MUD LOGGING

Directional drilling
1. n. [Drilling]

ID: 1341

The intentional deviation of a wellbore from the path it would

naturally take. This is accomplished through the use of whipstocks,
bottomhole assembly (BHA) configurations, instruments to
measure the path of the wellbore in three-dimensional space, data
links to communicate measurements taken downhole to the surface,
mud motors and special BHA components and drill bits. The
directional driller also exploits drilling parameters such as weight
on bit and rotary speed to deflect the bit away from the axis of the
existing wellbore. In some cases, such as drilling steeply dipping
formations or unpredictable deviation in conventional drilling
operations, directional-drilling techniques may be employed to
ensure that the hole is drilled vertically. While many techniques
can accomplish this, the general concept is simple: point the bit in
the direction that one wants to drill. The most common way is
through the use of a bend near the bit in a downhole steerable mud
motor. The bend points the bit in a direction different from the axis
of the wellbore when the entire drillstring is not rotating. By
pumping mud through the mud motor, the bit turns while the
drillstring does not rotate, allowing the bit to drill in the direction it
points. When a particular wellbore direction is achieved, that
direction may be maintained by rotating the entire drillstring
(including the bent section) so that the bit does not drill in a single
direction off the wellbore axis, but instead sweeps around and its
net direction coincides with the existing wellbore. Rotary steerable
tools allow steering while rotating, usually with higher rates of
penetration and ultimately smoother boreholes.

Rathole
1. n. [Drilling]

ID: 1537

A storage place for the kelly, consisting of an opening in the rig

floor fitted with a piece of casing with an internal diameter larger
than the outside diameter of the kelly, but less than that of the

upper kelly valve so that the kelly may be lowered into the rathole
until the upper kelly valve rests on the top of the piece of casing.

Mousehole
1. n. [Drilling]

ID: 1492

An opening in the rig floor near the rotary table, but between the
rotary table and the vee-door, that enables rapid connections while
drilling. The mousehole is usually fitted underneath with a length
of casing, usually with a bottom. A joint of drillpipe that will be
used next in the drilling operation is placed in the mousehole, box
end up, by the rig crew at a convenient time (immediately after the
previous connection is made). When the bit drills down and the
kelly is near the rotary table, another piece of drillpipe must be
added for drilling to continue. This next piece of pipe is standing in
the mousehole when the kelly is screwed onto it. Then the kelly
and the joint of pipe in the mousehole are raised to remove the pipe
from the mousehole, the mousehole pipe screwed onto the rest of
the drillstring, and the drillstring lowered, rotated, and pumped
through to continue drilling. Another piece of pipe is put in the
mousehole to await the next connection.

Blowout
1. n. [Drilling]

ID: 1224

An uncontrolled flow of reservoir fluids into the wellbore, and

sometimes catastrophically to the surface. A blowout may consist
of salt water, oil, gas or a mixture of these. Blowouts occur in all
types of exploration and production operations, not just during
drilling operations. If reservoir fluids flow into another formation
and do not flow to the surface, the result is called an underground
blowout. If the well experiencing a blowout has significant
openhole intervals, it is possible that the well will bridge over (or
seal itself with rock fragments from collapsing formations)
downhole and intervention efforts will be averted.

Blow out at rig site

BOP (blow out preventer)

Kick
1. n. [Drilling]

ID: 1454

A flow of reservoir fluids into the wellbore during drilling

operations. The kick is physically caused by the pressure in the
wellbore being less than that of the formation fluids, thus causing
flow. This condition of lower wellbore pressure than the formation
is caused in two ways. First, if the mud weight is too low, then the
hydrostatic pressure exerted on the formation by the fluid column
may be insufficient to hold the formation fluid in the formation.
This can happen if the mud density is suddenly lightened or is not
to specification to begin with, or if a drilled formation has a higher
pressure than anticipated. This type of kick might be called an
underbalanced kick. The second way a kick can occur is if dynamic
and transient fluid pressure effects, usually due to motion of the
drillstring or casing, effectively lower the pressure in the wellbore

below that of the formation. This second kick type could be called
an induced kick.

