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Cement slurry design
The properties of Portland cements must often be modified to meet the demands of a particular well application. These modifications are accomplished by the admixing of chemical compounds commonly referred to as additives that effectively alter the hydration chemistry.
Overview of additives
An overview of the most common cementing additives is given in Table 1.[1]
Table 1-Summary Of Oilwell Cementing Additives
Table 1 Continued
The table also includes an indication of the primary uses and benefits, along with the cements that they can be used with. The primary effects of the cement admixtures on the physical properties of the cement, either as a slurry or set, are presented in Table 2.[1] This is a quick reference, and individual additives in a given category may not agree in total with the effects as given. It is also typically defined for individual additives, the properties and effects of which can be modified when additive combinations are used.
Table 2-Effects of Cement Admixtures on the Physical properties of Cement
Many chemical compounds have proved to be effective in modifying the properties of Portland-cement slurries. These compounds, when used alone, will have a primary effect upon the cement slurry that is considered to be beneficial. They will also exhibit at least one secondary characteristic that may be either beneficial or detrimental to the cement-slurry performance properties. The effects of the additives are reduced or enhanced by modifying the additive or by using additional additives. For most downhole requirements, more than one additive is needed. This give-and-take relationship between additives is the basis of cement-slurry design.
Effects of additives
The reaction of these additives with the cement and the interaction between them is not well defined chemically. What is actually known are the physical effects of these additives on the slurry performance properties. The slurry performance properties that are measured include:
- Thickening time
- Compressive strength
- Rheology
- Fluid loss
- Free fluid
- Slurry stability
Cement manufactured to American Petroleum Institute (API) depth and temperature requirements can be purchased in most oil-producing areas of the world. Any properly made Portland cement (consistent from batch to batch) can be used at temperatures up to 570°F. For example, Class H cement with the proper additives has routinely been used at depths up to 20,000 ft.
In addition to the cement, other factors, such as the correct Bottom Hole Circulating Temperature (BHCT), should be considered when designing a cement slurry to meet well requirements. In formulating a cement slurry, the designer must consider not only the temperature but also the other downhole conditions, such as permeability and water-sensitive formations.
A slurry should be designed for its specific application, with good properties to allow placement in a normal period. The ideal cement slurry should:
- Have no measurable free water
- Provide adequate fluid-loss control
- Contain adequate retarder to help ensure proper placement
- Maintain a stable density to ensure hydrostatic control
Do not add dispersants or retarders in excess of the amounts indicated by wellbore conditions, and provide just enough fluid-loss control to place the cement before it gels.
Slurry design is affected by the following criteria:
- Well depth
- Quality of mix water
- BHCT
- Fluid-loss control
- (define acronym) BHST
- Flow regime
- Drilling fluid’s hydrostatic pressure
- Settling and free water
- Type of drilling fluid
- Quality of cement
- Slurry density
- Dry or liquid additives
- Lost circulation
- Strength development
- Gas-migration potential
- Quality of the cement testing
- Pumping time
- Laboratory and equipment
When estimating job time, include the mixing time on the surface, especially if the job is going to be batch-mixed. Calculate the actual job time, using the slurry volume and average displacement rate. Limit the amount of trouble time to 1 to 1.5 hours. To calculate the approximate thickening time for slurry design, add 1 to 1.5 hours to the job time.
Categories of additives
The additives used to modify the properties of cement slurries for use in oilfield well-cementing applications fall into the following broad categories:
- Accelerators
- Retarders
- Extenders
- Weighting agents
- Dispersants
- Fluid-loss control agents
- Lost-circulation agents
- Strength-retrogression prevention agents
- Free-water/free-fluid control
- Expansion agents
- Special additives
The demand for new additives with special properties and tuned performance continues to increase. These demands include:
- Density range of application
- Temperature stability
- Economics
- Viscosity range
- Singular function
- Multifunction
- Rate of solubility
- Synergism with co-additives
- Resistance to cement variability