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8/12/2019 4 Clays Chemistry and Problems
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CLAYS CHEMISTRY
AND PROBLEMS
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INDEX
• CLAYS CHEMISTRY…. Pag. 03• CLAYS PROBLEMS …. Pag. 41
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In drilling fluids industry, certain clay minerals as smectite, a major component of
bentonite, are used to provide viscosity, gel structure and fluid-loss control.
Formation clays are unintentionally incorporated into the mud during drilling and in
this case, they may cause various problems
INTRODUCTION
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• Clay minerals can be beneficial (bentonite) or harmful (gumbo) to the fluid system.
• Gumbo clays are encountered in superficial drilling and they are often soft and plastic
when wet, hard when dry and they cause problems of plug in the shale shakers and
flow line or extremely increase the solid contents in the mud if dissolved.
• Bentonite is the term used for a particular smectite (clay) sodium montmorillonite,
used as additive for mud.
INTRODUCTION
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TYPES OF CLAYS
There is a great deal of clay minerals, but in drilling fluids, they can be categorized
into three types:
• Attapulgite and sepiolite.
• Needle-shaped, non swelling clays. Used for their shape and their colloidal stability
in sea water based mud.
• Illite, chlorite, kaolinite which have a flattened shape, do not swell out again orcan have a reduced swelling.
• Montmorillonites which have a flattened shape but large swelling properties. In
nature, it is often calcium montmorillonite, but the use in drilling fluids is in thesodium montmorillonite quality (Wyoming).
This is the only type of clay intentionally added to the mud, while the other clay
types can be considered contaminants
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Clays exist in nature with a stacked or layered structure, with each unit layer roughly 10
angstroms ( Ǻ) thick. This means there are about a million layers of clay per millimeter of
thickness. Each clay layer is highly flexible, very thin and has a huge surface area. Anindividual clay particle can be thought of as being much like a sheet of paper or a piece of
cellophane. One gram of sodium montmorillonite has a total layer surface area of 8,073
ft2 (750 m2)!
In freshwater, the layers adsorb water and swell to the point where the forces holding
them together become weakened and individual layers can be separated from the packs.
Separating these packs into multiple layers is known as dispersion. This increase in
number of particles, with the resulting increase in surface area, causes the suspension to
thicken.
TYPES OF CLAYS
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Photo of bentonite particles
The characteristic shape of the particlesproduces the so-called shingling effectwhich is very important for the fluid-losscontrol
TYPES OF CLAYS
Ideal particle of montmori llonite
Three-layer clays are built of unit layerscomposed of two tetrahedral sheets oneither sides of one octahedral sheet,like a sandwich
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Pyrophyllite with neutral chargeThis neutral clay is very similar to the following negativelycharged montmorillonite
TYPES OF CLAYS
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The substitution of Mg2+ for Al3+ creates a particle with a negative chargeThe net negative charge is compensated by the adsorption of cations (positive ions)on the unit layer surfaces, and are called the exchangeable cations of the clay. Thequantity of cations per unit weight of the clay is measured and reported as the CEC.
Montmorillonite (Three-layer clays)
TYPES OF CLAYS
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Smectite structure (Montmori llonite)The most typical property of
montmorillonites is that of interlayer swelling (hydrating) with water.
TYPES OF CLAYS
Montmori llonite (Three-layer clays)
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Ill ites (Three-layer clays)
The structures of the illites are similar to the montmorillonites structures but
they do not show interlayer swelling. Instead of thesubstitution of Mg2+ for
Al3+, as in motmorillonites, Illites have a subsitute Al3+ for Si4+.
The compensating cations are the potassium ions (K+).
The gap among the single layers is 2.8 Å because the ionic diameter of K+ is
2.66 Å, there is a prevention of swelling in presence of water .
TYPES OF CLAYS
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Chlorites (Three layer clays)
Chlorites are structurally connected to three layer clays. If pure they will not swell, but it
happens when there are alteration forms.
