Page 249 - Well Control for Completions and Interventions

Well Kill, Kick Detection, and Well Shut-In 243

Similarly, when reverse circulating, fluid is pumped down the annulus
and returns taken through the tubing. Pump pressure is a function of:
Surface line friction 1 annulus friction 1 friction through the BHA
1 tubing friction 1 tubing hydrostatic annulus hydrostatic:

If the tubing and annulus HP is balanced (both have the same fluid
density) then pump pressure is equal to friction whilst circulating or
reverse circulating. If tubing and annulus have fluid of different density,
the hydrostatic imbalance will register as an additional pressure increase
over and above friction pressure, or decrease below friction pressure.
When circulating or reverse circulating, pressure acting against the
formation is higher than under static conditions, owing to the addition of
friction pressure. This can be calculated as follows.
BHP during conventional (forward) circulation:

(7.1)
BHCP 5 HP 1 SP 1 FP ann
where:
BHCP: bottom hole circulating pressure;
HP: hydrostatic pressure;
SP: surface pressure;
FP ann : friction pressure in the annulus.
BHP during reverse circulation:

BHCP 5 HP 1 SP 1 FP tbg (7.2)
where:
BHCP: bottom hole circulating pressure;
HP: hydrostatic pressure;
SP: surface pressure;
FP tbg : friction pressure in the tubing.
During drilling operations, it is normal to carry out slow circulation
rate tests to accurately establish frictional pressure drop. The information
obtained would be used if a well kill had to be performed. When plan-
ning a workover, it is unlikely that empirical data will be available and so
for planning purposes, hydraulic modeling software can be used to esti-
mate bottom hole circulating pressure and frictional pressure drop.

7.3.2 Equivalent circulating density
Reservoir engineers will typically give reservoir pressure as a pressure,
i.e., psi, kPa, or bar. Drill crews will normally convert a given pressure to