OIL WELL CEMENT Application- 7 Most Common Oil Well Cement Additives

Oil Well Cement is a cement filling space between steel casing of well and rocks and is manufactured to suit the special requirements of the oil or a gas well.

Let us try to understand what these requirements are and where oil well cement is actually used in an oil/gas well in this post. 

Cement in Oil Well

You may be wondering why steel casing of oil well is to be cemented with the rocks. Let us see the purpose of cementing the steel casing in an oil/gas well-

• To prevent the unwanted migration of fluid from one rock formation layer to another
• To prevent the pollution of oil zone, which is valuable
• To protect the casing from external pressure which may result in collapse of the casing
• To prevent the casing from possible damage due to corrosive gases and water

As you may know, oil wells are drilled in sedimentary rocks at a depth until oil is reached. Of course, this depth is not near to the ground. It is too deep beyond 6100 metres, where high temperature and pressure may be found.

During oil mining, there is a possibility of oil getting stuck. If this happens, oil may escape from the space between steel casing and rock formation.

Therefore, this annular space needs to be sealed off. For this purpose, cement slurry is used. It fills up the fissures and cavities in the rock layers.

Requirements of Oil Well Cement

The slurry described above is to be placed at a great depth and pumped into position. Here, the temperature rise up to 175 °C and pressure may be up to 1300 kg/cm². 

Cement slurry has to remain sufficiently flowy for several hours to be pumped in these conditions.

Therefore, water-cement ratio is kept more. Also, the setting of cement should be reduced. The setting time of cement can be increased by reducing-

• C₃A content
• Fineness

Then again, the slurry has to set and harden quickly once pumped into place.

The cement slurry also has to resist the corrosive condition because of sulphur gases or water containing dissolved salts.

The cement fulfilling all these requirements can be used in oil well and is called oil well cement. Modified ordinary portland cement are manufactured to serve this purpose.

Oil Well Cement Composition

To obtain the properties described above, following methods can be used-

1. Adjusting compounds of cement

2. Adding retarders to ordinary portland cement 

Retarders prevent the quick setting of cement. this keeps the slurry in flowy condition. Hence, slurry gets enough time to penetrate into all the fissures and cavities.

There are many admixtures at present, which are patented under retarders. Some of the common retarders are starch, cellulose products, acids, etc.

3. Adding workability agents. 

Workability agents are also added sometimes to increase the flowabiltiy of concrete to facilitate the easy penetration of slurry into fissures.

Development of Oil Well Cement 

Initial oil well cement was developed by using coarse ground cement. Afterwards, it came into light that cement needs to be retarded further if it is to be used at temperature above 82 ° C.

Cement setting time can be further retarded by adding–

• Lignosulphonates
• Cellulose products
• Salts of acids with one/more hydroxyl groups

Another crucial requirement as seen above was that cement must gain enough strength upon hardening. 

With time, it was also discovered that lowering CaO/SiO₂ ratio of cement hydration products helps in achieving high strength. 

At temperature above 110 ° C, CaO/SiO₂ ratio is lowered below 1.3. This helps in attaining high strength after hardening. For this purpose, silica flour is added.

On this basis, numerous additives were developed that can improve the properties and qualities of oil well cement.

Classes of Oil Well Cement

Both American Petroleum Institute and Indian Standards have given nine classes of oil well
cement from A to H & J. But the specification of cement falling under a particular class is
different in both the standards. Let us have a look at this classification.

As per American Petroleum Institute

Nine classes of cement are formed based on their characteristics for the cement used in oil well. This classification is specified in American Petroleum Institute Standard 10A.

API Class A-C: 

Cement belonging to these classes has –

• Low C₃A content
• No retarder 

It is used for well depths up to 1830 metre, where temperature ranges from 27 to 77 ° C.

API Class F: 

Cement belonging to this class has–

• Low C₃A content
• Retarder is present

It is used for well depth ranging from 3048 m to 4877 m, where temperature ranges from 110 to 160 ° C.

