cementing oil wells
TRANSCRIPT
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Cementing
DRI LL ING ENGINEERING
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CEMENTING
The purposes of this chapter are to present:
1. The primary objectives of cementing
2. The test procedures used to determine if thecement slurry and set-cement have suitable
properties for meeting those objectives.
3. The common additives used to obtain the
desirable properties under various wellconditions.
4. The techniques used to place the cement at
the desired location in the well.
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3.1 Composition of Portland Cement
Portland cement made by burning limestone and clay.
Oxides of Ca, Al, Fe, Si react at high temperatures in theKlin (26002800 oF).
When it cools, it becomes balls of cement clinker.
After aging in the storage, the seasoned clinker is taken tothe grinding mills where gypsum is added to (CaSO4.2H2O)
to retard setting time and increase ultimate strength.
It is sold in units of barrels = 376 lbm or four, 94 lbm sacks.
TYPES OF CEMENT
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Cement is thought to be made up of four crystalline components
in the clinker that hydrate to form a rigid structure.
1. Tricalcium silicate (3 CaO.SiO2 or C3S)
2. Dicalcium silicate (2 CaO.SiO2or C2S)
3. Tricalcium Aluminate (3 CaO.Al2O3or C3A)4. Tetracalcium aluminoferrite(4CaO.Al2O3.Fe2O3C4AF)
The reaction is exothermic and generates a considerable quantity
of heat.
The main cementing compound is 3CaO.2SiO2.3H2O or
tobermorite gel = it has extremely fine particle size.
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Manufactur ing of Portland Cement
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3.2 Cement Testing
API : Recommended Test procedures
Test Equipment
1. Mud balance: to determine slurry density.
2. Filter press: to determine fi l tration rate.
3. Rotational viscometer: to determine rheological properties.
4. Consistometer: to determine thickening rate characters.
5. Cement permeameter: to determine permeabil i ty ofthe set cement.
6. Specimen molds and strength testing machines for
determining the tensile and compressive strength.
7. Autoclave : to determine the soundness of cement.
8. Turbidimeter : to determine the f ineness of cement.
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3.3 Standardization of Dri l l ing Cements
API has defined eight standard classes and three standard
types of cement for use in wells.
Classes are designated by letters A to H .
Types are designated by O, MSR, HSR
To provide uniformity in testing it is necessary to specify
the amount of water to be mixed with each type of cement.
Water content ratio, or normal water content or API
waterof the cement class. Table 3.6
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Well depth and cementing time relationship used in
def ini tion of API cement classes.
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10Physical Requi rement of API Cement Types.
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11Normal Water Content Of Cement Recommended by API
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For each wt% of bentonite added the water content should
be increased by 5.3%
For each wt% of barite added 0.2% of water should be
added.
For 3.5 : Cement mixing time = 20 cuft/min
Displacement rate = 50 cuf t/min
Casing OD = 7.0 in,
Area of Casing = 33.57 sq.in.
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Protect and support the casing Prevent movement of the fluid through the annular
space outside the casing
Stop the movement of fluid into regular or fractured
formations. Close an abandoned portion of the well.
Cement slurry is made by mixing powdered cement and
water.
It is placed by pumping it to the desired location.
The hardened-reacted-cement slurry becomes set cement
a rigid solid that exhibits strength.
PROPERTIES OF CEMENT
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At present the cement classes G and H can be modified easily
through the use of additives to meet almost any job
specifications economically.
Types of cement additives:
(1)Density contr ol additives(2)Setting time control additives
(3)Lost cir culation additives
(4)Fi l tration control additives
(5)Viscosity contr ol additives
(6)Special additives
Yield of cement: the volume of slurry obtained per sack of
cement used.
Percent mix: Content of water expressed as weight percent.
3.4 CEMENT ADDITIVE
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3.4.1 Density Control:
The density of the cement slurry must be high enough toprevent the higher pressured formation fluids from flowing
into the well during cement operation. Yet not so high as to
cause fracture of the weaker formations.
Cement density is reduced by using a high water cementratio, or adding low specific gravity solids, or both.
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Low specific gravity solids used to reduce slurry densityinclude:
1. Bentonite2. Diatomaceous earth3. Solid hydrocarbons4. Expanded perlite5. Pozzolan
Slurry density usually is increased by using a lower watercontent or adding high specific gravity solids. High specificgravity solids used to increase slurry density include:
(a) Hematite
(b) Ilmenite
(c) Barite
(d) Sand
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EXAMPLE 3.5 : Use hemati te to increase the density of
cement to 17.5 lbm/gal. I f the water requi rement are 4.5
gal/94 lbm class H cement and 0.36 gal per 100 lbm
hemati te compute the amount of hemati te that should be
blended with each sack.