Desander
1. n. [Drilling]

ID: 1328

A hydrocyclone device that removes large drill solids from the

whole mud system. The desander should be located downstream of
the shale shakers and degassers, but before the desilters or mud
cleaners. A volume of mud is pumped into the wide upper section
of the hydrocylone at an angle roughly tangent to its
circumference. As the mud flows around and gradually down the
inside of the cone shape, solids are separated from the liquid by
centrifugal forces. The solids continue around and down until they

exit the bottom of the hydrocyclone (along with small amounts of

liquid) and are discarded. The cleaner and lighter density liquid
mud travels up through a vortex in the center of the hydrocyclone,
exits through piping at the top of the hydrocyclone and is then
routed to the mud tanks and the next mud-cleaning device, usually
a desilter. Various size desander and desilter cones are functionally
identical, with the size of the cone determining the size of particles
the device removes from the mud system.

Kelly
1. n. [Drilling]

ID: 1447

A long square or hexagonal steel bar with a hole drilled through the
middle for a fluid path. The kelly is used to transmit rotary motion
from the rotary table or kelly bushing to the drillstring, while
allowing the drillstring to be lowered or raised during rotation. The
kelly goes through the kelly bushing, which is driven by the rotary
table. The kelly bushing has an inside profile matching the kelly's
outside profile (either square or hexagonal), but with slightly larger
dimensions so that the kelly can freely move up and down inside.

Drawworks
1. n. [Drilling]

ID: 1352

The machine on the rig consisting of a large-diameter steel spool,

brakes, a power source and assorted auxiliary devices. The primary
function of the drawworks is to reel out and reel in the drilling line,
a large diameter wire rope, in a controlled fashion. The drilling line
is reeled over the crown block and traveling block to gain
mechanical advantage in a "block and tackle" or "pulley" fashion.
This reeling out and in of the drilling line causes the traveling
block, and whatever may be hanging underneath it, to be lowered
into or raised out of the wellbore. The reeling out of the drilling
line is powered by gravity and reeling in by an electric motor or
diesel engine.

Desilter
1. n. [Drilling]

ID: 1329

A hydrocyclone much like a desander except that its design

incorporates a greater number of smaller cones. As with the
desander, its purpose is to remove unwanted solids from the mud
system. The smaller cones allow the desilter to efficiently remove
smaller diameter drill solids than a desander does. For that reason,
the desilter is located downstream from the desander in the surface
mud system.

Shale shaker
1. n. [Drilling]

ID: 1568

The primary and probably most important device on the rig for
removing drilled solids from the mud. This vibrating sieve is
simple in concept, but a bit more complicated to use efficiently. A
wire-cloth screen vibrates while the drilling fluid flows on top of it.
The liquid phase of the mud and solids smaller than the wire mesh
pass through the screen, while larger solids are retained on the
screen and eventually fall off the back of the device and are
discarded. Obviously, smaller openings in the screen clean more
solids from the whole mud, but there is a corresponding decrease in
flow rate per unit area of wire cloth. Hence, the drilling crew
should seek to run the screens (as the wire cloth is called), as fine
as possible, without dumping whole mud off the back of the
shaker. Where it was once common for drilling rigs to have only
one or two shale shakers, modern high-efficiency rigs are often
fitted with four or more shakers, thus giving more area of wire
cloth to use, and giving the crew the flexibility to run increasingly
fine screens.

Kill line
1. n. [Drilling]

ID: 1458

A high-pressure pipe leading from an outlet on the BOP stack to

the high-pressure rig pumps. During normal well control
operations, kill fluid is pumped through the drillstring and annular
fluid is taken out of the well through the choke line to the choke,
which drops the fluid pressure to atmospheric pressure. If the
drillpipe is inaccessible, it may be necessary to pump heavy
drilling fluid in the top of the well, wait for the fluid to fall under
the force of gravity, and then remove fluid from the annulus. In
such an operation, while one high pressure line would suffice, it is
more convenient to have two. In addition, this provides a measure
of redundancy for the operation. In floating offshore operations, the
choke and kill lines exit the subsea BOP stack and run along the
outside of the riser to the surface. The volumetric and frictional
effects of these long choke and kill lines must be taken into account
to properly control the well.

Choke line
1. n. [Drilling]

ID: 1287

A high-pressure pipe leading from an outlet on the BOP stack to

the backpressure choke and associated manifold. During wellcontrol operations, the fluid under pressure in the wellbore flows
out of the well through the choke line to the choke, reducing the
fluid pressure to atmospheric pressure. In floating offshore
operations, the choke and kill lines exit the subsea BOP stack and
then run along the outside of the drilling riser to the surface. The
volumetric and frictional effects of these long choke and kill lines
must be considered to control the well properly.

kelly bushing
1. n. [Drilling]

ID: 1449

An adapter that serves to connect the rotary table to the kelly. The
kelly bushing has an inside diameter profile that matches that of the
kelly, usually square or hexagonal. It is connected to the rotary
table by four large steel pins that fit into mating holes in the rotary
table. The rotary motion from the rotary table is transmitted to the
bushing through the pins, and then to the kelly itself through the
square or hexagonal flat surfaces between the kelly and the kelly
bushing. The kelly then turns the entire drillstring because it is
screwed into the top of the drillstring itself. Depth measurements
are commonly referenced to the KB, such as 8327 ft KB, meaning
8327 feet below the kelly bushing.

