Page 69 - Drilling Technology in Nontechnical Language

60 Drilling Technology in Nontechnical Language Second Edition

the mud flow has to fill 50 ft (15 m) of hole each minute. The capacity

of 50 ft (15 m) of this annulus will be 50 ft × 26.6 gal/ft = 1,330 gal
(15 m × 329 l/m = 4,935 l). In this large hole, at least a rate of 1,330
gallons per minute (GPM), or 4,935 l, is required to give the minimum AV

of 50 FPM (15 m/min). In our pilot hole of 12¼" (311 mm), the flow rate
is much less; for 50 FPM (15 m/min), only 255 gal (965 l) are needed, and
the preferred 100 FPM (30 m/min) is easily achieved. In soft rock, up to
a limit, the more GPMs pumped, the faster drilling can progress. Thus, a

12¼" (311 mm) diameter hole is generally drilled with a flow rate of around
600 GPM (2,271 l).
Several things limit the volume of mud that can be pumped. One limit
is how fast the pumps can go at the pressure required.
Another is that if pumping very fast in softer rock, the force of the mud
hitting the bottom of the hole below the bit can cause erosion of the side of
the hole, making it bigger than the drillbit diameter.

Fig. 3–13. Circulating mud around the well

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