Page 161 - Introduction to Petroleum Engineering

148 DRILLING

In most drilling cases, the operator chooses to pump mud of density sufficient that
wellbore pressure exceeds formation pressure. This “overbalanced” condition pushes
wellbore fluids into the formation. Larger particles in the mud cannot penetrate the
pores of the formation, and so they collect on the wall of the bore forming mud cake.
Liquids and very small particles in the mud can penetrate the formation and can alter
the properties of the rock adjacent to the well. An operator may choose to drill under-
balanced by pumping low‐weight mud. In this case, formation fluids will move into
the wellbore. If mud weight is too low, a blowout or sloughing and collapse of the
formation can occur. If mud weight is too high, unintended fractures and mud loss
into the formation can occur.
If a well is left inactive or shut‐in, the solid particles in the drilling mud can settle
to the bottom of the well and may harden. This process is called “sagging.” If drill
pipe is still in place, the hardening mud may lead to sticking.

example 8.3 Mud Volume in Pipe
What volume of mud is needed to fill a section of pipe with inner diameter
.
3
ID = 35 in. and length L = 6000 ft? Express your answer in ft .
Answer
ID
2
V = π r L, ID = 0 2917. , ft r = = 0 1458. ft
2
2
2
V = π r L = 3 14159 0 1458. ( . ft ) 66000ft = 400 9ft. 3

8.2.6 Well Control System

Mud of proper weight is the first defense against blowouts. Other parts of the well
control system are the blowout preventer (BOP), the kill line, and the choke line and
choke manifold. The BOP is used to shut in the well in emergencies. As shown in
Figure 8.9, the BOP is a stacked sequence of two to four hydraulically actuated
valves or preventers. The top preventer is an annular preventer, which functions
much like a rubber sleeve for measuring blood pressure. The annular preventer can
squeeze around the drill pipe or the kelly to close the annular space. The next pre-
venter is a pair of pipe rams that slide from opposite sides of the BOP to close around
a pipe. The half‐circle sealing elements on the pipe rams must have the same diam-
eter as the pipe in order to properly seal. Blind rams are designed to close an open
hole and cannot shut the well if the pipe is in the hole. The final preventer is a shear
ram. Shear rams are designed to cut any pipe in the hole and seal the well. The
drilling crew installs the BOP on top of the casing head, which is attached to the top
of the surface casing after it is cemented in place.
In addition to the BOP, a kill line and a choke line are connected at the wellhead.
The kill line can be used to perform well integrity tests and to inject high‐density
mud into the wellbore to block fluid flow up the wellbore. The choke line has a choke