Centralizer is a steel apparatus that is secured around the casing at different locations in the borehole. The device is fitted with bowsprings and a hinged collar to keep the casing centralized in the borehole. This helps efficient cement placement between the casing and the bore wall preventing uneven and imperfect seal that can cause fluid to be pushed up the borehole contaminating aquifers and upper level strata.
Trenchlesspedia Explains Centralizer
After drilling is complete and the drill string is withdrawn, the centralizer is attached to the casing string and lowered into the borehole.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.