Bottom hole pressure is reduced due to swabbing effect.
Bottom hole pressure is increased due to surging effect.
The calculation steps are as follows:
1. Determine pressure around drill pipe
2. Determine pressure loss around drill collar
3. Determine total pressure loss by summation of the figures in the 1st and 2nd step
4. Determine surge and swab pressure
The First Step: determine pressure loss around drill pipe.
1. Determine n:
Where:
n is the power law exponent.
Θ600 is a value at 600 viscometer dial reading.
Θ300 is a value at 300 viscometer dial reading.
2. Determine K
Where:
K is the fluid consistency unit
Θ300 is a value at 300 viscometer dial reading.
n is the power law exponent.
3. Determine fluid velocity around drill pipe
For closed-ended pipe (plugged flow):
For open-ended pipe:
Where:
Vdp is the fluid velocity around drill pipe in ft/min.
Dp is drill pipe diameter in inch.
Dh is hole diameter in inch.
Di is inner diameter of drill pipe in inch.
4. Maximum pipe velocity
Vm = Vdp x 1.5
Where;
Vdp is the fluid velocity around drill pipe in ft/min.
Vm is maximum pipe velocity.
5. Pressure loss around drill pipe
The Second Step: determine pressure loss around drill collar.
You also need to consider pressure loss around drill collar or BHA as well because they have different OD which sometimes creates significant surge/swab pressure.
1. Determine fluid velocity around drill collar
For close-ended pipe (plugged flow)
For open-ended pipe
Where:
Vdc is the fluid velocity around drill collar in ft/min.
Dc is drill collar diameter in inch.
Dh is hole diameter in inch.
Dci is inner diameter of drill collar in inch.
2. Maximum pipe velocity
Vm = Vdc x 1.5
Where:
Vdc is the fluid velocity around drill collar in ft/min.
Vm is maximum pipe velocity.
3. Pressure loss around drill collar
The Third Step: Total Pressure Loss = Pdp + Pdc
The Final Step:
For surge pressure, you need to add this figure to hydrostatic pressure of mud.
Bottom hole pressure = Hydrostatic pressure + Total Pressure Loss
On the other hand, if you want to get swab pressure, the pressure sure must be subtracted from the hydrostatic pressure.
Bottom hole pressure = Hydrostatic pressure – Total Pressure Loss
Where: n is the power law exponent.
Where:
Where:
Where:
