Post on 03-Apr-2018
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1
Day Two
Objectives :
• What is Pore Pressure?• Abnormal Pressure Origins
• Pore Pressure Evaluation• ROP & Drilling Exponents
• MWD and Wireline Logs
• Shale Density and Factor
• Formation Gases
• Temperature
• Borehole Condition
• Pore Pressure Estimation
Formation Pressure
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2
WHAT IS POREPRESSURE?
Formation Pressure
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What is Pore Pressure?
• Pore pressure is the pressure exerted by pore fluids.
• Normal pore pressure = Normal hydrostatic pressure
• Subnormal pore pressure < Normal hydrostatic pressure
• Abnormal pore pressure > Normal hydrostatic pressure
Formation Pressure
Water Type Salinity
Cl-
mg/l
Salinity
NaClmg/l
Water Density
gm/cc
Fresh Water 0 to 1500 0 to 2500 1.00
Sea Water (Example)
18000 30000 1.02
FormationWaters
(Example)
Salt Water
saturated in NaCl
1000036000
4800060000192667
1650060000
80000100000317900
1.011.04
1.051.071.20
• Normal pore pressure
reflects the water density in
the basin of deposition.
• Fluid density is a function
of the concentration of dissolved salts. Varying
salinity causes varying fluid
density.
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What is Normal Pore Pressure?
Formation Pressure
Pore Pr. = Hyd. Pr.
• Pore pressure will remain
normal if there is good
hydraulic communication
between the sedimentsand the depositional basin.
• Fluids will escape during
compaction and the rock
grains accept all of the
overburden stress.• Pore fluids will maintain a
normal hydrostatic
pressure.
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What is Abnormal Pore Pressure?
Formation Pressure
Pore Pr. > Hyd. Pr.
• Abnormal formation
pressures develop when
some process limits the
hydraulic communication.• In this case, the trapped
pore fluids accept a
greater share of the
overburden stress.
•
This has the effect of raising the pore pressure
above normal hydrostatic
pressure.
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How to Measure Pore Pressure?
Formation Pressure
• Production Samples such as DST’s.
• RFT Samples - restricted to potential reservoir areas.
• TesTrak LWD data.
•Mud Chlorides - show gross changes in pore water salinity.
• Resistivity Logs - used to calculate Rw.
• Rw Tables
– Rw is the total resistivity of the water and assumes thepresence of NaCl. It does not differentiate between other saltsand dissolved gases with different densities.
• Offset Data - regional curves or formation water densitytables generally only give an approximation for the area.
• When all else fails ... a bucket and a mud balance.
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Magnitude of Pore Pressure
• The magnitude of pore pressure will
depend on one or any of ...
• Surface communication
• Concentration of dissolved salts
• Percentage of effective porosity
• Degree of overburden
• Geothermal gradient
• Percentage of gas
• Most important of these is Surface Communication
• Once communication has been halted the other factors will take effect.
Formation Pressure
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• Offset Well Data
• Rw Catalogues
• Resistivity Logs
• Bucket on a Rope
Formation Pressure
PORE WATER DENSITY
rf
Normal Hydrostatic Pr.Normal Pore Pressure
“P” in psi, bars, atm
ThePlan
(2)Pore
Pressure
(Fluid Properties)
Normal Pore Pr. Gradient(“P” / TVD from water level)
Formation Balance Gradient(“P” / TVD from flowline)
Gas,
Dxc,
Elogs,
Temp.
Flows,
Kicks,
etc.
Estimated PorePressure and
FB Gradient
MINIMUM
STATIC MUD
DENSITY
“S” (from 1)
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ORIGINS OF ABNORMALPRESSURE
Formation Pressure
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Abnormal Pressure Environments
Formation Pressure
• There are several geologic
conditions favorable to the
development of abnormal
pressure. – Young sediments
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Abnormal Pressure Environments
Formation Pressure
• There are several geologic
conditions favorable to the
development of abnormal
pressure. – Young sediments
– Large total thickness
– Presence of clay rocks
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Abnormal Pressure Environments
Formation Pressure
• There are several geologic
conditions favorable to the
development of abnormal
pressure. – Young sediments
– Large total thickness
– Presence of clay rocks – Interbedded sandstones
of limited extent
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Abnormal Pressure Environments
Formation Pressure
• There are several geologic
conditions favorable to the
development of abnormal
pressure. – Young sediments
– Large total thickness
– Presence of clay rocks – Interbedded sandstones
of limited extent
– Rapid loading and burial
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Abnormal Pressure Mechanisms
Formation Pressure
• Abnormal pressure develops when de-watering is restricted.
