Page 157 - Drilling Technology in Nontechnical Language

148 Drilling Technology in Nontechnical Language Second Edition

while not exceeding the optimum drilling parameters so that the bit drills
for a reasonable time.
The procedure for optimizing the drilling parameters is called a drilloff
test.

Effect of mud hydrostatic pressure on ROP

Normally, the hydrostatic pressure of the mud is greater than the
formation pore pressure. This is deliberate to maintain control of the well.
This overpressure gives rise to a condition known as chip hold down.
As a tooth penetrates into the formation, a chip of rock is dislodged.
Underneath the chip is the native formation pore pressure, and above it
is mud hydrostatic pressure. This pressure differential tends to hold the
chip in place. If this happens and the chip’s departure from the bottom is
delayed by a small fraction of a second, the next tooth down may hit the
chip instead of virgin rock. The overall effect is to slow the progress of the
bit. In practice, with tricone drill bits, it can be seen that there is in fact a
strong correlation between mud overbalance and rate of penetration.
This phenomenon can be turned to advantage. If the bit penetrates a
region where pore pressures start to increase, the rate of penetration will
also increase. As the natural tendency of a bit in a particular formation is to
decrease the rate of penetration with depth, an increase (unless explained
by a change in lithology) is one signal that a kick might be imminent, and
it should be investigated.
PDC bits are less affected by chip hold down. Thus, in exploration
wells, tricone bits are preferable so that changes in the pore pressure regime
can be identified.

Effect of mud solids content on ROP

A low solids content mud will give a better rate of penetration in a
similar fashion to low mud overbalance. The reason is not clear, but it is
speculated that the mud solids may slow down the equalization of pressure
under the chip. With a solids-free system, the drilling fluid can more easily

penetrate past the chip to reduce the hold down effect.

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