The cationic exchange capacity of chlorites varies from10 to 20 meq/100 g.Usually, the distance among the intermediate layers of the chlorites is around 14 Å.
Chlorite is often found in old, deeply marine sediments and normally does not cause
significant problems.
TYPES OF CLAYS
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Kaolinites (Two-layer clays)
Kaolinite is a non-swelling clay, its layers are closely tied through hydrogen bond.
This prevent the expansion of the particles because water is unable to penetrate the
layers.
Kaolinite has a relatively low cationic exchange (5 to 15 meq/100 g).
Kaolinites are commonly found as minor to moderate constituents in sedimentary
rocks.
TYPES OF CLAYS
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Group Structure Tension Cationic
exchange
InteratomicDistance
(Å) Hydratability
Kaolinite 1:1 Layer No No 7.2 No
Talc 2:1 Layer No No 9.3 No
Smectite 2:1 Layer 0.3 - 0.6Na+,Ca2+, K+,Mg2+
11 - 15 Variable
Vermiculite 2:1 Layer 1.0 - 4.0 K+
, Mg2+
14 - 15 Variable
Illite 2:1 Layer 1.3 - 2.0 K+ 10 No
Mica 2:1 Layer 2.0
K+ 10 No
Chlorite 2:2 Layer Variable 14 No
Sepiolite 2:1 chain No No 12 No
Palygorskite
2:1 chain Minor No 10.5 No
Most common clays
TYPES OF CLAYS
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Comparison of clay structure
TYPES OF CLAYS
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CATIONIC EXCHANGE CAPACITY
CEC
The compensating cations that are adsorbed on the unit-layer surface may be exchanged
for other cations and are called the exchangeable cations of the clay. The quantity of
cations per unit weight of clay is measured and reported as the CEC. The CEC is
expressed in millequivalents per 100 g of dry clay (meq/100 g). The CEC of
montmorillonites is within the range of 80 to 150 meq/11 g of dry clay. The CEC of illites
and chlorites is about 10 to 40 meq/100 g, and for kaolinites it is about 3 to 10 meq/100 g
of clay.
The Methylene Blue Test (MBT) is an indicator of the apparent CEC of a clay.
In order to know which cations will replace the other cations, observing the scheme eachcation on the left will replace a correspondent cation on the right
H+ > Al3+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+ > Li+
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• The commercial clays such as Bentonite, are added to water during the preparation
of a water-based mud. They can:
1) give the mud the right viscosity;
2) contribute to create a film which will impermeabilize the formation;
• In the water/clay-muds, water is used as a liquid in a continuous phase in which
certain materials are maintained in suspension and where other materials are
dissolved.
CLAY/WATER-BASED MUD COMPOSITION
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Additives for muds used in order to obtain determined characteristics can be divided
in three categories:
1. The continuous phase of the mud is water;
2. The base of the reactive solids is composed of commercial clays, incorporated
hydratable clays and clays coming from drilling cuttings then maintained insuspension in the fluid phase. Some of this solids are treated chemically to
control the properties of the mud;
3. Inert solids are solids in suspension without chemical reaction such aslimestone, dolomite and sandstone. Barite is added to drilling mud in order to
increase its density and it is also an inert solid (high-density solid).
CLAY/WATER-BASED MUD COMPOSITION
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Clay CEC (meq/100 g)
Smectite 80 - 150
Illite 10 - 40
Chlorite 10 - 40
Kaolinite 3 - 10
cec range for pure clay mineral materials
CLAY/WATER-BASED MUD COMPOSITION
Clays hydration
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CLAY/WATER-BASED MUD COMPOSITION
Depending on the cationspresent, the interlayer spacing of
dry montmorillonite will bebetween 9.8 (sodium) and 12.1 Å (calcium) and filled with tightlybound water. When dry claycontacts freshwater, the
interlayer spacing expands, andthe clay adsorbs a large“envelope” of water. These twophenomena allow clays togenerate viscosity. Calcium-
base bentonites only expand to17 Å, while sodium bentoniteexpands to 40 Å.
