Everybody knows why bumping the top plug at the end of the displacement is an essential element of each successful primary cement job.
- It ensures only the shoe track is left with cement, and;
- It allows the pressure testing of the casing.
First of all, a couple of questions for you:
What is the reason for a shoe track and how is the length (1, 2 or 3 casing joints?) estimated?
Hint: Don’t be surprised if we find soft cement inside.
Now I am going to detail the most common reasons to explain why top plugs don’t always bump, even after pumping the allowable extra displacement volume.
The casing dimensions and weights to estimate displacement volumes are typically nominal values as defined by API Specification 5CT. These values have tolerances.
- Casing OD tolerances are +1.0% with an absolute maximum of 0.125” and –0.5%
- Casing Weight tolerances are +6.5% and –3.5%
We derive the ID tolerance from the above. For example: for 9 5/8” casing, there would be up to a 2 bbl difference for each 305m. This value can be greater, depending on casing length, than the typical displacement excess volume (½ the shoe track volume). For example 20 bbl for 9 5/8” casing at 3050m.
This information has no use in the field. However, it is important to realize that casing IDs are different from nominal values. More than likely, the ID will be larger than what is in the tables (or in any calculation sheet/handbook). Consequently, the calculated displacement volume (based on nominal values) would get underestimated.
In the field, this can be avoided by taking casing ID measurements to increase the accuracy of the displacement volume. Typically, the average of 10 random samples for each 100 – 150 casing joints should be good.
Displacement Fluid Compressibility
This is particularly the case when using oil-based mud (OBM), and it derives from the combined effect of pressure (compressing) and temperature (expansion). With compression forces surpassing, the total effect still accounts for a reduction in volume and the need to pump extra barrels to land the top plug successfully.
The drilling fluids company can typically provide the compressibility values for the mud in use; this allows easy calculation of the required extra displacement volume to successfully bumping the plug.
While OBM is undoubtedly compressible, the effect of entrained air can affect both OBM and water-based mud (WBM), mainly when the mud is delivered to the cement unit displacement tanks. Another source of ‘entrained air’ is bacteria growing in the mud.
This ‘additional compressibility’ however can be reasonably compensated in the field by multiplying the displacement volume by the ratio between the density measured with the pressurized mud balance and the density from the atmospheric mud balance.
Rig pumps efficiency
When a rig’s pumps get used for displacement, their volumetric efficiency must be calculated before the cement job to ensure accurate displacement volume.
For triplex pumps the theoretical pump output is calculated from the following equation:
This calculation assumes an unrealistic 100% efficiency. In practice, rig pumps efficiency is anywhere between 90 to 99%, with 95% being an average.
The actual efficiency of a pump gets determined by measuring the suction pit. With this pit isolated, the mud gets pumped out of that pit only for a certain number of pump strokes, then the volume pumped out of the pit is divided by the total number of pump strokes.
Most of the time, the pump efficiency is assumed to be a specific fixed value (written on the board in the company office); however, it can occasionally vary depending on factors like any maintenance done or pending on the triplex pumps or cleanliness of the mud (suction). A safe practice, particularly for the long string, is to estimate the pumps efficiency before the cementing operation.
Most common human errors
- Displacement volume not individually calculated by more than one person on-site for comparison.
- Length errors in the casing or pipe tally.
- In the case of liner cementing, not including the effect of the tool joints (reduction in volume).
- If the cement unit gets used for displacement (smaller jobs); miscounting displacement tanks or not filling/emptying tanks properly.
During my early years in eastern Venezuela, there was a lesson learned for deep wells with long 9 5/8” intermediate casings rigorously enforced by the customer: the sole use of Nitrile-based wiper plugs to prevent the effect of temperature and aniline point. This practice was part of the guidelines to avoid wet shoes, including minimizing the rat hole (2 – 3 m Max.); use of self-centralized up-jet float shoes and efficiency of the rig pumps, among other precautions.
The displacement philosophy was simple ‘pump till bump’, but only with nitrile-based wiper plugs. The use of this particular type of material (nitrile) was due to some suspected cases where the displacement fluid (OBM) bypassed the top plug before reaching its landing point. (When its wiping effect disappeared due to wear, temperature and high aniline point (diesel-based OBM) in the mud).
Since then I have rarely seen a company enforcing the ‘pump till bump’ practice. In normal circumstances with a fixed maximum displacement volume (+ ½ shoe track capacity) even if the wiper plug gets damaged and fluid bypasses, the chances of over-displacing (creating a wet shoe) are limited (plug is only left behind).
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Do you have any more causes to explain why wiper plugs don’t bump? Please share