BAKER HUGHES - Drilling Fluids Reference Manual

HYDRAULICS
∑P a = sum of the pressure loss in each annular interval, psi
TVD = true vertical depth, ft
If the equivalent fluid weight due to the surge pressure exceeds the fracture gradient, at some point
in the open hole, loss of circulation may occur. Likewise, if the equivalent fluid weight due to swab
pressure is less than the formation pore pressure, at some point in the open hole, a kick, or even a
blowout, may occur.
These preceding steps and equations are useful approximations for surge and swab calculations
when the time required running or pulling pipe can be measured. For planning purposes, it would
also be useful to calculate the time required so as not to exceed a fracture gradient or pore pressure.
This type of calculation is done through iterations of time (t) and is one of the programs contained
SM
within ADVANTAGE Engineering.
Gel-Breaking Pressure
When pipe is started back in the hole after a trip, the fluid will have been at rest for some period of
time. The pressure required to break the downhole gel strength of the fluid can be significant,
especially if the gel strengths are progressive. The primary reason for measuring 30-minute gel
strength is to determine the progressive or fragile nature of the gel strengths. If the gel strengths are
significantly high and progressive, the resulting surge or swab pressure could lead to well control
problems.
Gel strengths are a function of many factors, including time and temperature. By utilizing the 30-
minute gel strength, the time factor can be partially taken into account for fragile gels, as long as
the fluid is temperature stable at the bottomhole temperature. If the calculated swab or surge
pressure is less than the sum of the gel-breaking pressures for each section of annulus, the gelbreaking
pressures should be used.
To find the gel-breaking pressure:
P g
4Lτg
= -----------------------------------
1200( D – D )
2
1
where,
P g = gel breaking pressure, psi
L = Length of annular section, ft
τ g = 30-minute gel strength, lb f /100 ft 2
D 2 = hole diameter, in.
D 1 = outside pipe diameter, in.
The equivalent fluid weight is calculated using the earlier equations.
CUTTINGS TRANSPORT
One of the primary functions of a drilling fluid is to bring drilled cuttings to the surface. Inadequate
hole cleaning can lead to a number of problems including fill, packing-off, stuck pipe, and
excessive hydrostatic pressure.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 9-23