Inefficient wellbore construction

As part of an effort to access a reservoir in the Permian Basin, an operator was using a three-string well design in which the casing point was above the field’s high pressure CO₂ zone. This approach was effective, and it countered problems the operator had experienced using a two-string design. These problems included CO₂ kicks, stuck pipe, wellbore collapse, poor cementing, and NPT. However, the high cost of a three-string well design made it an unsustainable option.

Casing-while-drilling two-string well design

To maintain the economic viability of the project, Schlumberger recommended constructing a two-string well using a Smith Bits 7 7/8-in XCD casing-while-drilling alloy casing bit and Allegro casing- and liner-while-drilling services. They included MLT Rings, which were added to the operator’s 5 1/2-in buttress threaded coupled casing to provide positive makeup and increase torque. Hydro-formed centralizers were also used to stabilize the casing string during drilling and help position it for cementing.

High-quality wellbore construction

The casing-while-drilling two-string design used in five wells enabled the operator to come out of the conventionally set 8 5⁄8-in casing and drill below the high-pressure CO₂ zone to TD with 5 1⁄2-in production casing. The 5 1⁄2-in production casing was cemented, and the XCD casing bit was drilled out with a standard PDC bit. The operator experienced no kicks, stuck pipe, borehole collapse, or extended NPT during the operation. In addition, the Allegro services’ technology signature plastering effect strengthened wellbores in which production casing was successfully cemented, despite running through a high-pressure CO₂ injection zone. As a result of the operation, the expense of an additional string was eliminated in five wells, and cement bond logs confirmed the cementing job was a record best for the operator below a high-pressure zone.