# Circulation Rategallons Per Minute

Fig. 7.11. Annular pressure drop for turbulent flow of 3 cp, 9.5 lb/gal mud *(pipe sizes). Courtesy Hughes Tool Company.

(b) Rework (a) including the effect of 500 ft of 74 inch o.d. drill collars on the bottom of the drill string.

Length pipe in hole = 9500 ft

Pipe size = 44 in. o.d. (ignore drill collars) The pipe is being lowered at 400 ft/min.

Solution:

Solution:

4001 60

• 1.08) (35) + 1.08V (35)2 + (9.3)(10)(4.5)2(15)
• 10)(4.5)
• 5.0 ft/sec FromEq. (7.16):

400/1 (4.5)' \ " 60 \2 (9)2 - (4.5)7 flow is turbulent.

### Apap for turbulent condition: R = (2970) (10) (5.6) (4.5) = ^ m e 35

• 0.0084 from Figure 7.1, curve IV _ (0.0084) (10) (9500) (5.6)2 = 21g due to friction aPap (25.8) (4.5)
• 0.0083 _ (0.0083) (10) (500) (18.5)2 Pac ~ (25.8) (1.5)

The total instantaneous bottom hole pressure is: p = 215 + (0.052) (10,000) (10) = 5415 psi

### Pressure surge = 203 + 370^ 570 psi Total p = 570 + 5200 = 5770 psi

• c) What would the instantaneous bottom hole pressure be if the pipe were being removed at the same rate as in parts (a) and (b)?
• 1) p = 5200 - 215 = 4985 psi.
• 2) p = 5200 - 570 = 4630 psi.

In other words, the running pressure and pulling suction are equal but of opposite sign.

2000 6000 10,000 14,000 18,000

Well depth, ft

Fig. 7.12. Circulating and static wellbore temperatures for Gulf Coast wells of various depths. Courtesy Halliburton Oil Well Cementing Company.

These problems serve to illustrate the potential hazards of lowering or removing the drill string too fast. The importance of such calculations lies not in the precise magnitude of the numerical answers but in the realization of the problem which is involved. Slowing of pipe movement means longer trip time, but in many areas such action is well worth this expense. It should be noted that ignoring the presence of drill collars caused an error of approximately 100%. Closer hole-collar clearance will greatly increase this error, since turbulent pressure loss varies as v2. Note also that Ap varies a» the length of pipe in the hole. Consequently the string may be pulled (or run) faster as length decreases. However, once the drill collars are in the hole, that section exerts a large and constant effect.

The use of yield point in the previous calculation may also introduce errors, particularly for muds which exhibit large gel strengths with quiescent time. A more realistic value would be the actual gel strength at the time involved, if it were known.

7.8 Air, Gas and Aerated Mud Drilling

Air Requirements

The circulation volume required in air or gas drilling is governed by the annular velocity necessary to lift the

 Average temperature of Gulf Coast wells / T 7 / t A 1 f A f 3 / r O f / oí / f / *1 / / / f t t 9> / r / * t > f 4 i f c<*

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