Comparison of swelling for Calcium and Sodium Montmorillonite
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Cationic influence of the hydration
• As mentioned previously, the substitutive force of a cation with another cation is as
follows:
H+ > Al3+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+ > Li+
Cation Ion Diameter (Å)
HydratedDiameter
(Å)Li+ 1.56 14.6
Na+ 1.90 11.2
K+ 2.66 7.6
NH4+
2.86 5.0Mg2+ 1.30 21.6
Ca2+ 1.98 19.2
Al3+ 1.00 18.0
Ionic radius and hydration radius of
the most recurrent cations. A cation may serve as a bond to holdthe clay mineral particles together,thereby decreasing hydration.Multivalent cations tie layers together
more firmly than monovalent cations,usually resulting in aggregation of theclay particles. Potassium, amonovalent cation, is the exception tothe rule.
CLAY/WATER-BASE MUD COMPOSITION
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CLAY/WATER-BASE MUD COMPOSITION
Clay reactions with potassium ions (K+)
The chemical reactions between clay and potassium ions are unique if compared to
other ions. The use of K+ is increasing and it is well known to stabilize reactive
shales, also when concnetration of K+ is below 5%.
According to Eberl (1980), there are two ways that K+ can become associated with
clay minerals:
1. Ion Exchange.
2. Ions Fixing.The ion exchange rate depends on the concentration of the K+ ion; the higher the
ratio of K+ ion to Na+ ion, the faster is the rate of exchange.
The fixation of K+ in montmorillonite clays can happen in particular conditions and is
due to a sort of alteration known as Burial Diagenesis. This diagenetic alteration ina two step reaction, creates a conversion from smectite to illite by the fixation of K+.
The complete reaction to transform smectite to illite, occurs if a large amount of K+
is present.
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Clay Particles associated inone of the following states:
• Aggregation• Dispersion• Flocculation• De-Flocculation
CLAY/WATER-BASED MUD COMPOSITION
Clay – Particle – Linking processes
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• Aggregation: leads to the formation of packets. The result is a decrease in the number of
particles and in plastic viscosity. It could happen when drilling in gypsum or anhydrite
formation, due to divalent cations Ca++ . It also occurs when drilling out cement.
• Dispersion: The opposite of aggregation. The increase in the number of particles gives a
higher plastic viscosity value.
The degree of dispersion depends on the electrolyte content of the water. Time, temperature
and exchangeable cations on the clay. Dispersion increases with time, with increase in
temperature and when the water has low salinity and hardness.
• Flocculation: the association of particles edge-to-edge and edge-to-face resulting in a
structure similar to “House of Cards”. This situation is promoted by everything that increases
the repelling force between the particles (e.g. addition of divalent cations, high temperature).
Flocculation causes increase in viscosity, gel strength, fluid loss.
CLAY/WATER-BASED MUD COMPOSITION
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De-Flocculation: when the electrochemical charges on the clays are neutralized,
the attraction between the particles is removed. The result is a decrease in
viscosity and an improvement in the formation of the filter cake to reduce the fluid
loss. The chemicals used as deflocculating action are also called mud thinners.
CLAY/WATER-BASED MUD COMPOSITION
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CLAY/WATER-BASED MUD COMPOSITION
Clay performance (Yield of clay)
The yield of a clay is defined asthe number of barrels of 15 cPmud that can be obtained from
1 ton of dry material in thisgraph. You can see that for bentonite, only 20 lbs/bbl isrequired to produce a 15 cPviscosity mud. It would contain
less than 6% solid by weight(2,5% by volume) and the yieldis 100 bbl/ton.By comparison, if sub-bentonite is used to prepare a
15 cP visc. Mud, 75 lbs/bbl arerequired, It would contain 18%solid by w. (8,5% by vol.) andthe yield only bbl/ton.
