Make Sure Insufficient Circulating Time Before Making a New Connection

When carrying out any form of drilling it is critical that you know how fast the cutting are travelling up the annulus or as it is properly described, knowing your “Up Hole Velocity” (UHV).

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We are always drilling with mud pumps that are too small for the diameter that we want to drill. For example a very common size of mud pump used is the 5” x 6”, which has a 5” diameter piston and a 6” stroke. This pump will deliver 580 liters per minute at maximum rpm. So for practical purposes you are looking at about 80% of this figure which will be around 464 liters per minute. If we are drilling a 12-1/4” hole with 3-1/2” drill rods and the pump is producing 464 liters per minute then the UHV of the fluid rising up the annulus will be 20ft per minute or 6 meters per minute.

To calculate your annular volume per meter you need to deduct the volume of the drill rods from the volume of the hole. So now you have an idea of the speed at which the mud is travelling up the hole. If you are unsure of what capacity your mud pump is giving then measure the flow coming out of the bore hole by timing how long it takes to fill a container with a known volume in liters.