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Permalink Reply by ji on August 4, 2012 at 1:41am

Centralizes are placed on the casing to keep it the center of the bore hole.

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Permalink Reply by Brian Powers on August 5, 2012 at 2:52am

ji & Monroe-

Both of your posts are correct- higher quality cement jobs occur if the pipe is poperly centralized. The centalizers that I am most familiar with : flexible spring steel (bowspring) or solid (rigid type)

All holes drilled into the ground have a high and low side; and all pipe ran into a drilled hole falls to the low side due to gravity. A rounded steel guide shoe is attached to the bottom joint of casing to help guide the casing to bottom. The type, number, and placement of centralizers are modeled by computer. The flexible centralizers are fitted over the casing and held in place by stop rings and hold-down screws that dimple the steel in the casing and theoretically limit upn movement.

Rigid centralizers are generally threaded with the same thread as the casing coupling and are screwed in between the casing joints to form  an intregal casing string. One of the worst things that happens is that the slip on centralizers move and bunch up as the casing is being run or that the rigid centralizers jam into a rock ledge or other tight spot downhole; both these situations usually impede further downward movent of the casing. Most casing jobs that I've neen involved with are with larger diameter, heavier weight pipe that cannot be fully supported by the derrick. For most, the team a"point of no return" depth where we would stop and cement.

Well centralized pipe leaves an an annular gap around the circumference of the pipe that initially contains mud that is displaced by the cement "pre-flush" and then the cement itself. Effective removal of both the mud and its filter cake is essential to ensure adequate bonding of the cement to the steel and formation for maximum integrity.

As Monroe stated, a variety of method are used to determine the cement quality or coverage. Setting cement generates heat (exothermic chemical reaction), so it was once common to run a temperature gradient survey the cement was curing and look for the interface depth between the natural geothermal gradient and the curing cement. While this gives a rough top of cement depth, it says nothing of the quality of cement. This is where the cement bond log or CBL is used, coupled with an internal casing integrity hydrostatic test.

 

Lack of adequate centralizers and a poorly executed cement job likely contributed to the BP Gulf of Mexico disaster.

 Have a great Sunday, all!

Brian  

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