• There are three main mechanisms
• Ineffective pore space (volume) reduction
• Volume expansion
•
Fluid movement Mechanisms within Mechanisms
• Compaction Disequilibrium
• Aquathermal Pressuring
• Clay Diagenesis
• Sulphate Diagenesis
•Salt Diapirism
• Tectonic Activity
• Hydrocarbon Maturation & Placement
• Piezometric Changes
• Osmosis
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Normal Sediment Compaction
Formation Pressure
• Under normal conditions sediments will de-water withburial.
• Overburden acts as the main cause for fluid expulsion.
• De-watering decreases the porosity and increases the
density of the sediment.• Normal clay compaction will depend on an overall balance
between :
• Clay permeability
• Sedimentation and burial rate
•Drainage efficiency
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Normal Sediment Compaction
Formation Pressure
Porosity can vary from 80% to less than 10% over a 5,000m interval.
Using data for the Gulf Coast of Mexico, at a depth of 3,000m the total volume of water
expelled is more than 75% of the original volume of the argillaceous sediment.
Interstitial Water
(% initial vol.)
Surface 300m 1000m 3000m
75.9
4.1
20
7366.6
80 13.3 7
202020
Expelled Water
(% initial vol.)
Solid(% initial
vol.)
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Compaction Disequilibrium
Formation Pressure
• Some sort of seal must be
in place.
• De-watering is stopped or
slowed down.• Overburden pressures are
transferred to the pore
fluids rather than normal
grain to grain contact.
•This has the effect of increasing the formation
pore pressure
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Differential Compaction
Formation Pressure
• Can occur in interbedded
sand/shale formations.
• Water escapes along the
path of least resistance.• Shales next to the sands
de-water more readily,
become compacted and
less permeable.
•Eventually further de-watering from within the
shale body stops.
• Pore pressure increases.
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Volume ExpansionDue to Hydrocarbon Generation
Formation Pressure
• With temperature and
pressure, kerogens are
converted to oil and gas.• This conversion is
associated with a volume
expansion.
• This will give rise to an
increase in the porepressure.
}
VolumeIncrease
Type IIKerogen
> Oil
> Wet Gas,Condensate
> Dry Gas
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Volume ExpansionDue to Aquathermal Pressuring
Formation Pressure
• If temperature is applied to pore water it
will increase in volume.
•The amount of pressure rise will dependupon
– density of the fluid
– amount of temperature increase
– effectiveness of the seal
The resultant pressure increase for a fluid of 1.0 SG (8.34 PPG) with a
rise in temperature from 50C to 75C is 5,600psi.
There is the question whether the seal can actually withstand the
aquathermal pressure. It is more likely that this is an extra drive to
break seals and keep systems dynamic.
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Clay Diagenesis
Formation Pressure
• Smectite rich clays dehydratelosing their interstitial water ascompaction occurs.
• As the clays compact, space is
created which is filled by thereleased water.
• This water may be able toescape or it may be trapped bythe now low porosity/lowpermeability Illite.
• Abnormal pressure may becaused by the presence of trapped water and the formationof impermeable seals by Illite.
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Sulphate Diagenesis
Formation Pressure
• Gypsum is the precipitated form of CaSO4
• Gypsum will transform to anhydrite above 40 C.
• In the presence of halite this may be around 25 C.
• Gypsum dehydrates to form anhydrite and free water.
• CaSO4.2H2O << >> CaSO4 + 2H2O
• Gypsum << >> Anhydrite + Water
• Up to 38% of the original water volume is released so
abnormal pressure can develop if this fluid cannot escape.
• Rehydration is accompanied by an increase in volume which
may also generate abnormal pressures.
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Tectonic Activity
Formation Pressure
• Tectonic activity will cause stress regimes which will
extensional or compressional.
• Extension causes fractures to open and therefore fluid
dissipation or movement to other zones• Compression has two main effects:
• The easy expulsion of fluids, leading to compaction and
therefore the formation of normal fluid pressures.
• The difficult expulsion of fluids, which causes
undercompaction, the formation of abnormal pressures.
