GAS SAMPLING
A sampling device
called a gas trap
removes gas from
drilling returns.
Ideally, the trap
should be capable
of extracting gas
from the drilling
fluid regardless of
the mud density,
viscosity, or gel strength. In practice, only 25%
to 75% of gas is removed.
The gas trap is located either in the possum
belly of the shale shaker or in a sluice box on
a diversion line. The trap must be clear of
cuttings, lost-circulation materials, congealed
mud and other solids at all times since reliable
HC logging needs a consistent flow rate. The
magnitude of carbide tracer responses is a
good indicator of the efficiency of the trap.
MUD SAMPLING
Mud samples are usually caught at the same
time as cuttings. Consistency of procedure,
cleanliness of containers and avoidance of gas
and oil evaporation are the key sampling
problems.
CUTTINGS SAMPLING
Cuttings are used to determine the
stratigraphic
interval, estimate reservoir
characteristics, and identify gas and oil
saturated intervals.
Special samples should be taken whenever
gas-in-mud levels increase and one lag
(estimated as pump stroke) after a drilling
break.
Rapid processing of samples for gas and oil
analysis is essential to avoid fluid loss by
evaporation.
GAS-IN-MUD
After removal of gas from drilling fluid or
cuttings, samples are processed through
instruments design to detect and analyze
hydrocarbon composition.
The typical instruments include:
• catalytic combustion or hot-wire detectors
• thermal conductivity detectors
• flame ionization detectors
• gas chromatographs
• infrared analyzers
• mass spectrometry
The first three instruments provide continuous
total gas analysis. Of these total gas
instruments, flame ionization detectors have
the best sensitivity.
OIL-IN-MUD
At low gas-oil ratios, gas-in-mud analysis may
not detect oil reservoirs. Friable reservoirs may
totally disaggregate and fail to provide any
composite chips to show oil staining.
In these circumstances, oil-in-mud analysis
may be the only basis for recognizing oil
saturation.
Oil can sometimes be seen in drilling fluid as a
colour sheen or as globules on the surface
after dilution with water. Examination of the
diluted mud sample under ultraviolet light may
also detect oil as a surface fluorescence.
Procedures are available to distill out the oil
fraction for liquid-liquid chromatography or
infrared analyzers.
GAS-IN-CUTTINGS
Fresh cuttings are routinely examined for gas
content. A measured weight of sample chips is
pulverized in a fixed amount of water.
The vapour above the pulverized slurry is
analyzed with at total gas detector. The
resulting readings are logged as cuttings gas
or microgas. Note is also made of any oil film
or globules on the surface of the slurry.
Significant amounts of gas-in-cuttings are
characteristic of gas saturation in low
permeability rocks.
OIL-IN-CUTTINGS
Procedures for detecting and evaluating oil-incuttings include:
• hydrocarbon odour
• description of oil staining
• ultraviolet fluorescence
• solvent cut test
• acid (HCl) test
• hot water immersion
• pyrolysis chromatography
• iridescence
HYDROCARBON ODOUR
Very small amounts of HC can be detected by
odour under favourable conditions. Because
odour can dissipate rapidly, immediate
evaluation is best. Sample descriptions should
record odour in fresh cuttings as "none",
"slight", "fair" or "good".
OIL STAINING
Oil-in-cuttings is indicated by a colouration
called staining. The colour and nature of the
stain depends on the nature of the oil.
Low-gravity oils tend to stain dark brown or
black. Light oils stains can be almost
colourless.
Samples are examined immediately after
recovery and while still wet. A binocular
microscope with standard illumination source
used in conjunction with standard colour
charts is recommended for consistency.
Distribution of the stain with respect to grains,
pores and fracture surfaces etc should be
recorded.
Description of oil staining is not well
standardized and comparisons are difficult but
some estimate of the percentage of stained
chips should be made.
FLUORESCENCE
Fluorescence has been used as a detection
method for crude oil since the earliest days of
mud logging. Fluorescence is the ability of a
substance to emit light after absorbing
electromagnetic energy from an external
source.