Solution:
Assume X = lbm of hematite / sack of cement Total water requirement of slurry = 4.5 + .0036 X
X = 18.3 lbm hematite / sack of cement
volumetotal
masstotal
)0036.05.4()34.8(02.5)34.8(14.3
94
)0036.5.4(34.8945.17
XX
XX
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3.4.2 Bentonite
Use for building drilling fluid viscosity.
Also used extensively as an additive for lowering
cement density.
The addition of bentonite lowers the slurry densitybecause of its lower specific gravity and because
its ability to hydrate permits the use of much
higher water concentration.
In addition to lowering slurries density, the
addition of bentonite lowers slurry cost.
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3.4.3 Diatomaceous
A special grade of diatomaceous earth is used in
portland cements to reduce slurry density.
Lower specific gravity than bentonite.
Permits higher water/cement ratios without
resulting in free water.
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3.4.7 Hemati te
Reddish iron oxide core (Fe2O3) having s specific
gravity of approximate 5.02.
Can be used to increase the density of a cement
slurry to as high as 19 lbm/gal.
The water requirement for the hematite is
approximately 0.36gal/100 lbm hematite.
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3.4.9 Barite
Barite or barium sulphate is extensively used for
increasing the density of a cement slurry.
Water requirement for barite is about 2.4 gal/100
lbm of barite.
The large amount of water required decreases the
compressive strength of the cement and dilutes the
other chemical additives.
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3.4.10 Sand
Sand having low specific gravity of about 2.63,sometimes used to increase slurry density.
Sand requires no additional water to be added to
the slurry. Has little effect on the strength or pumpability of
the cement, but causes the cement surface to berelatively hard.
Also used to form a plug in an open hole as a basefor setting a whipstock tool used to change thedirection of the hole.
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3.4.11 Setting Time Control
The cement must set and develop strength before
drilling activities can be resumed.
Compressive strength = 500 psi common
Tensile strength = 40 psi common
For shallow, low temperature wells it may be necessary
to accelerate the cement hydration so that the waiting
period after cementing is minimized.
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Commonly used accelerators:
1. Calcium chloride (upto 4.0% T < 125oF)*
2. Sodium chloride (upto 5%) **
3. Hemihydrate form of gypsum (T=low)
4. Sodium Silicate (upto 7%)
Cement setting time is also a function of:
Cement composition Fineness Water content
Increases compressive strength (generally) at
saturations > 5% it acts as retarders used to cementsalt and shale formations.
NaCl, CaCl2, MgCl2, at concentrations present in sea
water all act as accelerators.
At T > 160
o
F use retarders when using sea water.
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3.4.12 Calcium Chloride
Concentration up to 4% by weight commonly is
used as a cement accelerator in wells having
bottomhole temp < 125oF.
Available in regular grade (77% calcium chloride)and an anhydrous grade (96% calcium chloride).
Anhydrous grade is in more general use because it
absorbs moisture less readily and is easier tomaintain in storage.
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3.4.13 Sodium Chloride
An accelerator used in low concentration.
Max. accelerator occurs at a concentration of
about 5% (by weight of mixing water) for cements
containing no bentonite.
Saturated sodium chloride cements are used
primarily for cementing through salt formations
and through shale formation that are highlysensitive to fresh water.
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3.4.16 Retarders:
Deflocculants ( lignosulfonates)
(thinners, dispersants)
Halliburton (HR-12)
Borax
CM HEC
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3.4.18 F il tration Control Additives
Functions:(1) Minimize hydration of formations containing
water-sensitive shales.
(2) Prevent increases in slurry viscosity.
(3) Prevent formation of annular bridges which canact as a packer to remove hydrostatic pressureholding back high pressure zones.
(4) Reduce rate of cement dehydration when pumping intoabandoned perforated intervals allowing longer plugs.
Commonly used:
Latex Bentonite with a dispersant CMHEC Various organic polymers, such as Halliburton HALAD-9
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EXAMPLE 3.6
Bi l = 17 in OD = 13.375 in. csg
I D = 12.415 in csg
Depth = 2500 ft
high strength cement column at bottom = 500 ft
composed of class A cement + 2% CaCl2.
upper 2000 ft low density slurry class A cement + 16%
bentoni te + 5% sodium chloride
Water cement ratio = 13 gal/sack
Excess factor = 1.75
Compute the slurry volume and number of cement sacks.
Annular capacity
sftin
ft 22
222
6006.4.14)375.1317(4
SLURRY DESIGN
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Volume of slurry required = 2000 (.6006) (1.75)
= 2102 cu. Ft.
