Cement Additives (function and definition)

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This article describes the function of the various classes of cement additives and also to highlight which chemicals are used for which purpose.

The additives to be used and their concentration shall be decided by the Cement Contractor, after testing the slurry using representative samples of cement, additives and mixwater despatched from the rig. This has to be then approved by the Operations Engineer and Superintendent.

No deviationfrom the recipe, as determined by the Cementing Contractor is permitted unless specific written orders have been issued by the Superitntendent.

 Note: % BWOC it means “percentage by weight of dry cement”. One sack of cement is 94lbs.

Accelerators

Accelerators may be added to the mixwater to reduce the thickening and setting times of the slurry, with the purpose of avoiding unnecessary time spent waiting on cement.

 Calcium Chloride is the most common type of accelerator used. Other, less frequently used accelerators are Sodium Chloride and Potassium Chloride.

Additive

Formula

Concentration

Comments

Calcium Chloride (Accelerator)

CaCl2

1 - 2 % BWOC

High gels may form at higher concentrations. Flash setting may occur above 3 %.

Sodium Chloride (Accelerator)

NaCl

Up to 20 lbs/bbl of mixwater

Acts as an accelerator and adversely affects other additives at higher concentrations than 20 lbs/bbl.

Sea Water (Accelerator)

 

Use as mixwater

Contains NaCl in the correct concentration range. Main application is shallow depth cementation offshore.

Potassium Chloride

KCl

As required

Equivalent to NaCl and CaCl2 but more expensive.

Notes:

  1. In general accelerators increase the viscosity of the cement slurry and decrease the effectiveness of most other additives
  2. Calcium Chloride can cause skin burns and sever irritation to eyes, nose and lungs. Gloves, goggles and respirators shall be used during mixing. Calcium Chloride is available in 50lb sacks.

Retarders

As the temperature increases, the chemical reaction between cement and water is accelerated which, in turn, reduces the pumpable time. Increased depths and formation temperatures may require the use of retarders in order to extend the pumpable time of the cement.

Thixotropic slurries may require retarders at shallow depths and low temperatures.

Effects on Viscosity

Most retarders affect the viscosity of the cement as follows:

Type of Retarder

Effect on Viscosity

Lignin Derivatives (HR 5, HR 6 etc)

Reduce

Organic Acids

Reduce

Cellulose Derivatives (CMC)

Increase

Note:   The combined use of retarders and accelerators in the same mix should be    avoided.

High Density Additives

There are two main methods of increasing the gradient of the cement slurry:

  • reducing the water/cement ratio
  • addition of a weighting material.

Reducing the Water/Cement Ratio

This is the preferred method of increasing cement gradient, particularly when cementing across gas zones. A maximum gradient of approximately 17.0 ppg can be attained by this method.

Addition of a Weighting Material

The preferred material is hematite (iron oxide) but barite may also be used. It is preferable to batch mix the cement and the weighting material rather than to use pre-blended cement since weighting materials additive in pre-blended cement may settle out during storage. Furthermore, handling different blends of cement on one rig may result in operational mistakes.

Note: When drilling with a mud gradient of 14.8 ppg or more, a batch mixing unit and sufficient weighting material must be on site as a contingency. If very large volumes need to be mixed,  batch mixing is not achievable

Low Density Additives (Extenders)

Low gradient slurries are used for cementing weak formations or when there is a possibility that the casing could collapse or float if a heavier slurry was used.

Preparation

A low gradient slurry is prepared by adding extra mixwater. Bentonite is usually pre-mixed at a concentration of ½ % BWOC in the freshwater to reduce free water separation of the resultant slurry.

Bentonite must be fully hydrated before any other additives are added to the mixwater. To achieve this, premix may be cut back using industrial water or dry bentonite may be hydrated in industrial water. In the latter case, the minimum hydration period is six hours, but may be longer, depending on the type of bentonite used. Consult the Cementing Contractor for expert information. The recommended type of bentonite for best results is Wyoming bentonite.

The bentonite content must be checked using the methylene blue test (MBT).

Compressive Strength

As a result of the decreased cement concentration of extended slurries, the compressive strength of the hardened cement will be lower than that of neat slurries.

Friction Reducers (Dispersants or Thinners)

Friction reducers are dispersing agents which can be added to the slurry to reduce its viscosity and thus the frictional pressure losses in the system while displacing (or squeezing) the cement. As a result, higher pumping rate are possible and higher displacement efficiencies may be achieved

Fluid Loss Control Additives

Fluid loss control additives are added to cement slurries for the following reasons:

  • To reduce the possibility of dehydration opposite porous zones and consequently flash setting of the cement.
  • Loss of fluid from the slurry will result in increasing slurry viscosity and gradient and higher circulating pressures.
  • Excessive fluid loss will reduce slurry volume and give less cement fill.
  • When squeeze cementing it is desired to get an effective squeeze against the entire formation and not just squeeze cement filtrate into it.

Most fluid loss additives tend to viscosify the slurry and consequently, dispersants are often added at the same time to control this effect

Additives for Thixotropic Slurries

The main application of thixotropic slurries is cementing in lost circulation environments.

During pumping the slurry behaves as normal, however, a gel structure develops rapidly when static. Such slurries also find application in cementing across gas zones.

The major disadvantage of thixotropic slurries is their relatively high viscosity which may adversely affect displacement efficiency

Defoamers and Antifoams

While mixing cement, and/or when preparing mixwater, containing retarders, salts, fluid loss additives and/or bentonite, foaming is often experienced. To control such problems antifoams and defoamers are available.

Antifoams should be used before adding any other chemicals.

Defoamers remain effective when added after foam-causing chemicals.

In practice, such distinction may be difficult to ensure and therefore both antifoams as well as defoamers should be added in advance in re-circulating cement mixer.

Gasblocking Additives

During the hardening process, the cement slurry passes through a semi-solid phase in which the liquid has gelled up. As a result, the overburden pressure will be lost thus permitting gas migration into, and through, the cement matrix.

Surfactants added to the slurry will form a stable foam with the gas bubbles preventing their transport through the cement slurry.

Additives may be incorporated into the slurry which act to block the gelled cement pore structure preventing gas migration.

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