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Components Parts per Milion
(mg/l)Sodium 10,550
Chloride 18,970
Sulphate 2,650
Magnesium 1,270
Calcium 400
Potassium 380
Bromide 65Other components 80
CLAY/WATER-BASED MUD COMPOSITION
Factors which influence the clay performance
NOTE: Brackish water could have the same components but with differentconcentrations
Hydration and dispersion of dryclay are Influenced by the ionscontained in the water used to build
the mud.Many drilling muds off-shore, areprepared using SW for economyand convenience.
Typical seawater composition
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Effects on viscosity by additionof bentonite to water whichcontains various concentrationsof salt or calcium.The hydration of clays in freshwater decreases rapidly withincreasing concentrations ofthese ions.The following figures can better explain the different hydration
(and consequent swelling) indifferent water used.
CLAY/WATER-BASED MUD COMPOSITION
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Bentonite hydration with salt water
Bentonite hydration with fresh water
CLAY/WATER-BASED MUD COMPOSITION
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Pre-hydrated bentonite in salt water
CLAY/WATER-BASED MUD COMPOSITION
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Effect of calcium on pre-hydratedbentonite.The initial increase of viscosity is dueto the flocculation caused by theaddition of divalent cation Ca++.
When the concentration of Ca++
reaches the breakover point, itcauses the aggregation of theparticles. The dehydration withreduction of number of particles
decreases also the viscosity.
CLAY/WATER-BASED MUD COMPOSITION
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Salt effect on pre-hydratedbentonite.
This is similar to the previouspicture but due to highconcentration of Na+ cation.
CLAY/WATER-BASED MUD COMPOSITION
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pH effect on Wyoming bentoniteUsually, all the muds work in pHalkaline (above 7). This is for obvious reasons such as safety and
corrosion.Looking at this picture, it isunderstandable why the majority ofmuds are maintained in a range ofpH between 8 and 9.5. Above this
value, the dispersion of clayincreases and as a consequencethe viscosity.
CLAY/WATER-BASED MUD COMPOSITION
pH effect
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CHEMICAL TREATMENT PRINCIPLES
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Phosphates
•The main types of phosphates used in drilling fluids are:
1. Sodium pyrophosphate acid with a pH of 4.8
2. Sodium tetra phosphate with a pH of 8.0
All are powerful anionic dispersant and a small amount can strongly reduce the viscosity.
The addition of these products rarely exceed 0.2 lb/bbl (0.5 kg/mc)
They are used in combination with NaOH to avoid alkalinity decrease. They remove Ca++
and Mg++. They cannot be used over the temperature of 175°F (80°C) because of their
transformation in orthophosphates that are flocculants.
CHEMICAL TREATMENT PRINCIPLES
CHEMICAL TREATMENT PRINCIPLES
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CHEMICAL TREATMENT PRINCIPLES
Lignite
Acid lignite (pH 3.2) is used to control viscosity. The mud needs to be run
in alkaline range to be effective.
On site the ratio between caustic soda and lignite varies from 1:6 to 1:2.
The best lignite performance is obtained in the muds with a pH from 9 to10.5.
They are moderately effective at higher salt concentration and not effective
at high calcium conc.
CHEMICAL TREATMENT PRINCIPLES
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CHEMICAL TREATMENT PRINCIPLES
Lignins
Lignins are an assembling of products similar to lignite and lignosulfonates and
they are obtained chemically. Their aim is to fluidize the mud and to control the
filtrate in low pH muds.
CHEMICAL TREATMENT PRINCIPLES
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CHEMICAL TREATMENT PRINCIPLES
Linosulfonates
Lignosulfonates include:• Chromium lignosulfonates
• Ferrochromium lignosulfonates.
• Many applications in deflocculated water based-muds.
Characteristics:
• Keep their characteristics in presence of high level of calcium
• Good performance at all alkalinity level
• Can be used with high salt concentrations
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CLAYS PROBLEMS
Clays problems
INTRODUCTION
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The borehole instability during shales drilling is a common problem.
