Selection of Long or Short Production Casing on HPHT Wells

Abstract

Drilling in high-pressure high-temperature (HPHT) wells present many economical, technical and operational challenges. The wells are usually located in deep water¬¬¬––where the total well cost and trip times are increased. Well planning of these wells require special considerations of three factors: safety, efficiency and cost.

The production string is one of the most important aspects of well planning in HPHT wells. A failure of the production string may have disastrous results, since the string serves as the backup protection for the tubing.

The selection of a long casing string versus a liner tieback string is a common discussion in the oil industry. Several factors have to be accounted for prior to the final selection. The liner option will for instance provide more barriers against annular flow, but it is also a more complex and time-consuming operation to perform. Long casing string solution provides better well integrity over the lifetime of a well, but is dependent upon a successful primary cement job.

It is easier to achieve a successful primary cement job with the liner than with the long casing string. The space between the casing and drill pipe is much bigger when running in hole with the liner, which will generate less surge pressure and open up for a higher flow rate during the cement job. The high flow rate during cementing, normally improves the quality of the cement job.

In HPHT wells there is in general tighter clearance in the operating drilling window. Proper control of Equivalent Circulating Density (ECD) is of major importance when it comes to drilling safely and efficient. Liner provides reduced ECD, resulting in less risk for losses both while running in hole and cementing.

Although the liner comes with several advantages, the long string option is operationally easier and is less time consuming. My opinion is that long string option is the preferred one, assuming that the formation strength is sufficient for the cement job.