API Classes of Cement
Types of Cementing
Performing a good cementing job
Cement is made of limestone and clay/or shale mixed in the right proportions
Cement (oxides of Ca, Al, Fe, and Si) is heated in a rotary kiln to between 2,600 oF to 2,800 oF.
The result from the kiln is called Clinker.
Clinker is ground with a controlled amount of gypsum (CaSO4.2H2O) to make Portland cement.
Gypsum retards setting time and increases ultimate strength.
The classes used today are primarily Classes A, C, G, and H.
Classes A, B, and C are used for surface casing job.
Class A is used in some areas because it is very similar to construction cement and can be obtained locally.
Classes A and C are already ground with an accelerator.
Classes G and H are basic cements.
Class G is a finer grind and requires more mix water resulting in a lower density.
Class G is more common around the world.
Classes E and F are used for deep wells under high temperature and pressure.
Cement additives will be sub-divided into these functional groups:
Density of cement should be high enough to prevent the high-pressured formations from entering the well.
Density of cement should not be so high as to cause fracture of the weaker formations.
Density of normal slurry (cement mixed with normal amount of water) is usually too high for formation strength.
It is desirable to lower slurry density by:
Lowering slurry density reduces cost.
Methods of lowering slurry density:
Increase water/cement ratio
Add low-specific-gravity solids
Effect of high (excess) water/cement ratio:
Increase thickening time
Increase free water
Reduces compressive strength
Lowers resistance to sulfate attack
Increases permeability of the set cement
Common low-specific-gravity solids in use are:
Most common light-weight additive
Ability to hydrate permits the use of high water concentration.
High amount will reduce cement strength and thickening time.
Cement strength retrogress with at high temperature above 230 oF.
Diatomaceous earth (Diacel D)
Lower specific gravity than bentonite
Permits higher water/cement ratios without resulting in free water.
Like in bentonite, high amount will reduce cement strength and thickening time.
More expensive than bentonite.
Gilsonite (lustrous asphalt) and Kolite (crushed coal)
Almost no effect on slurry thickening time.
Higher cement compressive strengths than other types of low-density solids.
Siliceous and aluminous materials that react with lime and water to a form a calcium silicates that possesses cementitious properties.
Natural pozzolans are volcanic ash
Artificial pozzolans include glass, furnace slag and fly ash (residue from chimneys of coal-burning power plants.
Only slight reduction in slurry density is achieved.
Volcanic glass bubbles that has sometimes been used in geothermal wells because of its insulating properties.
Considerably more expensive
If there is need for higher slurry density, additives that are used are:
Reddish iron oxide ore (Fe2O3)
Very common because of its high specific gravity (5.02)
For smaller increases in density
Requires more water to keep slurry pumpable
Additional water not needed to be added to the slurry
Little effect on strength and pumpability of the cement
Accelerate cement hydration
Decrease thickening time
Applicable for shallow, low-temperature wells.
They are inorganic compounds:
Little amount is needed. If in excess, it will retard the cement.
In offshore drilling: NaCl, CaCl2, MgCl2in seawater act as accelerators.
Used in concentration of up to 4%
Anhydrous type is preferable because it absorbs moisture less readily
Best result is achieved at a concentration of 5%
Saturated type are used in salt formations or reactive shale formations.
Special grade of gypsum hemihydrate cement is mixed with portland cement.
Maximum working temperature is 140 oF for the regular grade, and 180 oF for the high-temperature grade.
Also called thinners or dispersants
Increase the thickening time of cement
Applicable for deeper, high temperature wells.
Are typically organic compounds
Most common retarder
Very low concentration is needed
Organic acids can be added for high temperature jobs
Sodium tetraborate decahydrate
At high concentrations
CMHEC (carboxylmethyl hydroxyethyl cellulose)
Works with all portland cements
High slurry viscosity will:
Increase pump horsepower
Increase annular frictional pressure loss, which may lead to formation fracture.
Common viscosity-control additives are:
Organic deflocculants (e.g. calcium lignosulfonate)
Certain long-chain polymers
Silica flour –to form stronger, more stable, and less permeable cements at high temperature.
Hydrazine–oxygen scavenger to control corrosion.
Radioactive tracer –to determine where cement has been placed.
Nylon–to make cement more impact resistant
Paraformaldehydeand sodium chromate –to counter the contamination effect of organic deflocculants from drilling muds.
Casing cementing –cementing of casing to the borehole
Liner cementing –cementing of liner. More complex than casing cementing
Plug cementing –to separate lower section of a well from upper section. Open hole or cased hole.
Squeeze cementing –to plug off abandoned perforation, repair annular leaks, and plug severe lost circulation zones.
Performing a Good Cementing Job
Placed on the outside of the casing to help remove mudcake from the borehole walls, either by reciprocating the casing or by rotating the casing.