• Abnormal pressures can progress to induce hydraulic
fracturing, leading to the expulsion of pore fluids and
ultimately the formation of normal pore pressure.
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Tectonic Activity
Formation Pressure
Amount of
Shortening Possible Geopressured Zones
Extension Extension
Compression
Compression
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Salt Diapirism
Formation Pressure
osmosis osmosis
trapped pressure
and confinement
uplifted
paleopressure
isolated rafts with
paleopressure
trapped pressure
under salt sheets Banff Diapir
& Salt Field
• Salt diapirism can cause the formation of abnormal pressure in a variety
of ways, most of which are noted in the cross-section above.
• Note that besides pressure in the isolated rafts, you may also have
dangerous gases such as hydrogen sulphide trapped.
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Piezometric Changes
Formation Pressure
• A low water table, or an aquifer with an outcrop below the water table, will show a pressure that is subnormal for drilling purposes.This is not as dramatic as abnormal pressure but the resulting lostcirculation will cause a loss of control of the hydrostatic pressurein the well which could result in well control problems.
• A water table above the
height of the rig will have
abnormal pressure on
penetrating the aquifer,
which will cause the fluid to
rise to the piezometric level
to equalise the pressure
imbalance.
When the seal is punctured fluids in the
aquifer will rise to this level to equalise
the pressure.
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Osmosis
Formation Pressure
• Osmosis is the spontaneous
movement of ions in water
down a concentration gradient
from fresh to saline.
• Movement will continue untilthe salinity’s are equal or
pressure prevents further
movement.
• That pressure may be up to
4000psi where shales act as
semi-permeable membranes. SALTS
Clays
MWD Resistivity
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PORE PRESSUREEVALUATION
Formation Pressure
i
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Pore Pressure Evaluation
• All methods are a function of mechanism.
– Compaction
– Tectonic
– Thermodynamic • Compaction techniques are best developed.
• The techniques only provide expectations.
• Developed for argillaceous rocks.
• Use direct or indirect porosity
determination.
Of course, operator experience is very important
Formation Pressure
F i P
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Pressure Evaluation Tools
• Prior to drilling
– Surface geophysical
– Regional geology
– Seismic – Offset data
• While drilling
– Drilling parameters - drill rate, torque, dxc, pump pressure etc.
– MWD / LWD / PWD - gamma ray, resistivity, sonic, density etc.
– Drilling fluid - gas, temperature, pit volume, salinity etc.
– Geology - shale density, volume, shape, size, shale factor etc
• After drilling
– Wireline logs
– Pressure tests
–Data analysis
Formation Pressure
F i P
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Formation Pressure
Pressure Evaluation Tools
F i P
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Formation Pressure
x
x
Resistivity
Porosityshould
decreasewith depth
Resistivityshouldincrease
withdecrease
of porosity
Trendreversal
mayindicate
porepressureincrease
Overpressure
Normal Trend
• Of the many techniques
used to detect and quantify
abnormal pore pressure,
all generally have the same
features in common
– Use clay/shale lithologies.
– Give indirect
measurements of porosity.
– Provide estimations based
on expected normalcompaction.
Pressure Evaluation Techniques
F ti P
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Formation Pressure
ROP’s and Drilling Exponents
• All factors being equal, the penetration rate will gradually
decrease with increasing depth due to the decreasing
formation porosity.
•
ROP can be used in abnormal pressure detection providingthe following factors are taken into account:
• lithology
• compaction
• differential pressure
•weight on bit
• rotary speed
• torque
• hydraulics
• bit type and wear
•
personnel and equipment
F ti P
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Formation Pressure
ROP’s and Drilling Exponents
• Drilling Exponents are used to
“normalise” the ROP.
• They aimed to eliminate the effects
of drilling parameter variations andtry to give a measure of formation
“drillability”.
• Historically the Dxc (Corrected
Drilling Exponent) we use came
from: – Bingham (1964)
– Jorden & Shirley (1966)
– Rehm & McClenden (1971)
Normal trend
Normal
Observed
F ti P
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Formation Pressure
ROP’s and Drilling Exponents
• Rehm & McClenden (1971) suggested correcting the “d”exponent to take into account the effects of differential pressurebetween the formation pressure and the regional hydrostaticpressure:
d = log10 R/ 60N x NFBG R = ft/hr ECD = ppg
log10 12W / 106
D ECD N = RPM NFBG = ppgW = pounds
D = inches
d = 1.26 - log10 R / N x NFBG R = m/hr ECD = SG
1.58 - log10 W / D ECD N = RPM NFBG = SG
W = tonnes
D = inches
• This had the effect of removing the “masking effect” when mud
weights were increased and also emphasised the shift in Dxc
values when entering an abnormal pressure zone.
