Page 160 - Introduction to Petroleum Engineering

ROTARY DRILLING RIGS 147

Mud ows from the pump to the swivel, then
to the kelly, and down the drill string

Shale Desander Desilter
shaker cyclones cyclones
Mud
pump

Cuttings Sand Silt
to pit to pit to pit

Drill pipe Annulus
Bit

FIGuRe 8.8 Illustration of the circulation system.

TABLe 8.1 Drilling Mud Density
Component Density
Water 8.3 lbs/gal (1 g/cm or 62.4 lb/ft )
3
3
Unweighted mud 9–10 lbs/gal
Barite‐weighted mud 11–20 lbs/gal

the first well in a new region are usually an experiment. Over time and with experi-
ence, the mud can be modified to make it more effective.
Water is the primary component in water‐based drilling mud, the most often used
mud. Bentonite clay is added to increase the cutting‐carrying capacity of the water.
Water‐soluble chemicals are added to adjust the properties of the suspended clay.
Other solids, such as barite, are added to increase the density of the mud. Water‐
based mud can cause subsurface clays to swell and adversely affect reservoir
properties. Table 8.1 illustrates the density of two types of mud and compares the
mud density to water density.
Diesel oil is used in oil‐based drilling mud. Designed to be stable at high temper-
ature, oil‐based systems are used to protect shale and can help keep the drill pipe
from sticking. They can also be used for native‐state coring, which is an attempt to
retrieve a rock sample in its original state. Oil‐based systems are messy and can have
a negative environmental impact if spilled.
Synthetic oil can be used for a synthetic‐based drilling mud. Synthetic mud act
like oil‐based mud but are more environmentally friendly. They are expensive and
tend to be used in environmentally sensitive areas.
In some cases foam, air, and mist can be used to drill a well. This increases the ROP
but it does not control movement of formation fluids into the wellbore. Furthermore,
this type of drilling fluid provides very little mechanical support for the wellbore.