While using conventional surface launch cementing plugs during cementing operations performed on large diameter surface pipe, significant costs are incurred during the pumping of cement and displacement fluids because of the large volumes involved in the job. Additionally, high differential lifting pressures sometimes exceed the safe collapse load that can be applied to the large casing; thereby preventing the operator from checking the float valves after displacement is complete.
For several operational reasons, the operator elected to cement subsequently installed surface pipe using the inner-string method of displacement.
To help reduce overall job execution time and provide safer means of checking the floats after displacement is complete, a versatile surface packoff was incorporated into cementing operations on many large-diameter surface jobs. The time required to run the work string and install the surface packoff was easily justified with reduced costs associated with actual job execution. This paper documents the operators cost analysis along with operational benefits realized from the use of inner-string cementing and surface packoff methods.
In cementing large-diameter surface pipe, the commonly used surface-launch cementing method has shown to present several operational problems:
Long pumping time.
High collapse pressure on casing after displacement.
Free-falling slurries apply a high collapse load to casing.
Requirement for surface launch equipment and plugs (which must be drilled out).
This paper presents solutions to the problems:
Inner-string cementing reduces slurry volume required.
Pumping time is drastically reduced.
Free fall is confined to the inner string, which eliminates the collapse load applied to the casing.
Plug-drillout time is eliminated.
A quick-connect casing head (QCH) provides a seal or packoff (at the rig floor level) between tubing (or drillpipe) and the large-diameter casing.
Inner-string cementing (ISC) equipment allows large-diameter casing strings to be cemented through drillpipe or tubing that is inserted into and sealed in floating equipment. ISC (Fig. 1) offers the following advantages:
Less costly than cementing large casing with the conventional plug-displacement method.
Does not require large-diameter casing plugs.
Reduces cement contamination.
Reduces amount of cement to be drilled out.
Decreases cementing displacement time.
Reduces casing collapse when surface packoff is used.