For large diameter casing, such as conductors and surface casing, conventional cementing techniques result in:
• The potential for cement contamination during pumping and displacement
• The use of large cement plugs which can get stuck in the casing
• Large displacement volumes
• Long pumping times
• Large volume of cement left inside the casing between float collar and shoe.
An alternative technique, known as a stinger cement job, is to cement the casing through a tubing or drillpipe string, known as a cement stinger, rather than through the casing itself.
In the case of a stinger cement job the casing is run as before, but with a special float shoe (Figure 15) rather than the conventional shoe and float collar. A special sealing adapter, which can seal in the seal bore of the seal float shoe, is attached to the cement stinger. Once the casing has been run, the cementing string (generally tubing or drillpipe), with the seal adapter attached, is run and stabbed into the float shoe. Drilling mud is then circulated around the system to ensure that the stinger and annulus are clear of any debris. The cement slurry is then pumped with liquid spacers ahead and behind the cement slurry. No plugs are used in this type of cementing operation since the diameter of the stinger is generally so small that contamination of the cement is unlikely if a large enough liquid spacer is used.The cement slurry is generally under-displaced so that when the seal adapter on the stinger is pulled from the shoe the excess cement falls down on top of the shoe.This can be subsequently drilled out when the next hole section is being drilled.Under-displacement however ensures that the cement slurry is not displaced up above the casing shoe, leaving spacer and drilling mud across the shoe. After the cement has been displaced, and the float has been checked for backflow, the cement stinger can be retrieved. This method is suitable for casing diameters of 13 3/8″ and larger. The main disadvantage of this method is that for long casing strings rig time is lost in running and retrieving the inner string.
Institute of Petroleum Engineering, Heriot-Watt University