Shales are sedimentary rocks generally deposited in marin basins.
Shales with sands are called→sandstone shales
The Fluid Supervisor controls the→hydration degree of the shales together with
the cementing materails that connect the shales.
INTRODUCTION
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Shales are composed of structurally different clay minerals
The most common are:
Smectite Montmorillonite Illite Chlorite Kaolinite
SHALES: DEFINITIONS
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• Shale is a generic term used to describe clay-rich sedimentary rocks. The correctgeneric term is mudrock.
Other terms used to describe shale include:
• Clay
• Claystone
• Mudstone
• Argillite
• Gumbo
• Shales can be massive, interbedded, fissile, silty, swelling, dispersive etc…
• It is estimated that over 75% of footage drilled is through shales
CLASSIFICATION OF SHALES
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Shales can be classified according to
• Age
• Mineralogy
• Hardness
• Moisture content
• Type of reaction with drilling fluids / mode of wellbore failure
• Depth of burial / diagenetic history
CLASSIFICATION OF SHALES (AFTER MONDSHINE)
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( )
2.5-2.75-30Illite,
Kaolinite,
chlorite
2-50-3BrittleE
2.2-2.520-30Illite +
possiblesmectite
5-153-10HardD
2.3-2.720-30Illite+mixed layer 2-1010-20Firm-hardC
1.5-2.220-30Illite+
mixed layer
15-2510-20FirmB
1.2-1.520-30Smectite+
illite
25-7020-40Soft A
Density
gm/cc
Clay
Wt%
Clay
Minerals
WaterContentwt%
CEC
meq/100 g
TextureClass
CLAYS INSTABILITY FACTORS
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• Reactive or chemically stressed shale
• Mechanical stress
• Geopressure
• Hydro-pressure
• Overburden stress
Instability in the formation can be due to:
CONSEQUENCES OF SHALE INSTABILITY
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• Hole closure
Repeated reaming
Stuck pipe (in mobile
formations)Stuck casing
Logging difficulties
• Hole enlargement
Stuck pipe due to hole packingoff (cavings)
Poor hole cleaningLoss of directional control
Poor quality coment jobs
Logging difficulties
Poor quality log data
It is estimated that shale problems cost the industry over $600 million / year
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CHEMICAL STRESSReactive Clay
Causes:
• Clay susceptibility to water and mud with null or insufficient inhibition.
• Clay, hydrating with water, expands in the borehole
• Time
Chemical problems during drilling shale interval, can be defined as chemical
interactions between shale minerals and mud filtrate
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FIG 1. Reactive clays
Clays folding 1 DAY EXPOSITION
ABSORBEB
BY SH ALE
ABSORBEB
BY SH ALE
Hole Wall
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3 DAYS EXPOSITION
The reaction depends on time.Clay balls
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5 DAYS EXPOSITION
Overpull during POOH.
Impossible or limited circulation.
S T U C KP AC K O F F
O V E R
P U L L
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Reactive clays indicators
Pressure increase→circulation beginningSwabbingMud loss
IncreaseLight increase withweightOverpull without slips
Circulation
Pressure increase→circulation beginning
SwabbingMud loss
IncreaseLight increaseIncrease without slipsBack Reaming
It begins in depthwhere problems arise.Probable mud loss
Light increase withweightOverpull withoutwedges
Running
SwabbingLight increaseWithout wedgesPullin Out
Back pressure beforetheBack flow connection
Increase→ circulationbeginning
Without slipsConnection
Pressure increasingGradual ROP
decreasingProbable mud loss
IncreaseLight increaseLight increase
Drilling
Other PressureTorsionOverpull
Reactive Cuttings IndicatorsPhase
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High viscosity funnel. & YP. PV, low gravity solids & increasingCEC. Probable mud density increase.mud eng
Large quantities of hydrated clay cuttings. High value inflating claystest
mud logger
Soft aggregation of shales. Hydrated clay (gumbo). Flow lineblocking
shaker
Visual Indications
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Reactive Clays
Preventive actions
• Salts addition (potassium, sodium, calcium, etc.) to reduce water to salt
chemical attraction.
