BAKER HUGHES - Drilling Fluids Reference Manual

HYDRAULICS
Swab and surge pressures can be calculated by a method similar to that used for calculating annular
circulating pressures. The greatest problem is determining fluid flow rate in the annulus when the
pipe is open-ended since the distribution of flow between pipe bore and annulus cannot be found
by any simple method. However, assumptions can be made which will simplify the analysis.
If an assumption is made in which the pipe is closed, or the entire fluid flow rate is in the annulus,
then the analysis will lead to swab or surge pressures equal to or probably exceeding the true swab
or surge pressures.
Alternatively, to assume the pipe is open-ended and fluid levels in pipe and annulus remain equal is
a rarely justifiable and subsequent swab and surge pressure calculated will usually be too low.
Another alternative procedure considers the pipe bore and annulus where a U-tube fluid level in the
pipe and annulus are not equal – where the sum of pressure losses in the pipe bore and bit equals
the sum of pressure losses in the annulus. This procedure requires a trial and error solution due to
the uncertainty of the fluid level in the pipe bore.
Finally, when calculating swab or surge pressures, the calculated value should be checked against
the pressure required to break the gel strengths of the fluid. If the calculated swab or surge pressure
is less than the sum of the pressures required to break the gel strengths, the gel-breaking pressure
should be used.
ADVANTAGE SM Engineering contains swab and surge calculation modules. This is Baker Hughes
Drilling Fluids’ recommended tool for performing swab and surge analysis. The discussion below
explains some of the background to swab and surge calculations.
Equivalent Fluid Velocity
When moving the drillstring through a stationary fluid column, an equivalent fluid velocity due to
the speed of pipe movement can be calculated. Since the combination of fluid movement and
viscous drag has an effect on the swab or surge pressure, Burkhardt's constant is introduced in
order to account for the viscous drag effect. It is common practice to assume Burkhardt's constant
is equal to 0.45, as any error introduced by this assumption will be on the side of safety.
To find the equivalent fluid velocity in the annulus:
where:
⎛ (D
V m = 0.45 + -------------------------------- 1
⎜
--- 2 2⎟ ⎞ V p
⎝ ( D 2) – ( D 1) ⎠
) 2
V m = equivalent fluid velocity, ft/min
D 1 = outside diameter of the pipe, in.
D 2 = hole diameter, in.
V p = average maximum speed of pipe movement, ft/min
The average speed of pipe movement, V P , can be determined by one of the following
methods:
1. Measurement of the time required to run or pull one stand of pipe from slips to slips, divided
into 1.5 times th e stand length.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 9-21