mold-on type casing centralizer
Casing centralizer is a mechanical device secured around the casing
at various locations to keep the casing from contacting the
wellbore walls. As a result of casing centralization, a continuous
annular clearance around the casing allows cement to completely
seal the casing to the borehole wall.
Casing centralization is one of the key elements to ensure the
quality of a cementing job by preventing mud channeling and poor
zonal isolation. Centralizers can also assist in the running of the
casing and the prevention of differential sticking.
Centralizer’s usage is extensive! It is estimated that 10 million
centralizers are manufactured and used every year globally.
Centralizer manufacturers likely want to increase the demand for
centralizers. However, operators on the other hand, may wonder:
“Should we use that many?”
While centralizers are used extensively, wellbore problems continue
to arise due to poor cementing jobs. Centralizer properties and
placements directly or indirectly affect the quality of the
The challenge that both operators and service companies face is to
choose the right type of centralizers and place the right amount at
the optimum positions on the casing to achieve a good standoff
There are 4 types of centralizers (Fig. 1): bow-spring, rigid,
semi-rigid, and mold-on; each with its own pros and cons.
Since the bow springs are slightly larger than the wellbore, they
can provide complete centralization in vertical or slightly
deviated wells. Due to the flexibility of bows, they can pass
through narrow hole sections and expand in the targeted locations.
The shape and stiffness of the bows determine the restoring force,
which is defined as the resistance force when a bow is compressed
by 1/3 of its uncompressed height. The effectiveness of this type
of centralizer is heavily dependent on the restoring force.
When the casing is heavy and/or the wellbore is highly deviated,
they may not support the casing very well. For example, on a riser
tieback casing string, a helically buckled casing could create a
side force of 50,000 to 100,000 lbf (222 to 445 kN), well beyond
the capabilities of the spring-bow centralizer. A solid centralizer
would be able to meet the requirements.
Rigid centralizers are built out of solid steel bars or cast iron,
with a fixed blade height and are sized to fit a specific casing or
hole size. This type is rugged and works well even in deviated
wellbores, regardless of the side force. They provide a guaranteed
standoff and function as bearings during the pipe rotation, but
since the centralizers are smaller than the wellbore, they will not
provide a good centralization as the bow-spring type centralizers
in vertical wells.
|Casing Size||Hole Size||Max O.D.|
|5- 1/2||8- 1/2||256||10,079|
Semi-rigid centralizers are made of double crested bows, which
provide desirable features found in both the bow-spring and the
rigid centralizers. The spring characteristic of the bows allows
the semi-rigid centralizers to compress in order to get through
tight spots and severe doglegs. The double-crested bow provides
restoring forces that exceed those standards set forth in the API
specifications and therefore exhibits certain features normally
associated with rigid centralizers.
The mold-on centralizer blades, made of carbon fiber ceramic
materials, can be applied directly to the casing surface. The blade
length, angle and spacing can be designed to fit specific well
applications, especially for the close tolerance annulus. The
non-metallic composite can also reduce the friction in extended
reach laterals to prevent casing buckling.