• Encapsulant polymer addition (coating) to reduce water/clay contact.
• Use of oil muds or synthetic base oils to cut water/clay contact out.• Minimize OH permanence.
• Provide reaming at regular intervals or at first signs of reactive clays.
• Correct hydraulic handling for bit and well cleaning.
• Keep ideal mud properties and minimize low gravity solids.
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MECHANICAL STRESS
Geo-pressured clay (figure 2)
Cause :
• Drilling of pressurized clays with insufficient mud density• Caving of fractured clays in the hole
Attent ion :
First signs arise when reaching the clays:• Mud logger record indicating porosity pressure increase
• ROP increase at the commencement of drilling
• Connections overpull and torsion increase
• Settlings at the connections, swellings, caving of fractured clays
• Probable gas increase
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Indicators :
• It often happens in trip, sometimes in drilling
• Pack-off, caving possibility
• Circulation difficulty or impossibility
First actions :
• Apply pressure moderately (200 – 400 psi)
• Apply torsion, run-in with the maximum weight
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FIG 2. Geo-Pressured Clay
H ole W allH ole W all
Hydrostatic
Pressure
6000 Psi5500 Psi
Formation pressure
5500Psi 6000 Psi
Drilling
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Trip
Pack-Off
Stuck
Ov er p ul l
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Preventive actions for mechanically stressed clays.
• Consider well data off-set a/o computer modules for the simulation of the clays
structural limits in the planning of each hole section.
• Increase the mud density with the increasing of the inclination of the well and TVD to
keep the hole stability.
• In the exploration wells, regularly consult the Mud Logger for formation pressure
changes. Increase gradually the mud density till the symptoms disappearance
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• If possible, increase mud density gradually (from 0.1 to 0.2 ppg per day) till the
desired value. The clays sensible to hydrostatic will be counerbalanced.
• Avoid mud density decreasing after more than one exposition day to the clays
sensible to hydrostatic. If this decreasing should be necessary , reduce the density
gradually during a period similar to the exposition one.• Reference to Shaker Handover Notes to determine cuttings volume, block and
cuttings shape
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• Keep mud properties during hole cleaning
• Sweeps usage for well cleaning
• Stop drilling and re-start it only after hole conditioning
• Keep OH permanence minimum
• Define action plan if problems arise
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Hydro-pressured clay (figures, 3 to 7)
Causes :
• Delayed intervention. The mud invades the formation overloading it
• Imprecise movement the battery can interfere with not stabilized clay
• The clay, folding, falls in the hole causing overpull or drill string stuck
Attention :
• Normally, it happens after a weight drop
• Torsion and overpull increase
• Shaker clay
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Indications :
• Probably in trip or drilling phase
• Pack-off, caving possibility• Difficulty or circulation impossibility
First Actions :• Apply pressure moderately (200 – 400 psi)
• Apply torsion, to run-in with maximum weight
• Impossible or drastically limited circulation
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Preventive Actions :
• Use OBM, SBM or oil mud in case of trouble
• If necessary, drop mud weight, gradually during the circulation cycles
• Minimize swab / purge pressure
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5000 PsiHydrostatic
Pressure
6000 Psi
Stabilized
Shale
FIG 3. Hydro-Pressured clays pattern
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Caving formation :
Clay Stability Effect : a good conditions borehole, can loose its stabilitybecause of formation pressure increasing during the swab/surge trips
.