All crude oils fluoresce (because they contain
cyclic and polycyclic HCs or asphaltenes.
These compounds are most abundant in
aromatic crudes and least abundant in
paraffinic crudes. The intensity of fluorescence
depends
on
both
composition
and
concentration.
The colour of fluorescence can be used to
roughly estimate API gravity since the
fluorescence wavelength decreases as API
gravity increases.
SOLVENT CUT TEST
An important test to verify the presence of oil is
the solvent cut test. The sample is immersed
in a hydrocarbon solvent and the colour and
fluorescence of the resulting solution is
recorded.
The colour of the cut depends on the
characteristics of the oil. Low-gravity oils
produce dark-coloured cuts; high-gravity oils
produce amber or straw coloured cuts.
Cut fluorescence can detect very small
amounts of oil. A few chips are leached in a
clean porcelain dish or spot plate. The solvent
is allowed to evaporate and then the residue is
examined for fluorescence.
The cut solvent must dissolve HCs, be
colourless, nonfluorescent, and nonflammable. Chlorothene (1,1,1-trichlorethane) satisfies these requirements and is less toxic than
benzene or tetrachlorethane.
ACID TEST
Stain in carbonates and calcareous sands can
be detected by immersing cuttings in dilute
(15%) HCl. Bubbles will float oil-stained chips,
unstained chips remain submerged.
HOT WATER TEST
Immersion of unwashed cuttings in hot (>75oC)
water may release oil to float as a film than can
be detected with ultraviolet light.
PYROLYSIS CHROMATOGRAPHY
Heating cuttings and passing the gases
through a chromatograph can detect low
concentrations of HC in low permeability rocks.
IRIDESCENCE
Iridescence on wet cuttings without apparent
staining can indicate the presence of light oil
or condensate.
CUTTINGS TRANSPORT
Ideally, cuttings would be recovered in the
same order with the same composition as they
were cut. This does not happen because
particles travel at different velocities in the
annulus.
The spread of arrival times for material cut at
the same time is minimized by the use of high
density muds with the minimum gel strength
necessary to keep the cuttings in suspension.
Spread depends on particle size, shape and
density. The largest particles are usually from
borehole wall caving. The smallest particles
are likely to be the result of recirculation.
The location of the diversion line for sampling
cuttings can be used to eliminate large
particles from caving and avoid recirculated
fines.
CUTTINGS EXAMINATION
The AAPG Sample Examination Manual
provides a standard for cuttings description.
The manual lists the items to be recorded, a
set of standard abbreviations and a
recommended order in which items should
appear.
The more important items in the AAPG
recommended order are:
LOGGED ITEMS
Rock Type
Color
Hardness
Grain Size
Sorting
Cementation/Matrix
Porosity
Stain
Fluorescence
Cut
Cut Fluorescence
EXAMPLES
SS
SLTST
LS
LT GRY-BUFF
GRY-BRN
LT BRN
FM
V FRI
SFT
MED GR
F GR
C GR
PR SRTD
W SRTRD
MOD SRTD
NO CMT
SIL CMT
CALC CMT
NO VIS POR
INT GRAN POR
FR POR
OIL STN
DK OIL STN
LT OIL STN
YEL-BRN FLOR
M-DK BRN FLOR
YEL FLOR
RPD AMBER CUT
WEAK LT BRN CUT
DK STRAW CUT
BRT YEL CUT FLOR
GOLD CUT FLOR
BLU WH CUT FLOR
SIGNIFICANCE OF SHOWS
The significance of an oil or gas show depends
on a large number of factors including:
• Formation oil and gas content
interval porosity, interval saturation, hole diameter
• Flushing
overbalance, mud filtrate mobility, formation permeability,
drill bit design (jets), drilling rate
• Mud volume
circulation rate, drilling rate
• Reservoir P-T conditions
volume changes moving to surface
• Retained oil saturation
oil retained in cuttings and not released to mud
GAS LOG ZEROS
For calibration purposes, the CCD (catalytic
combustion detector) true zero (or pure air
background) is found by passing a stream of
air through the filament chamber.
