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Cement slurry design testing
When determining a slurry’s characteristics and performance, these testing procedures are recommended:
Test to the highest simulated Bottom Hole Circulating Temperature (BHCT) with a variety of retarders, densities, and temperatures.
Test to the actual bottomhole pressure (BHP) thickening time. [Note: The slurry to be tested should include surface time required (if batch mixed) and calculated time to bottom.]
Compressive strength at the following top-of-liner (TOL)
Ensure certain conditions are met:
- Simulated temperature and pressure
- Lowest simulated BHCT used with longest thermal recovery
- Ultrasonic cement analyzers set for simulated temperature recovery and calculated pressure not American Petroleum Institute (API) minimum (3,000 psi).
Investigate and standardize:
- Order of addition
- Time taken to add
- Holding of mix water
- Time to mix at surface
- Surface mixing temperature/shear effects
- Slurry stability
- Sedimentation test
- High-pressure/High-temperature (HP/HT) rheology (where available).
The methods of testing cement for downhole application are based on performance testing. Testing methods are usually performed according to API specifications, though specifically designed and engineered equipment or tests are also used. The choice of additives and testing criteria is dictated primarily by the specific parameters of the well to be cemented. Performance testing has proven to be the most effective in establishing how a slurry will behave under specific well conditions. There is no direct means of predicting cement performance from the properties of cement, and no technique has yet been established that would correlate cement composition and cement/additive interaction with performance.
Performance testing is not adequate in troubleshooting downhole problems where the integrity of the cement blend is in question. There are diagnostic analyses that can be performed to evaluate the cement powder, but there are no definitive tests for chemically analyzing the composition of a cement once it has been mixed with additives, either as a dry blend, a slurry, or a set cement. The primary reason for this is the low concentration of additives used in the slurry or set cement. This concentration in set cement can be even lower than that of the original slurry, if the additive is consumed and/or modified during the cement hydration reaction. The content of samples taken from downhole is often questionable in that it is not clear exactly where they were obtained, or if they were contaminated with drilling fluid, formation waters, or during retrieval. Many of the techniques used for understanding the chemistry of cement are designed for laboratory-prepared specimens and applications, and are not applicable to field samples. However, depending on the sample and the concentration of additives, some qualitative analysis can sometimes be achieved.
Analysis of dry-blended samples is somewhat different from that of the slurry or set cement. If sufficient quantity is available for performance testing, this would be the most appropriate to compare the actual blend with that designed. If this is not the case, then the blend would require dissolution in an extracting solvent. This usually includes water and, inevitably, cement hydration will occur, with some of the additive component being removed by the hydration products. As the contact time is less, more additive should be extracted, and will more likely be detectable through one of the methods previously discussed. After cement and additives are blended, it is usually not possible to separate the additive from the dry sample unless it has a significantly greater particle size or heavier density than that of the cement.