F ti P
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Formation Pressure
ROP’s and Drilling Exponents
• Drilling Exponents do not take into
account
– Lithology, unconformities etc.
–
Mud hydraulics – Bit type and wear
– Post 1970’s technology
• highly deviated long reach wells.
• rotary closed loop systems.
• improved mud systems, better hole
cleaning control.
• PDC bits.
• improved cutting efficiency of insert
bits.
Normal trend
Normal
Observed
F ti P
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Formation Pressure
Drilling Exponents
Soft Clay
Calc Clyst
Claystone
Calc Clyst
Claystone
N o r m a l P r
e s s u r e
G e o p r e s s u r e
Silty Clyst
Interpretation
• In soft clays the bit jets away theformation and gives a low &scattered dxc curve.
• The trend line has beenestablished in the normallypressured claystones.
• Geopressure can be seen below
the calcitic claystone. Caremust be taken to confirm thatthe trend shift is due to pressureand not just the lithology.
F ti P
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Formation Pressure
Drilling Exponents
Interpretation
• Drilling through an alternating
sequence of shales and sands a
trend line can be established for
the shales only in a normallypressured zone.
• When entering the abnormally
pressured zone then shales will
show deviations from its trend
line.
• Sands may or may not show asimilar change in trend. Do Not
try to draw trends through
sandstones.
N
o r m a l P r e s s u r
e
G e o p r e s s u r e
F ti P
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Formation Pressure
Drilling Exponents
Interpretation
• When drilling with different
types of bit, trends can be
established for each bit run.
• These trends can be smoothedinto a continuous plot.
• Care must be taken when doing
this for if a geopressured zone
is entered at the start of a new
bit run then the data could be
misinterpreted as being normal.
Smoothed
DataRockBit
Insert
Bit
Rock
Bit
F ti P
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Formation Pressure
Drilling Exponents
Interpretation
• When a new hole section is
drilled there will be a shift in the
Dxc values to the right. Again
the data can be smoothed usingtrend lines.
Smoothed Data
12 1/4”
8 1/ 2”
F ti P
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Formation Pressure
Drilling Exponents
Interpretation
• Towards the end of a bit run, bit
wear can be seen as an increase
in the Dxc values, this is
because the bit is not drilling asefficiently as previously.
NB #3
NB #4
Bit wear
F ti P
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Formation Pressure
Drilling Exponents
Interpretation
• If an abnormally pressured zone
is drilled with a dull bit then the
magnitude of the shift to the left
is reduced when compared todrilling with a fresh bit.
Fresh Bit Dull Bit
Normally
Pressured
Over
Pressured
Formation Pressure
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Formation Pressure
Drilling Exponents
Beware the Failings of Dxc
• You have to be judge and jury on the quality of Dxc data
before you present you material to the drilling team.
• You have to look at every argument and set of evidence; both
in favour of, and against your prediction of pressure.
• Do not be afraid to say that you cannot make an estimation at
this time on the evidence so far presented - You can say that
the evidence is inconclusive but only as long as you have
looked at all of the evidence available on the well.
• As we will see later, trendline placement will be a key concern
in the evaluation!
Formation Pressure
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• All of the following e-logs
have and can be used to aid
in pressure evaluation.
–
Resistivity Logs – Sonic/Acoustic Logs
– Gamma Ray Logs
– Density Logs
• We will concentrate on
resistivity and sonic logsfor pressure evaluation.
• Gamma will be used to pick
the shale points.
Wireline and MWD Logs
Formation Pressure
Formation Pressure
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Formation Pressure
Resistivity Logs
• With compaction pore water
will be released and the
formation resistivity should
increase.
• In a homogeneousargillaceous formation an
increase in porosity may
indicate an increase in pore
pressure.
• This will be reflected by adecrease in resistivity.
x
x
Overpressure
Normal Trend
Formation Pressure
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Formation Pressure
Resistivity Logs
x
x
Picking Shale Points from Resistivity Logs
• Use the gamma ray (or SP) curve to identify shale beds.
• Shales should be clean and at least 30 feet thick.
