JAMES A. CRAIG

Cement Composition API Classes of Cement Cement Additives

Density control Accelerators Retarders Viscosity control

Types of Cementing Cementing Operation

Cementing equipment Cementing process 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.

Clinker

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 control Accelerators Retarders Viscosity control Others

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: Bentonite Diatomaceous earth Solid hydrocarbons Pozzolan Expanded perlite

Bentonite 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.

Solid hydrocarbons 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.

Pozzolans 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.

Perlite 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:

Hematite Reddish iron oxide ore (Fe2O3) Very common because of its high specific gravity (5.02)

Barite Barium sulfate For smaller increases in density Requires more water to keep slurry pumpable

Sand 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:

Calcium chloride Sodium chloride Gypsum Little amount is needed. If in excess, it will retard the

cement. In offshore drilling: NaCl, CaCl2, MgCl2 in seawater act

as accelerators.

Calcium chloride Used in concentration of up to 4% Anhydrous type is preferable because it absorbs

moisture less readily Sodium chloride

Best result is achieved at a concentration of 5% Saturated type are used in salt formations or reactive

shale formations. Gypsum

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

Lignins Borax Sodium chloride Cellulose derivatives

Lignins Calcium lignosulfonate Most common retarder Very low concentration is needed Organic acids can be added for high temperature jobs

Borax Sodium tetraborate decahydrate

Sodium chloride At high concentrations

Cellulose derivatives CMHEC (carboxylmethyl hydroxyethyl cellulose) Commonly used 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) Sodium chloride 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 Paraformaldehyde and sodium chromate – to

counter the contamination effect of organic deflocculants from drilling muds.

Primary cementing Casing cementing – cementing of casing to the borehole Liner cementing – cementing of liner. More complex

than casing cementing

Secondary/remedial 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.

Cementing Equipment

Cementing Process

Performing a Good Cementing Job

Cement head

Guide shoe

Guides the casing past irregularities in the borehole wall.

Centralizer

They are placed on the outside of the casing to help hold the casing in the center of the hole.

Float collar

Acts as a check valve to prevent cement from backing up into the casing.

Float shoe

Serves the function of both a guide shoe and a float collar when no shoe joints are desired.

Cement basket

Placed on the outside of the casing to help support the weight of the cement slurry at points where porous or weak formations are exposed.

Bottom plug

Top plug

Scratchers

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.

Cement excess

* Note: care should be taken to ensure cement does not reach the subsea well or mudline.

Casing centralization Pipe movement Drilling fluid condition Hole condition Displacement velocity Spacer fluids Mud-cement density difference Directional wells

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

( )( )

%Standoff 1 100%b c

C DR R

+= − × −

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?

Laminar flow: Has the smallest displacement efficiency (75% and less). Laminar flow gives inadequate cementations and should

be avoided when possible since it promotes channeling Plug flow:

Lowest annular velocity (30 to 90 feet/min, depending upon cement properties).

Very efficient 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.

Turbulent flow: 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 Cost effective

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 Cuttings bed

There may be cuttingsbeds on the bottom ofthe hole.

They should be removedbefore cementing.

Free water Should be zero to

minimize the possibilityof a free water channel on the high side of the hole.

Data: 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)Density (ppg) 13.8 15.8Height (ft) 2,000 1,000Yield (ft3/sack) 1.59 1.15Excess volume = 50%

Questions: 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?

2 2

Annular capacity,C183.35h COD

AD D−

=2 217.5 13.375183.35−

= 30.6946 ft /ft=

Determine number of sacks of LEAD:

Excess# Sacksyield

AC h× ×=

0.6946 2,000 1.501.59× ×

= 1,311 sacks=

2

Casing capacity,C183.35

CIDC

D=

212.615183.35

= 30.8679 ft /ft=

2

Casing capacity,C1,029.4

CIDC

D=

212.6151,029.4

= 0.1545 bbl/ft=

Determine number of sacks of TAIL:

Excess# Sacks (annular)yield

AC h× ×= 0.6946 1,000 1.50

1.15× ×

= 906 sacks=

# Sacks (casing)yield

A FCC H×=

0.8679 441.15

×= 33 sacks=

# Sacks (total) (annular) (casing)= + 906 33= + 939 sacks=

Determine barrels of mud to bump plugs:

( )Volume TVD FC CH H C= − × ( )3,000 44 0.1545= − × 456.7 bbls=

Determine number of strokes to bump plugs:

Volume# StrokesPF

=456.70.112

= 4,078 strokes=