Page 159 - Introduction to Petroleum Engineering

146 DRILLING

penetration (ROP). Replacing a bit is costly, so wise operators attempt to extend bit
life by managing mud composition and circulation rate, as well as bit revolutions per
minute (RPM) and weight on bit (WOB).
WOB is measured using sensors on the rig floor and in some measurement while
drilling (MWD) tools in BHA. With the drill string off bottom and rotating, the rig‐
floor WOB sensor is zeroed. Then as the rotating drill string is lowered to touch
bottom, the sensor reads the actual WOB. Drillers perform tests to find the WOB that
maximizes the ROP. Bit manufacturers specify a maximum WOB for avoiding
damage. High WOB and low rotation speed can cause the bit to stick and slip causing
damaging vibrations. Low WOB and high rotation speed can cause the bit to whirl
without cutting yet causing more damaging vibrations. The stable zone for “smooth
drilling” has moderate WOB and rotation speed. The optimum WOB and rotation
speed depend on the type of bit, the BHA, the mud weight, and the rock being drilled.
Replacing the bit (or any part of the BHA) is a time‐consuming and costly process.
It requires pulling the entire drill string out of the wellbore (tripping out), changing
the drill bit, and lowering the drill string back into the wellbore (tripping in).
Depending on the skill of the crew, trip time is about one hour per thousand feet of
pipe. For an 8000 ft drill string, the trip time would be about 8 hr each way. And to
complicate matters, pressure kicks are more likely to occur during tripping.
Efficient drilling is reflected in the cost per foot drilled. To minimize that cost,
rig downtime must be kept to a minimum. Downtime includes time for repair of
power generators and mud pumps, trips for bit replacement or other changes to the
drill string, or delays for stuck pipe or lack of inventory. With experience drilling in
an area, operators learn how to maximize ROP and bit life. Managing a rig to
minimize cost is a complex task that requires a wide variety of skills, including
organizational skills.

8.2.5 Circulation System
A system for circulating mud is illustrated in Figure 8.8. It is a continuous system so
that the operator can reuse the mud. The mud is mixed in an aboveground tank and then
pumped through the kelly and down the drill pipe. The mud passes through nozzles on
the drill bit and up through the annular gap between the drill pipe and the rock wall of
the wellbore. Rock cuttings are carried up with the mud to a shaker on the surface
where a well site geologist can evaluate samples. The rock cuttings provide information
about the geologic environment. The mud falls through the shaker to the first in a series
of mud tanks, separated by baffles. Pumps (not shown) circulate mud through the
desander and desilter to remove undesired solids. Finally, the reconditioned mud is
returned through the swivel and kelly and to the top of the drill string.
Drilling mud has several functions. It lifts cuttings and contents of drilled
formations to the surface, controls formation pressure, lubricates the drill string and
bit, cools the bit, mechanically supports the wellbore, and transmits hydraulic power.
The mud can also prevent movement of fluids from one formation to another.
There are several types of drilling mud. The choice depends on performance,
environmental impact, and cost. The composition and properties of mud used to drill