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Mud Hydraulics Optimization 97

in Figure 4.7. It shows that the pressure drop at the bit is always the
major component.
When a drilling string or casing is moved, it displaces the mud in the
hole, which leads to pressure variations. A pressure increase due to a
downward pipe movement is called surge pressure, whereas a pressure
decrease due to an upward pipe movement is called swab pressure.
Excessive swab pressure may initiate a kick, while surge pressure is detri-
mental in that it frequently is large enough to fracture a formation. This
is particularly true for ERD wells, slim holes, and deepwater offshore

Pump P Maxi. Pump P.
25000

20000
Pump Pressure (psi) 15000

10000

5000

0
200 300 400 500 600 700 800
Flow Rate (gpm)
Figure 4.6 Pump pressure versus mud flow rate.

Pipe Annulus Bit
20000

Pressure Drop (psi) 15000

10000

5000

0
200 300 400 500 600 700 800
Flow Rate (gpm)
Figure 4.7 Pressure components versus mud flow rate.