What Does Casing Centralizer Mean?
A casing centralizer is a steel apparatus that is secured around the casing at different locations in the borehole to prevent it from contacting the wall. The casing centralizer is fitted with bowsprings and a hinged collar to keep the casing centralized in the borehole. There are two classes of a casing centralizer; namely, bow-spring design and rigid blade design.
A casing centralizer helps efficient cement placement between the casing and the bore wall preventing uneven and imperfect seal that can cause drilling fluid to be pushed up the borehole contaminating aquifers and upper level strata.
Trenchlesspedia Explains Casing Centralizer
The bow-spring design casing centralizer is slightly larger than the wellbore, and it provides complete centralization even if the well is deviated. But in some cases, it may not be able to support the casing weight. A rigid blade design needs the wellbore to be in good condition or it may cause trouble downhole.
A casing centralizer is smaller than the wellbore and is good for deviated bores, but don't provide as strong support as the bow-spring design in vertical wells. After drilling is complete, and the drill string is withdrawn, the casing centralizer is attached to the casing string and lowered into the borehole.
The centralizer prevents the casing from resting on the borehole edge and is attached to every third or fourth knuckle joint. The even spacing between the casing string and the borehole wall allows even distribution of cement filled in the annular space preventing thin areas. An un-centered casing due to uneven cementing is a principal cause for groundwater contamination and can be observed as a mud cake channel being formed from thickened drilling mud that finds its way up the annulus.