Pressure differential effects on clays stabili ty
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FIG 4. Pressure Penetration in Sandstone and Clay
• Pressure differentialcreates filter cake
• Filter cake prevents
pressure transmission
• Pore presure constant
• Flow due to pressure
differential
• Pressure transmitted
• Pore pressure increases
with time
t2
SANDSTONE
Mud Pressure
Borehole
wall
Pore Pressure Distance from
well bore
t2
SANDSTONE
Mud Pressure
Borehole
wall
Pore Pressure Distance from
well bore
t2
SANDSTONE
Mud Pressure
Borehole
wall
Pore Pressure Distance from
well bore
SANDSTONE
Mud Pressure
Borehole
wall
Pore Pressure Distance from
well bore
t0 t1 t2
SHALE
Mud Pressure
Boreholewall
Pore Pressure Distance from
well bore
t0 t1 t2
SHALE
Mud Pressure
Boreholewall
Pore Pressure Distance from
well bore
t0 t1 t2
SHALE
Mud Pressure
Boreholewall
Pore Pressure Distance from
well bore
SHALE
Mud Pressure
Boreholewall
Pore Pressure Distance from
well bore
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FIG 5. Pore Pressure in Clay Distribution (water)
1
1
10
20
30
40
50
60
3 5 7 9 11 13 15 17 19 21
Kshale
= 10-7 Darcy
1
1
10
20
30
40
50
60
3 5 7 9 11 13 15 17 19 21
Kshale
= 10-7 Darcy
1 Day
7 Days
45 Days
P r e s s u r e ( B a r )
Normalised distance from borehole wall r/R
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FIG 6. Porous clay pressure overloads for hydrostatic over balance effect.
6000 PSI5000 PSI 5000 PSI
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FIG 7. Porous clay pressure overloads for hydrostatic over balance effect
S T U C K
P AC K O F F
O V E R
P U L L
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Overburden stresses
Causes :
• Insufficient Mud Density
• Mud Density not adequate to the angle increase
• Clays caving in the hole
Attention :
• Borehole cleaning problems
• Overpull and torsion increase
• Clay at the shaker
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Indications
• Probably in drilling or trip phase
• Pack-off, caving possibility
• Circulation difficulty or impossibility
First Actions:
• Apply pressure moderately (200 – 400 psi)
• Apply torsion, to drop with maximum weight
Preventive Actions :
• Correct mud density to stabilize the overburden
• As the inclination of the well increases, the mud density increases
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FIG 9 Overburden stress
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Overburden stress
• Mud density→ not
sufficient to support the
overburden.
• Mud density→ not correct
for the angulation
• Clays caving in the
borehole.
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Tectonic Stresses (figure 10)
Causes :
• Lateral natural strngth recurrent in the formation
• Fractures in the stressed clays, the consequent cavings “stick” the string
• The sandstones thrust cause borehole tight
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Attent ion :
• Mountainous places
• Known Tectonic• Abnormal torsion and overpull
• Clays blocks subject to caving
• Elliptic hole tendency
Indications :
• Probable in trip or drilling.
• Impossible or limited circulation
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First Actions :
• Apply pressure moderately (200 – 400 psi)
• Apply torsion, to run-in with maximum weight
Preventive Actions :
• If possible, increase mud density
• Circulate sweeps at high density
• Minimize swab
• Minimize permanence in OH
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FIG 10 Tectonic stresses
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Aqueous Mud
Generally, we have aqueous muds stability through ionic inhibition, encapsulation and
physical block. The stability degree won’t be high as in oil-base mud. However, a water
treated mud, can be used with excellent results also with problematic clays.
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*Not available
*1.14 Al+++
17.62.86Ba++
19.22.12Ca++
21.61.56Mg++
7.62.86NH4+
7.62.66K+
11.21.96Na+
HydratedNot hydrated
Ionic Diameter (Angstroms)Ion
Ionic diameters
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Oil-base Muds
Crumbly, dispersible and hydratable clays are sensible to water. The instability can be
reduced when mud water is not in contact with the clay.
A solution is to use an oil-based mud where water is emulsified in the oil-phase
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Capil lary Effects
WpPp
WpPp
PpWp PpWp Wp = Wellbore Pressure
Pp = Pore Pressure
Dp = Differential Pressure
OBMWBM
Capillary Action