• Plot consistent resistivity values from the curve.
•
Don’t use values within 10 feet of the top of a sand.• Values above 3500 ft may be influenced by possible
freshening of formation pore water and low temperatures.
• Age boundaries and unconformities will probably cause a
shift in trends.
• Shales near salts may give values that are too low.
• Shale gas may give values that are too high.
• Again, trendline placement may be a key concern.
Note : It’s better to have a few, good, hand picked data points, than hundreds of
data points from a wireline file. The latter STILL need evaluation for shale points.
Formation Pressure
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Formation Pressure
Sonic / Acoustic Logs
x
x
• Sonic logs can be used both for bulk density calculations
as we have seen, and pore pressure evaluation.
• Sonic logs register the Transit Time (Delta T) of a
formation. Delta T is measured in usecs/ft.
• The delta t for a rock is a measure of its porosity• Lower transit times = faster acoustic velocity
= lower porosity = higher density
• On encountering a zone of abnormal pressure, the Delta T
will increase due to increased porosity.
•
As with Dxc and resistivity, only shale points should beused for pressure evaluation.
Formation Pressure
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Formation Pressure
The Other Logs
• Gamma Ray : it has been proposed
that greater porosity reduces the
strength of the gamma ray & this
may be used to calculate pore
pressure. The method was notsuccessfully proven, so GR is used
mostly to identify lithology.
• Density Logs : can be used for OBG
and pore pressure evaluation.
Limitations include their infrequent
use during the well and their shallow
depth of investigation and need for
caliper correction.
Formation Pressure
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Formation Pressure
Temperature Analysis
• With uninterrupted drilling, the
flowline temperature should
increase with depth.
• The earth’s core radiates heat
outwards.
• The rate of temperature increase
with depth is the Geothermal
Gradient.
• Fluid in pore spaces cause
abnormalities in the heat transfer.
• An overpressured zone will have
high fluid content and will distort
the temperature gradient.
Formation Pressure
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Formation Pressure
Temperature Analysis
• Evaluations based on geothermal
gradient reflects changes in the
return mud flow.
• Any changes seen in the mud
temperature after correction mustindicate changes in the borehole.
• Therefore the temperature probe
in the possum belly must be kept
clear of cuttings and other debris.
• Temperature data from MWD tools
may also be used when available.
Formation Pressure
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Formation Pressure
Temperature Analysis
• Temperature may be affected by
• Lithology
• Penetration rate
• Mud additions
• Flow rate & pump speed
• Hole size
• Depth
• Mud type
• Length of riser
• Type of bit
• Hole and string geometry• Surface temperature
• Breaks in rotation
Formation Pressure
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Formation Pressure
Temperature Analysis
SEALED GEOPRESSURED ZONE
Good Heat Absorption
Good Insulator but
Poor Heat Conductor
ISOTHERMS
Formation Pressure
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Formation Pressure
Temperature Analysis
Transition Zone
GEOPRESSURE
Geothermal GradientAverage = 3 degrees C / 100m
When plotted, you may see a
decrease in the gradient in the
transition zone, followed by a sharp
rise in the pressured zone itself.
Mud Temperature Out
Formation Pressure
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Formation Pressure
Gas Analysis
• For the mudlogger this is probably the
most important evidence for pressure
evaluation available.
• Note that any evaluation is dependant
on lagtime.• Be aware of ...
• Drilling Rate against Gas
• background gas
• Static Mud against Gas
• connection, trip & swab gas
• Mobile Drillstring against Gas
• background gas & swab gas • ECD against Gas
• background gas
Formation Pressure
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Formation Pressure
Gas Analysis
• Cuttings Gas - gas released from the
drilled formation.
• Produced Gas - gas issuing from the
borehole walls. May be due to caving,
swelling ordiffusion if differential
pressure is negative.• Recycled Gas - gas that is returned
down the hole from surface if, for
example, degassers are not working.
• Contamination Gas - gas from
petroleum products in the mud or
from thermal breakdown of additives.
Breakdown of organic matter such asshales or thermal effects of the bit
can also give rise to hydrocarbons.
Caving
Swelling
Gas
Diffusion
Gas in Water
Shale Gas
Eruptive Oil
Cuttings
Invasion
Formation Pressure
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Formation Pressure
Gas Analysis
%GASLithologyROP ft/hr Depth0 10400 0
3250
3300
3350
3400
3450
3500
Here you see the background
gas rising gradually as the
rates of penetration increase.