Calculate the yield of cement
=
For Lead (low strength)= Volume of one sack of cement (A) + Volume of added
bentonite per sack (B) + Volume of salt water per sack (C)
cementofsack
slurryofftcu ..
sack
f t
f tlbm
sacklbmwt
awcv
3
3 4797.0/)4.62(14.3
/9494
)(
sack
f tBentoniteofwtb
bentonite
3
0910.0)4.62(65.2
)94)(16(..)(
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(c) =Volume of water = Wt. Of 5% NaCl
= .05 (94) = 4.7 lbm
Water- cement ratio = 13 gal/sack
Water wt. = 13 g.(8.34 ppg)/sack
= 108.4 lbm/sack
Wt. of fraction of NaCl =
From Table 2.3, NaCl= 1.0279 by interpolation
Volume of salt water
Yield = 0.4797 + 0.0910 + 1.7633 = 2.334 cuft/sack
No. of sack = 2102 cuft/2.334 cuft/sack = 901 sacks
0415.07.44.108
7.4
sack
ftwaterofwt
salt
2
768.1)4.62(0279.1
7.44.108
)4.62.(
.
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H igh strength tai l slur ry volume
= (.6006) (500) (1.75) +
= 559.2 cuft
Yield = volume/sack
Volume = vol. of cement (one sack) + Vol. of CaCl2
Cement Volume
Wt. of CaCl2= (0.02) (94) = 1.88 lbm
Wt. Water = (5.2) (8.34) = 43.4 lbm
Wt. Fraction =
144
40)45.12(
4
2
sackft /4797.0)4.62)(143(
94 3
0415.04.4388.1
88.1
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By interpolation from Table 2.4
Volume of salt water (Brine)
Yield = 0.4797 + 0.7025 = 1.182 cuft/sack
No. of sacks of cement sack of cement
Total slurry volume = 2102 + 559.2 = 2661.2 cu.ft.
Total no. of sack of cement = 901 + 473
= 1,374 sacks Answer
0329.12CaCl
7025.0)4.62(0329.1
88.14.43
473182.1
2.559
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Cement Casing Conventional
Equipment:
guide shoe f loat col lar bottom plug top plug
Outside casing: centralizers scratchers cement basket
SUB-SURFACE CASING EQUIPMENT
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Common Cement Placement Requirements.
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Conventional Placement Technique
used for cementing casing
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Guide Shoe (Courtesy World Oils Cementing Handbook)
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F loat collar (Courtesy World Oils Cementing Handbook)
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Centralizers: (a) Bow springs welded on end r ings (b) centralizer with
ref lector vanes (c) sl im-hole centr alizer(Hall iburton Sales and Service Catalog)
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(a) Rotating and (b) reciprocating wall scratchers
(Courtesy World Oils Cementing Handbook)
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Cement baskets (a) in place within the casing and (b) with
limit rings(Courtesy World Oils Cementing Handbook)
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Cementing plugs: (a) top and (b)bottom plugs(Courtesy World Oils Cementing Handbook)
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Different cementing placement techniques are used for:
Cementing casing strings Cementing liner strings Setting cement plugs Squeeze cementing
3.5.2 Stage Cementing
To avoid fracturing formations by reducing cement column
length.
To make sure cement is not lost in low-pressure highly
permeable zones.
3.5.3 I nner-Str ing Cementing
To reduce cementing time and amount of cement left in the
shoe joint of large diameter casing.
Performed using drill pipe or tubing.
3.5 CEMENT PLACEMENT
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3.5.4 Annular-Cementing through tubing:
It is used to bring cement top of the previously placedcement to the surface
Or to repair casing.
3.5.5 Mul tiple Str ing Cementing
It is a multiple completion method that involves
cementing several strings of tubing in the hold withoutthe use of an outer casing strings.
3.5.6 Reverse-Circulation Cementing
It is used when extremely low-strength formation werepresent near the bottom of the hole.
The cement is displaced (pumped) down the annulusand the mud is displaced back through the casing.
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3.5.7 Delayed-Setting Cementing
It is used to obtain a more uniform mud displacement. Use retarded cement slurry having good filtration property
in the well bore before running the casing.
Cement placement is achieved (accomplished) down the
drill pipe and up the annulus. The drill pipe is then removed and casing is lowered to the
unset cement.
3.5.8 Cementing liners
Latch-down plug separator mud from cement.
When it reaches top of liner it actuates a special wiper
plug.
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When wiper-plug reaches the float collar a pressure
increase at the surface signifies the end of the cementdisplacement.
Liner setting tool are activated by:
1. mechanical device (drill pipe rotated and lowered)
2. hydraulic device : drill pipe rotated or a ball or a
plug is dropped and then set by applying pressure.
Tie-back liner = to the top. Stub-liner = up the liner but not to the top = to repair leak
at liner top.
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3.5.9 Plug Cementing
Prevent fluid communications between an abandonedlower portion of the well and the upper part of the well.
Placed using drill pipe or tubing.
Bridge plug is used to assist in forming a good
hydraulic seal.