Bottom rubber wiper plug is released to minimize cement contamination from drilling fluid.
Spacer fluid (or mud preflush) may be pumped also.
Desired volume of slurry is pumped
Top wiper plug is released
Drilling fluid displaces the top plug down the casing
When bottom plug reaches the float collar, its diaphragm ruptures.
The whole cement slurry has been fully displaced when the top plug bumps the bottom plug.
* Note:care should be taken to ensure cement does not reach the subsea well or mudline.
Centralization promotes a good cementing job
Centralizers are used to keep the pipe in the center of the hole
For a non-Newtonian fluid when pipe is not well centralized, the velocity on the narrow of the annulus is slower than the velocity on the wide (Popcorn theory).
The point between the centralizers is the hardest to centralize
The degree of centralization is called % Standoff.
100% -perfectly centralized
0% -pipe touching wall
A minimum standoff of
70% is a good rule-of-thumb
() ()%Standoff1100% bcCDRR + =−×−
Rule of thumb:
1 centralizer per 2 joints in vertical wells.
1 centralizer per 1 joint in directional wells
2 centralizers per 1 joint in high angle or horizontal wells.
Smaller hole clearance will require more centralizers
Pipe movement is rotation and/or reciprocation.
It increases the displacement efficiency
Drag forces associated with rotation will pull the cement into the narrow side of the annulus.
Reciprocation has a tendency to break the gel strengths of the mud in the narrow side of the annulus.
The pipe should be reciprocated from the time the pipe reaches bottom until the cement plug bumps.
Pipe sticking may be a problem
Landing the casing if it starts to stick
Higher viscosity mud is harder to displace
Viscosity of mud can be reduced prior to cementing by adding water (or thinners in a weighted mud).
Move the casing
The thicker the mud, the longer it should take to circulate.
A clean hole allows the ease of running casing and getting centralizers to bottom.
Washouts are hard to cement
If washouts are a problem, change the mud to try and minimize the washouts.
Which should be employed: turbulent, laminar or plug flow?
Has the smallest displacement efficiency (75% and less).
Laminar flow gives inadequate cementations and should be avoided when possible since it promotes channeling
Lowest annular velocity (30 to 90 feet/min, depending upon cement properties).
Limited to cementations of small volumes and where the mud in the hole is of low density.
Can be used when high displacement rate is possible because of ECD or hole size.
High displacement efficiencies.
Applicable to large volume cementations
Applicable where the mud and cement slurry weight are similar.
Limited in use by excessive bottomhole circulating pressure.
Can be limited by insufficient surface pump horsepower.
Primarily chosen because it requires shortest of all cementation times.
Cement and mud are not compatible and should be kept separate.
A spacer fluid is used to separate the cement and mud in the annulus.
The spacer fluid should compatible with both cement and the mud
The spacer volume should equal at least 500 ft in the annulus.
In water based muds (WBM), water is a very good spacer
Water is thin and will easily go into turbulent flow
Compatible with both cement and mud
In weighted muds, spacer may have to be weighted with barite.
Little effect on displacement efficiency
The cement should be at least 0.5 ppg greater than mud density to prevent movement after the pump stops
Difficult to get adequate centralization
There may be cuttings
beds on the bottom of
They should be removed
Should be zero to
minimize the possibility
of a free water channel
on the high side of
Casing setting depth, HTVD= 3,000 ft
Average hole size, Dh= 17-1/2 in.
Casing ID, DCID= 12.615 in.
Casing OD, DCOD = 13-3/8 in.
Float collar (from shoe), HFC = 44 ft
Pump factor, Fp= 0.112 bbl/stroke
Casing program:LEAD (or FILLER)TAIL (or NEAT)
Excess volume = 50%
How many sacks of LEAD cement will be required?
How many sacks of TAIL cement will be required?
How many barrels of mud will be required to bump plugs?
How many strokes of pump will be required to bump plugs?
22Annular capacity,C183.35hCODADD− = 2217.513.375183.35− =30.6946 ft/ft= Determine number of sacks of LEAD: Excess# SacksyieldACh××=0.69462,0001.501.59××=1,311 sacks= 2Casing capacity,C183.35CIDCD= 212.615183.35=30.8679 ft/ft= 2Casing capacity,C1,029.4CIDCD= 212.6151,029.4=0.1545 bbl/ft=
Determine number of sacks of TAIL: Excess# Sacks (annular) yieldACh××=0.69461,0001.501.15××=906 sacks= # Sacks (casing) yieldAFCCH×=0.8679441.15×=33 sacks= # Sacks (total) (annular) (casing)=+90633=+939 sacks=
Determine barrels of mud to bump plugs: ()VolumeTVDFCCHHC=−×()3,000440.1545=−×456.7 bbls= Determine number of strokes to bump plugs: Volume# StrokesPF=456.70.112=4,078 strokes=