During a trip, the drillstring is removed from
the hole, usually to replace a bit or for coring.
Drilling and circulation are halted for a
significant period. Cold and stagnant mud
degasses with time and the total gas recorder
approaches the pure air background.
When circulation is resumed, the total gas
rises as system sources of all kinds contribute
to the recorded level. This level is the system
background gas.
When a connection is made (new pipe
connected), circulation ceases and the gas log
records a reduced level used as a system
zero.
TRIP GAS AND CONNECTION GAS
When a trip occurs the swabbing (pumping)
action of the rods causes production from
previously drilled intervals. When circulation is
restarted after a trip a slug of accumulated
production called trip gas is moved to the
surface.
It is normal practice to carry out a carbide test
after a trip by adding acetylene-generating
calcium carbide to the mud. This tracer travels
down the drillstem to the bit and up the
annulus back to surface. The trip gas must
arrive ahead of the carbide test.
Diesel is often added to drilling mud as a
lubricant. This appears after one lag as an
increase in total gas making necessary to reestablish the system background.
Gas production during a connection appears
after one lag and can thus be recognized as
connection gas rather than a show.
KELLY CUT
After a trip and sometimes after a connection,
the drillstring may not be full of mud and a
slug of air is pumped around the circulation
system.
The air is compressed as it moves downhole
and produces a slug on aerated mud in the
annulus. The aerated mud scavenges gas and
results in an increase in the total gas recorded
at surface.
10
20
30
TOTAL GAS
T
I
M
E
40
50
60
MUD WEIGHT
RECYCLED TRIP GAS
KELLY CUT
TRIP GAS
CIRCULATION STARTED
70
This false show
is called a kelly
cut and can be
recognized from
its appearance
after one lag.
A reduction in
mud weight is
also
recorded
as the aerated
slug passes.
TOTAL GAS LOG
A gas show is a total gas peak that cannot be
attributed to one of the many system related
events that can produce false shows.
DRY GAS SHOW
Reservoirs saturated with gas:
•
•
•
•
•
•
•
increase in gas-in-mud
lack of oil stain in cuttings
little or no fluorescence
absence of oil-in-mud
high methane ratios C1:C2, C1:C3, .....
good penetration rate
visible porosity
The log shows three reservoirs at A, B, and C:
• A is a dry gas sand reservoir
• B is a water sand reservoir
• C is a gas-bearing low permeability sand.
GAS CAP OIL SHOW
An oil show in or near the gas cap of an oil or
condensate pool can be recognized by:
• increase in total gas (ditch gas)
• low to moderate oil in cuttings
• trace only of free oil-in-mud
• high methane ratios
• good penetration rate
• visible porosity
Mud conductivity or salinity should be
monitored to provide evidence of a bottom
water contact.
LIGHT-MEDIUM OIL SHOW
A light-medium gravity oil show in a reservoir
can be recognized by:
•
•
•
•
•
•
•
•
moderate gas-in-mud (ditch gas)
moderate to high gas-in-cuttings
abundant free oil-in-mud
abundant free oil-in-cuttings, light oil stain
moderate methane ratios
rapid solvent cut indicating mobile oil
good penetration rate
visible porosity
As oil gravity increases C3 and C4 may exceed
C2 in the chromatographic analysis, oil stain
becomes darker and solvent cut is less rapid.
HEAVY OIL SHOW
A low gravity heavy oil show in a reservoir can
be recognized by:
•
•
•
•
•
•
•
•
low to moderate gas-in-mud
low gas-in-cuttings
trace or no oil-in-mud
fair to good oil-in-cuttings, dark oil stain
moderate to low methane ratios
slow solvent cut indicating low oil mobility
good penetration rate
visible porosity
Reverse drilling breaks in the drilling rate log
indicates shale partings in the reservoir. Very
high drillings rates responsible for moderate
rather than low gas-in-mud.