Prime mover
1. n. [Drilling]

ID: 1532

The source of power for the rig location. On modern rigs, the prime
mover consists of one to four or more diesel engines. These
engines commonly produce several thousand horsepower.
Typically, the diesel engines are connected to electric generators.
The electrical power is then distributed by a silicon-controlledrectifier (SCR) system around the rigsite. Rigs that convert diesel
power to electricity are known as diesel electric rigs. Older designs
transmit power from the diesel engines to certain rig components
(drawworks, pumps and rotary table) through a system of
mechanical belts, chains and clutches. On these rigs, a smaller

electric generator powers lighting and small electrical

requirements. These older rigs are referred to as mechanical rigs or
more commonly, simply power rigs.

PHPA mud
1. n. [Drilling Fluids]

ID: 2197

A class of water muds that use partially-hydrolyzed polyacrylamide

(PHPA) as a functional additive, either to control wellbore shales
or to extend bentonite clay in a low-solids mud. As a shale-control
mud, PHPA is believed to seal microfractures and coat shale
surfaces with a film that retards dispersion and disintegration. KCl
is used as a shale inhibitor in most PHPA mud designs. In lowsolids muds, PHPA interacts with minimal concentrations of
bentonite to link particles together and improve rheology without
increased colloidal solids loading.

PDC bit (

FIXED-CUTTER, POLYCRYSTALLINE
DIAMOND BIT)

1. n. [Drilling]

ID: 1514

A drilling tool that uses polycrystalline diamond compact (PDC)

cutters to shear rock with a continuous scraping motion. These
cutters are synthetic diamond disks about 1/8-in. thick and about
1/2 to 1 in. in diameter. PDC bits are effective at drilling shale
formations, especially when used in combination with oil-base
muds.

Bit
1. n. [Drilling]

ID: 1216

The tool used to crush or cut rock. Everything on a drilling rig

directly or indirectly assists the bit in crushing or cutting the rock.
The bit is on the bottom of the drillstring and must be changed
when it becomes excessively dull or stops making progress. Most
bits work by scraping or crushing the rock, or both, usually as part
of a rotational motion. Some bits, known as hammer bits, pound
the rock vertically in much the same fashion as a construction site
air hammer

TRI- CONE BIT

Roller-cone bit
1. n. [Drilling]

ID: 1549

A tool designed to crush rock efficiently while incurring a minimal

amount of wear on the cutting surfaces. Invented by Howard
Hughes, the roller-cone bit has conical cutters or cones that have
spiked teeth around them. As the drillstring is rotated, the bit cones
roll along the bottom of the hole in a circle. As they roll, new teeth
come in contact with the bottom of the hole, crushing the rock
immediately below and around the bit tooth. As the cone rolls, the
tooth then lifts off the bottom of the hole and a high-velocity fluid
jet strikes the crushed rock chips to remove them from the bottom
of the hole and up the annulus. As this occurs, another tooth makes
contact with the bottom of the hole and creates new rock chips.
Thus, the process of chipping the rock and removing the small rock
chips with the fluid jets is continuous. The teeth intermesh on the
cones, which helps clean the cones and enables larger teeth to be
used. There are two main types of roller-cone bits, steel milledtooth bits and carbide insert bits.

Workover
1. n. [Drilling]

ID: 1666

The repair or stimulation of an existing production well for the

purpose of restoring, prolonging or enhancing the production of
hydrocarbons.

Degasser
1. n. [Drilling]

ID: 1324

A device that removes air or gases (methane, H2S, CO2 and

others) from drilling liquids. There are two generic types that work
by both expanding the size of the gas bubbles entrained in the mud
(by pulling a vacuum on the mud) and by increasing the surface
area available to the mud so that bubbles escape (through the use of

various cascading baffle plates). If the gas content in the mud is

high, a mud gas separator or "poor boy degasser" is used, because
it has a higher capacity than standard degassers and routes the
evolved gases away from the rig to a flaring area complete with an
ignition source.