Is this pressure related or
simply a function faster drilling
producing more cuttings which
produce more gas?
What other data would help
you decide if necessary?
Formation Pressure
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Formation Pressure
Gas Analysis
Background Gas stable with
sporadic Connection Gas.
This is not characteristic of
formation pressure variation.
This is indicative of swabbing,
lithology variations or gas
from cavings.
Depth
Background
Gas
Connection
Gas
Formation Pressure
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Formation Pressure
Gas Analysis
Background Gas stable with
increasing Connection Gas.
Characteristic of entering a
transition zone.
The stable background
indicates that there is still a
positive differential pressure
with ECD but the fact that the
connection gas is increasing
indicates this is in decline.
Depth
Formation Pressure
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Formation Pressure
Gas Analysis
Background and connection
gas are indicating that drilling
is proceeding into a negative
differential pressure condition
at the bit through entry into an
overpressured zone.
Depth
Formation Pressure
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Formation Pressure
Gas Analysis
%GASLithologyROP ft/hr Depth0 10400 0
3250
3300
3350
3400
3450
3500
Cxn Gas
with
Kelly System
Cxn Gas
with Top Drive
DANGER
Less Frequent
&Less Visible
Formation Pressure
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Cuttings Character
Formation Pressure
Broad face
Side view
Cross section
Typical cavings from drilling
underbalanced
Note the delicate, spikey shape.
Size: Starting small , 1 cm.
Growth dependant on amount of
underbalance and lithology.
Formation Pressure
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Cuttings Character
Formation Pressure
Broad face
Side view
Cross section
Stress relief or borehole failure
cavings.Typical blocky shape
showing fine cracks.
Failure due to rock mechanics
and borehole angle.
Size based on internal stresses
and rock competence.
Formation Pressure
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Cuttings & Borehole Condition
Formation Pressure
• Review how cuttings and cavings are behaving and tie this in to
what is happening in the borehole in terms of hole stability.
• Are you seeing torque build up from cuttings beds? The beds
generally form in hole angles from 45 and 55 degrees, is this so?
• Do you have caved material in block form illustrating that the well
direction is in one of the major stress directions ?
• Log the performance of each trip in and out. If the crew do
something that has a positive benefit then tell them. It is more
critical to drilling the well than being able to tell them that their performance got worse. Whatever they were doing right was cost
effective in a problem hole.
Formation Pressure
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Cuttings & Borehole Condition
Formation Pressure
Hookload Deviation -100 0 +100 Reamed Stuck Break Circulation Trip In
Trip Out NB #4 NB #5 NB #6 RRB #6 NB #7 NB #8 9 5/8” Csg
DEPTH
metres13
3/8”Csg 2000
-
3000
-
2500
-
3500
-
0 ANGLE 90
0 AZIMUTH 359
T R I P C O N D I T I O N L O G
Formation Pressure
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CALCULATING POREPRESSURE
Formation Pressure
Formation Pressure
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• The ratio method works on the principle that the difference between
the observed values for the compaction parameter and the normal
parameter extrapolated to the depth is proportional to the increase
in pressure. This means that the method works on the basic ratio
method.
• The ratio method is :
PPo = Dxcn x PPn / Dxco Where :
PPo = Observed pore pressure
Dxcn = Normal Dxc
PPn = Normal pore pressureDxco = Observed Dxc
Using Dxc - Ratio Method
Formation Pressure
Normal trend
Normal
Observed
Log Dxc
Formation Pressure
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• This method assumes that rock properties (porosity) at the depth of
interest will be essentially the same as those at a higher depth with
the same Dxc values. The difference will be higher pressure due to
overburden.
• The equivalent depth method is : PpA = [(OBGA x DA) - DB (OBGB - PpnB)] / DA
Where :
PpA = Observed pore pressure
PpnB = Normal pore pressure
OBGA , OBGB = Overburden pressure
DA , DB = Depth
Using Dxc - Equivalent Depth Method
Formation Pressure
Normal trend
B
A
Log Dxc
DB
DA
OBG
Formation Pressure
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• Showed the importance of using overburden gradient
• Showed the importance of using a variable overburden
• Depends heavily on a trendline placement
• Developed equations for a number of parameters
• Eaton’s method is : Po = S - [(S - Pn) * (Dxco / Dxcn)^1.2]
Where :
Po = Observed pore pressure
Pn = Normal pore pressureS = Overburden pressure
Dxco = Observed Dxc
Dxcn = Normal Dxc
Using Dxc - Eaton’s Method
Formation Pressure
Normal trend
Normal
Observed
OBG
Formation Pressure
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• Trendline placement is the biggest source of error
• This is true for any trendline method!