API Class G, H: 

Cement belonging to these classes has –

• Coarse-ground ASTM Type II & Type V cement
• No retarder 

It is used where well temperature is between 27 and 93 ° C.

API Class J: 

Cement belonging to this class has –

• Beta dicalcium silicate & pulverized sand
• No retarder 

It is used for well depth more than 6100 m, where temperature is more than 177 ° C.

It is developed recently. Even at high temperature, it can be used for cementing without adding any retarder.

The clinkers of beta dicalcium silicate are ground to attain Blaine fineness of 200 m²/kg. 40 % silica flour is added by mass for this purpose.

3 Grades of Oil Well Cement – (Based on sulphate resistance)

The three grades are-

Grade O: Ordinary Cement

Grade MSR: Moderate Sulphate Resistance

Grade HSR: High Sulphate Resistance

As per IS: 8229 – 1986, 9 classes of oil well cement are as follows:

Class A

• It can be used for depth up to 1830 metre. 
• This cement does not contain any special properties.

Class B

• It can be used for depth up to 1830 metre.
• It provides moderate to high sulphate resistance.

Class C

• It can be used for depth up to 1830 metre.
• It provides ordinary to high sulphate resistance. Besides this, it also provides high early strength.

Class D

• It can be used for the depth range from 1830 to 3050 metres. 
• It provides moderate to high sulphate resistance.
• It can be used under moderately high temperature and pressure conditions.

Class E

• It can be used for the depth range from 3050 to 4270 metres. 
• It provides moderate to high sulphate resistance.
• It can be used under high temperature and pressure conditions.

Class F

• It can be used for the depth range from 3050 to 4880 metres. 
• It provides moderate to high sulphate resistance.
• It can be used under extremely high temperature and pressure conditions.

Class G

• It can be used as basic cement up to 2440 metres. 
• It provides moderate to high sulphate resistance.
• It can be used with accelerators and retarders so that it can be applied to a wide range of depth and temperature conditions.
• For additives, only calcium sulphate/water can be blended with clinkers in the manufacture of this cement.

Class H

• It can be used as basic cement up to 2440 metres. 
• It provides moderate sulphate resistance.
• It can be used with accelerators and retarders so that it can be applied to a wide range of depth and temperature conditions.
•  For additives, only calcium sulphate/water can be blended with clinkers in the manufacture of this cement.

Class J

• It can be used as basic cement for the depth range from 3660 to 4880 metres.
• It provides moderate to high sulphate resistance.
• It can be used under extremely high temperature and pressure conditions.
• It can be used with accelerators and retarders so that it can be applied to a wide range of depth and temperature conditions.
•  For additives, only calcium sulphate/water can be blended with clinkers in the manufacture of this cement.

Manufacturing Process of Oil Well Cement

Oil well cement manufacture for different classes is given below:

Class A, B, C, G & H

Cement clinkers containing hydraulic calcium silicate is ground.

No other material is added except for calcium silicate or its forms during the grinding of clinker.

Class D, E, F

Set modifying agents are blended together with cement clinkers having calcium silicate. 

Class J

It is manufactured conforming to the requirements specified in the standard.

Oil Well Cement Additives

Petroleum industry generally makes use of basic portland cement with low C₃A content. However, the particles are kept coarse than OPC. API classes G & H are most popular.

In this cement, one or more admixtures are added at the site itself (the purpose is mentioned beside it):

1. Cement retarders- 

Retarders are used to reduce the setting of cement, thus increasing setting time. It allows the placing of cement in the well properly.

Retarders inhibit hydration of cement increasing the setting period of cement. As accelerators, retarders do not affect the ultimate strength of cement, but slow down the rate of strength development in earlier phase.

