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Mud Hydraulics Fundamentals 51

Illustrative Example 2.8
Calculate the surge pressure generated by a 10¾-in casing string under the
following conditions, and predict whether the total borehole pressure will
exceed the formation fracture gradient. Assume that the casing is effectively
“closed” with a float shoe and laminar flow in the annulus.
Casing depth: 6,400 ft (1,951 m) TVD
Fracture gradient: 0.82 psi/ft
Hole diameter: 14¾ in
Mud weight: 15.5 ppg
Plastic viscosity: 37 cp
Yield point: 6 lb/100 ft 2
Pipe velocity: –110 ft/min (the negative sign denotes downward velocity)
Solution

2
v m = − −110 10:75 = 2:08 f t/s
2
60 14:75 − 10:75 2
10:75

κ = 0:275 + 0:25 = 0:45
14:75
110

v e = 2:08 − 0:45 = 1:25 f t/s
60
Use the annular flow pressure loss equation for the laminar flow:
" #
Δp f = ð37Þð1:265Þ + 6 ð6,400Þ = 67 psi
2
200ð14:75 − 10:75Þ
1,000ð14:75 − 10:75Þ
The equivalent mud weight (EMW) is
EMW = 15:5 + 67 = 15:7 ppg
0:052ð6,400Þ
The equivalent mud weight of the fracture gradient (EMW f )is
EMW f = 0:82 = 15:8 ppg > 15:7 ppg
0:052
Therefore, the borehole will be safe during running the casing at this depth.

Illustrative Example 2.9
Using the Bingham plastic model, calculate the swab pressure generated by a
10-¾-in casing string under the following conditions, and predict whether the
(Continued )