• If a pressure point is known the true
position of the trend can be calculated
Dxcn = Dxco* [(S - Po) * (S - Pn)]^-0.833
Where :
Po = Observed pore pressure
Pn = Normal pore pressureS = Overburden pressure
Dxco = Observed Dxc
Dxcn = Normal Dxc
Using Dxc - Eaton’s Method
Formation Pressure
Normal trend
Normal
Observed
OBG
Formation Pressure
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• Eaton developed equations for three MWD or Wireline Logs
• Resistivity
• Conductivity
• Sonic
• Eaton’s equations : Po = S - [(S - Pn) * (Ro / Rn)^1.2]
Po = S - [(S - Pn) * (Cn / Co)^1.2]
Po = S - [(S - Pn) * (Dtn / Dto)^3 ]
Where : Ro, Rn = Observed & Normal Resistivity
Co, Cn = Observed & Normal Conductivity
Dto, Dtn = Observed & Normal Sonic Dt
Using E-Logs - Eaton’s Method
Formation Pressure
Normal trend
Normal
Observed
OBG
Formation Pressure
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• This is a petrophysical-mechanical method
• It is based on derived formation properties
• porosity from Archie’s equation • matrix stresses from Terzaghi
• assumes power law compaction
• uses formation fluid resistivity
• uses formation resistivity from logs
• It does not need a trendline!
Resistivity - Bryant’s Method
Formation Pressure
Shale Resistivity
OBG
Formation Pressure
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• Calculate the overburden pressure - (S)
• Find the normal pore pressure - (Pn)
• Calculate normal effective stress - (ESnorm)
ESnorm = S - Pn
• Calculate the actual effective stress - (ESact)
ESact =
11225 * [1-exp((ln Rw - ln Ro) / 2)]^7.47
•
Calculate the pore pressure - (P)If ESact > ESnorm … P = Pn
If ESact < ESnorm … P = S - ESact
Resistivity - Bryant’s Method
Formation Pressure
Shale Resistivity
OBG
At the depth of interest ...
Note : Rw will be in a range of 0.015 to 0.03 ohmms
Formation Pressure
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• Pore pressure will only ever be an “educated best guess” if
using wireline, MWD, Dxc or other formation evaluation data.
• The engineer should bring together as much
information as possible to present to the client.
Pore Pressure - Conclusion
o o essu e
• Whether using paper,
pencil and ruler,
spreadsheets or computer
programs, the end result
will depend on the
knowledge and experienceof the pressure engineer
evaluating the data.
Formation Pressure
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Formation Pressure Worksheet Eaton Dxc Pore Pressure Calculations
Air Gap 95.1 feet 29.0 metres
Water Depth 728.4 feet 222.0 metres 0.59
Normal PP 8.7 ppg 1.04 sg 0.000089
1.2
TVD (ft) TVD (m) OBG (sg) Dxc Trend (ppg) (sg)
984.3 300.0 1.09 0.80 0.68 8.6 1.03
1148.3 350.0 1.22 0.78 0.69 8.5 1.02
1312.3 400.0 1.32 0.98 0.71 7.6 0.91
1476.4 450.0 1.39 0.98 0.72 7.4 0.89
1640.4 500.0 1.45 0.88 0.74 7.9 0.94
1804.5 550.0 1.51 0.85 0.75 8.1 0.971968.5 600.0 1.55 0.88 0.77 7.9 0.95
2132.5 650.0 1.58 0.90 0.78 7.9 0.94
