Page 11 - Well Control for Completions and Interventions

6 Well Control for Completions and Interventions

1.1.2.2.2 Acid fracturing and acid matrix treatments
Carbonate (limestone and dolomite) formations are fractured using acid,
normally HCl. Acid fractures the formation, or enters existing fractures
where it dissolves the carbonate material, creating highly permeable path-
ways into the wellbore. Acid treatments are frequently performed as inter-
ventions in existing wells, often through coiled tubing.

1.1.2.3 Artificial lift
Many workovers are performed to install artificial lift. Gas lift is widely used,
and works by reducing the density of the fluid in the tubing. All other artificial
lift systems use a pump. There are several types of pump used, including electric
submersible pumps (ESP), beam pumps, and progressive cavity pumps (PCP).

1.1.2.4 Mechanical repairs
Completion equipment can fail. Some failures occur very early in the life
of the well, and are a result of poor design, wrongly specified equipment,
or damage during installation. Most failures occur late in well life because
of corrosion, erosion, and fatigue. A plot of failure frequency against time
resembles a bathtub (colloquially known as a bathtub curve) (Fig. 1.1).
Many completion components are barrier elements (Chapter 6. Well
Barriers), and their failure can compromise the primary or secondary well
barrier envelope. When this occurs, they must be repaired or replaced.
Some components are repairable or replaceable using through tubing
interventions with the well still live, e.g., replacement of a failed tubing
retrievable safety valve, or the replacement of a gas lift valve when the
check valves have failed. Other failures can only be remedied by replacing
the completion, a tubing collapse for example.

Failure rate Early life wear-out
Late life
failure
or
failure
failure

Useful life failure

Operating time
Figure 1.1 Bathtub failure curve.