Special Primary Cementing Methods
Several cementing methods designed to manage special situations deserve a brief description.
Inner-String Cementing
Cementing large-diameter casing sometimes requires some special considerations. Such casing, which has a relatively large cross-sectional area, is subject to being pumped out of the hole if significant pump and hydrostatic pressure at the casing shoe acting on this area exceeds the buoyed weight of the casing. Large casing can also be floated out of the hole if the weight of the casing and the mud in the pipe do not exceed the buoyancy provided by the annular column of cement after it is displaced.
Inner-string or stab-in cementing is a fairly common practice for large-diameter casing. The string is cemented through drill pipe inserted into a special sealing sleeve in the shoe (Animation 1).
External Cement-Filled Packer
When zonal isolation is particularly critical, external cement filled casing packers can be utilized. Extra-long (20- or 40-ft) elastomer-sheath covered inflatable packers can be run as part of the casing string. One packer (or more) is landed across the productive zone to be perforated and the primary job is completed conventionally (Animation 2).
When the initial slurry displacement is complete and the top plug is bumped, but before the cement sets, pressure is increased to open shear-pin controlled valves in the packers, and additional cement is pumped into the packer elements to expand them tightly against the borehole wall. This “inflation cement” is pumped down the casing between the first top wiper plug and a special second top wiper plug.
After curing, the cement-filled packer and the casing joint mandrel on which it is run are perforated by conventional methods. Two such cement-filled packers can be used effectively to straddle a productive interval for zone isolation.
External cement-filled packers offer these advantages in primary cementing:
- Mud channels across the producing zone are avoided completely by the squeezing of any remaining mud or mud cake from the packer rubber-formation interface.
- The producing formation is supported by the pressure-set cement packer and unconsolidated zones cannot dilate or “flow” with a production pressure differential.