UNDERSATURATED HEAVY OIL SHOW
An undersaturated (below bubble-point) heavy
oil show in a reservoir can be recognized by:
• no gas-in-mud
• no gas-in cuttings
• abundant free oil-in-mud
• abundant free oil-in-cuttings, dark oil stain
• slow solvent cut indicating low oil mobility
• good penetration rate
• visible porosity
The absence of gas indicates a water-wet
reservoir with undersaturated oil.
TIGHT OIL SHOW
An oil show in a tight formation displays:
•
•
•
•
•
•
•
•
•
low gas-in-mud peak
higher gas-in cuttings
fair oil-in-mud
fair oil-in cuttings
moderate to low methane ratios
slow solvent cut
higher crush cut indicating low perm
moderate penetration rate
no visible porosity
The drilling rate and lack of visible porosity
combined with the increased crush cut are the
diagnostic features of a tight formation.
MUD CAKE
The build-up of mud
solids on the borehole
face
in
permeable
formations is called mud
cake. This limits further
invasion of drilling fluids
into the reservoir.
A fluid phase called mud filtrate can pass through mud
cake to replace reservoir fluids in an annulus around the
borehole. The mud-impregnated zone is surrounded by
a larger infiltrated zone. Together these form the
invaded zone. In the flushed zone, all but residual HC
has been removed.
MUD CAKE FORMATION
Mud cake is an essential
protection for the reservoir
during drilling. When the
drilling is overbalanced,
there is a radial outward
pressure gradient from
the wellbore to the
formation.
Initially mud particles invade the reservoir and
begin to form a seal. As the mud particles clog
the pores, the passage of mud filtrate is more
progressively restricted.
Mud cake can be clearly
identified
on
many
wireline logs, particularly caliper.
Some micro-resistivity
logs are designed to
respond to mud filtrate.
BOREHOLE GEOMETRY
Rotary drilling creates a
cavity when mud replaces
the broken
formation.
Borehole size and shape
is measured with a caliper
tool. Tools may have from
one to four arms. Each
device measures different
properties.
1- or 2-armed devices
measure the longest axis
of the hole. 3-armed tools
measure some average
diameter. 4-armed calipers record the long-axis
and a short-axis at right
angles. Caliper devices
used with other wireline
tools when their response
depends
on
borehole
geometry.
TEMPERATURE DISTURBANCE
Drilling fluids are normally warmer than
shallow formations and cooler than deep
formations giving rise to temperature
disturbances around the hole.
This may effect some wireline logs where the
region sensed is close to the hole (e.g. microresistivity).
Bottom hole temperatures change with time
and when drilling circulation is stopped they
return to their natural equilibrium levels.
Temperature logs in wells are mainly used for:
• determination of geothermal gradients
• location of fluid inflows
• location of artificially fractured zones
• location of casing leaks
• location of primary cement tops
STRESS DISTURBANCE
Vertical stresses are readily estimated from
depth and specific weight:
σv = γ .h
Horizontal stresses are generally unknown but
can be deduced from well-wall damage in
deep wells called breakouts.
The elastic stresses
around
a
circular
opening
in
an
axisymmetric
stress
field
are
readily
calculated.
In weak materials, the
material around the
well may collapse
producing a "plastic"
annulus where the
stresses are reduced.
WELLBORE STABILITY
The borehole wall must support the loads from
both the in-situ stresses and the pressure of
the wellbore fluids. Failure can enlarge,
reduce, fracture or collapse the hole.
Fracing (hydrofracture)
occurs when the mud
weight exceeds the
effective
tangential
stress by an amount
equal to the tensile
strength of the rock.
Brittle elastic failure of the rock at the
circumference of the hole occurs if the stress
exceeds the compressive strength. This kind of
failure is called a borehole breakout.
Plastic yield or collapse may occur in weak
materials such as shales and unconsolidated
sands resulting in squeezing or caving.
Control of mud weight can prevent wellbore
failure.