2296.6 700.0 1.62 0.94 0.79 7.6 0.91
2460.6 750.0 1.64 1.00 0.81 7.2 0.87
2624.7 800.0 1.67 0.98 0.82 7.5 0.90
2788.7 850.0 1.69 1.05 0.84 7.0 0.84
2952.7 900.0 1.71 1.10 0.85 6.7 0.80
3116.8 950.0 1.73 1.15 0.87 6.4 0.77
3280.8 1000.0 1.75 1.10 0.88 6.9 0.83
3444.9 1050.0 1.76 1.20 0.90 6.2 0.74
3608.9 1100.0 1.77 1.15 0.91 6.7 0.81
3772.9 1150.0 1.79 1.28 0.93 5.7 0.69
3937.0 1200.0 1.80 1.25 0.94 6.1 0.73
4101.0 1250.0 1.81 0.95 0.95 8.7 1.05
4265.1 1300.0 1.82 0.85 0.97 9.6 1.16
4429.1 1350.0 1.83 1.00 0.98 8.6 1.03
4593.1 1400.0 1.84 0.90 1.00 9.5 1.14
4757.2 1450.0 1.85 0.85 1.01 10.0 1.20
Well - Bideford - 31/7 : Grossenschmuck : Celtic Petroleum
Pore Pressure
Eaton Slope & Tre nd Paramete rs
Displacement
Slope
Exponent
300.0
400.0
500.0
600.0
700.0
800.0
900.0
1000.0
1100.01200.0
1300.0
1400.0
1500.0
1600.0
1700.0
1800.0
1900.0
2000.0
2100.0
2200.0
2300.0
2400.0
2500.0
0.10 1.00 10.00
dxc/pp
d e p t h m
Formation Pressure
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Formation Pressure Worksheet Bryant Pore Pressure Calculations
Air Gap 95.1 feet 29.0 metres
Water Depth 728.4 feet 222.0 metres 0.028
Normal PP 8.7 ppg 1.04 sg
TVD (ft) TVD (m) OBG (sg) Res ES Norm ES Act (ppg) (sg)
984.3 300.0 1.08 0.80 18 2389 8.7 1.04
1148.3 350.0 1.21 0.78 82 2337 8.7 1.04
1312.3 400.0 1.30 0.98 149 2815 8.7 1.04
1476.4 450.0 1.38 0.98 218 2815 8.7 1.04
1640.4 500.0 1.45 0.88 289 2587 8.7 1.04
1804.5 550.0 1.50 0.85 362 2514 8.7 1.041968.5 600.0 1.55 0.88 437 2587 8.7 1.04
2132.5 650.0 1.60 0.90 513 2634 8.7 1.04
2296.6 700.0 1.64 0.94 592 2726 8.7 1.04
2460.6 750.0 1.67 1.00 672 2858 8.7 1.04
2624.7 800.0 1.71 0.98 754 2815 8.7 1.04
2788.7 850.0 1.74 1.05 838 2963 8.7 1.04
2952.7 900.0 1.76 1.10 923 3064 8.7 1.04
3116.8 950.0 1.79 1.15 1010 3161 8.7 1.04
3280.8 1000.0 1.81 1.10 1099 3064 8.7 1.04
3444.9 1050.0 1.84 1.20 1188 3254 8.7 1.04
3608.9 1100.0 1.86 1.15 1280 3161 8.7 1.04
3772.9 1150.0 1.88 1.28 1372 3396 8.7 1.04
3937.0 1200.0 1.90 1.25 1466 3344 8.7 1.04
4101.0 1250.0 1.92 1.00 1561 2858 8.7 1.04
4265.1 1300.0 1.94 1.00 1657 2858 8.7 1.04
4429.1 1350.0 1.96 1.00 1755 2858 8.7 1.04
4593.1 1400.0 1.97 1.00 1853 2858 8.7 1.04
4757.2 1450.0 1.99 1.23 1953 3308 8.7 1.04
Form ation Water Resistivity
Resistivity
Water resistivity used in
this method can be f rom
0.015 to 0.04 ohmms.
Bryant PPr.
Well - Bideford - 31/7 : Grossenschmuck : Celtic Petroleum
300.0
400.0
500.0
600.0
700.0
800.0
900.0
1000.0
1100.01200.0
1300.0
1400.0
1500.0
1600.0
1700.0
1800.0
1900.0
2000.0
2100.0
2200.02300.0
2400.0
2500.0
0.10 1.00 10.00
res/pp
d e p t h m
Formation Pressure
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0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
900.0
1000.0
1100.0
1200.0
1300.0
1400.0
1500.0
1600.0
1700.0
1800.0
1900.0
2000.0
2100.0
2200.0
2300.0
2400.0
2500.0
2600.0
0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.20 2.30
pore pressure s.g.
d e p t h m . Dxc
Res
Sonic
Bryant PPr.