Commonly used retarders are listed below:

• Lignosulphate is the most widely used retarder. 
• Cellulose derivatives like hydroxyethyl cellulose and carboxymethyl cellulose
• Sugar derivatives in small concentrations (At high concentration, it acts as accelerator)

2. Cement accelerators- 

Accelerators are used to increase setting of cement, if required. For permafrost zone, setting time needs to be reduced so that early strength can be developed.

When cement slurries are used in shallow wells having low temperatures, accelerators are required to speed up the drilling process.

Accelerators are not required in deep wells as high temperature increases the setting of cement.

Even though ultimate strength remains same, rapid rate of strength development at earlier stages may be observed by using accelerators. 

Accelerators are used in proportion of 2-4 % by weight of cement.

Calcium chloride and Sodium chloride are some of the common accelerators used in practice.

3. Light weight additives– 

Light weigh additives are used to reduce the weight of cement slurry. Slurry density is reduced by these additives. This may also reduce the ultimate compressive strength of cement slurry.

Bentonite is most commonly used as light weight additive in oil well cement. Besides reducing final strength, it also reduces resistance to sulphate attack.

4. Heavy weight additives- 

Heavy weight additives are used to increase the weight of cement slurry to restrain high formation pressures.

By adding heavy weight additives, density of cement slurry increases. It can be obtained by reducing the water content in cement.

Common heavy weight additives are:

• Hematite
• Ilmetite
• Barite 

5. Friction reducers- 

Friction reducers are also called dispersants. These are used to reduce the frictional pressure when placing cement. 

Thus, they allow the water content to be reduced without making it difficult for the cement to be pumped into the well. 

2-3 % bentonite clay or polynaphtalene sulphonate can be used as frictional reducers.

6. Low water-loss additives- 

To retain the water present in slurry when the pumped cement slurry passes through a permable zone. If this is not taken care of, sufficient water will not be available for hydration and the flowablitlity will start reducing as well. For this purpose, latex additives are added.

Common low water-loss additives include:

• Organic polymers like cellulose
• Carboxymethyl hydroxyethyl cellulose
• Hydroxyethyl cellulose

7. Strength retrogression additives- 

CaO/SiO₂ ratio in cement should be reduced at temperature above 100 ° C so that high strength can be obtained. 

Silica flour or pozzolana can be added to achieve this purpose.

Oil Well Cement Properties

The properties- setting time and strength of oil well cement is determined with the help of the procedure given in API RP – 10B recommended practice for testing oil well cements and cement additives.

Oil Well Cement Specification

Physical Requirements of Oil Well Cement

Characteristic	Requirement
	Class A	Class B	Class C	Class D	Class E	Class F	Class G	Class H	Class J
Water (% of mass of cement)	46	46	56	38	38	38	44	38	Recommended by manufacturer
Fineness Blaine specific surface (min)(m/kg)	225	225	225
SoundnessAutoclave (max)(%)	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8
Free water content in slurry(max) (% by vol)	–	–	–	–	–	1.4	1.4	–
Comp. Strength(min)(N/mm2)
8-HR CURING
38 ° C, atm pre	1.7	14	21	–	–	–	21	2.1
60 ° C, atm pre	–	–	–	–	–	–	10.3	10.3
24-HR CURING
38 ° C, atm pre	12.4	10.3	13.8	–	–	–	–	–

Chemical Requirements of Oil Well Cement

The chemical requirements are the values when the cement is tested according to the methods specified in IS: 4032 – 1985.

Chemical Requirements of Oil Well Cement

Storage of oil well cement has the same criteria as for ordinary Portland cement. To know the
complete details and precautions while storage, Read Storage of Cement.

Oil Well Cement Uses

Oil well cement CANNOT be used as structural cement. 

It is used for cementing the fissures and cavities between the steel casing and the rock formation.

Oil Well Cement Application

As the name indicates, oil well cement is used extensively in oil and gas wells. Cement slurry is
used to seal off the gaps between the casing and rocks.

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