Drillstem test
1. n. [Drilling]

ID: 1373

A procedure to determine the productive capacity, pressure,

permeability or extent (or a combination of these) of a hydrocarbon
reservoir. While several different proprietary hardware sets are
available to accomplish this, the common idea is to isolate the zone
of interest with temporary packers. Next, one or more valves are
opened to produce the reservoir fluids through the drillpipe and
allow the well to flow for a time. Finally, the operator kills the
well, closes the valves, removes the packers and trips the tools out
of the hole. Depending on the requirements and goals for the test, it
may be of short (one hour or less) or long (several days or weeks)
duration and there might be more than one flow period and
pressure buildup period.
2. n. [Well Testing]

ID: 8022

Well tests conducted with the drillstring still in the hole. Often
referred to as DST, these tests are usually conducted with a
downhole shut-in tool that allows the well to be opened and closed
at the bottom of the hole with a surface-actuated valve. One or
more pressure gauges are customarily mounted into the DST tool
and are read and interpreted after the test is completed. The tool
includes a surface-actuated packer that can isolate the formation
from the annulus between the drillstring and the casing, thereby
forcing any produced fluids to enter only the drillstring. By closing
in the well at the bottom, afterflow is minimized and analysis is
simplified, especially for formations with low flow rates. The
drillstring is sometimes filled with an inert gas, usually nitrogen,
for these tests. With low-permeability formations, or where the
production is mostly water and the formation pressure is too low to
lift water to the surface, surface production may never be observed.
In these cases, the volume of fluids produced into the drillstring is
calculated and an analysis can be made without obtaining surface
production. Occasionally, operators may wish to avoid surface

production entirely for safety or environmental reasons, and

produce only that amount that can be contained in the drillstring.
This is accomplished by closing the surface valve when the
bottomhole valve is opened. These tests are called closed-chamber
tests.

Drillstem tests are typically performed on exploration wells, and

are often the key to determining whether a well has found a
commercial hydrocarbon reservoir. The formation often is not
cased prior to these tests, and the contents of the reservoir are
frequently unknown at this point, so obtaining fluid samples is
usually a major consideration. Also, pressure is at its highest point,
and the reservoir fluids may contain hydrogen sulfide, so these
tests can carry considerable risk for rig personnel.
The most common test sequence consists of a short flow period,
perhaps five or ten minutes, followed by a buildup period of about
an hour that is used to determine initial reservoir pressure. This is
followed by a flow period of 4 to 24 hours to establish stable flow
to the surface, if possible, and followed by the final shut-in or
buildup test that is used to determine permeability thickness and
flow potential.

Rheology
1. n. [Geology]

ID: 430

Generally, the study of how matter deforms and flows, including

its elasticity, plasticity and viscosity. In geology, rheology is
particularly important in studies of moving ice, water, salt and
magma, as well as in studies of deforming rocks.
2. n. [Drilling Fluids]

ID: 2268

The science and study of the deformation and flow of matter. The
term is also used to indicate the properties of a given fluid, as in
mud rheology. Rheology is an extremely important property of

drilling muds, drill-in fluids, workover and completion fluids,

cements and specialty fluids and pills. Mud rheology is measured
on a continual basis while drilling and adjusted with additives or
dilution to meet the needs of the operation. In water-base fluids,
water quality plays an important role in how additives perform.
Temperature affects behavior and interactions of the water, clay,
polymers and solids in a mud. Downhole pressure must be taken
into account in evaluating the rheology of oil muds.

X-Y plots of rheological models.

Rheology. Fluids are described as Newtonian or non-Newtonian
depending on their response to shearing. Shear stress of a
Newtonian fluid (upper left) is proportional to the shear rate. Most
drilling muds are non-Newtonian fluids, with viscosity decreasing
as shear rate increases, and correspond more closely to one of the
other three models shown.

Appraisal Wells
1. n. [Geology]

ID: 35

The phase of petroleum operations that immediately follows

successful exploratory drilling. During appraisal, delineation wells

might be drilled to determine the size of the oil or gas field and
how to develop it most efficiently.

Reservoir heterogeneities
1. n. [Well Testing]

ID: 8148

The variations in rock properties in a reservoir. The variations can

result in directional variations in permeability. Geological
processes, such as sedimentation, diagenesis and erosion, act to
produce nonuniformities in rock formations. Because there are so
many types of reservoir heterogeneities, a unique interpretation of
test results from pressure data alone is often impossible. Expert test
interpreters rely heavily on experience, core analysis, well logs and
knowledge of the geology specific to the region.