic pressure of 4,500 psi at
8,000 ft, the kill mud density gradient (ρ ) can be calculated. (The Greek
2
letter ρ [rho] is used to signify a density gradient; ρ refers to the original
1
mud density, and ρ to the kill mud density.) This calculation is shown in
2
equation (11.2):

4,500 psi
ρ = ———— = 0.563 psi/ft (11.2)
2 8,000 ft

While preparations are made to kill the well, the mud engineer can
start to add barite to the mud in the active system to increase the density
gradient to 0.563 psi/ft. Meanwhile, some more calculations have to be
made before the kill can start.

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