Axial Force Due to the Pipe Weight
If the casing is set to a depth D and the top of cement is a depth x below the surface (Fig. 2.27), casing instability will occur first at the top of the cement. If this point is stable, all other points in the string will be stable. Thus, it is necessary to consider this point first.
Weight of the casing string carried by the joint above the top of the cement, Fau,, is given by:
Faw = weight of the casing below the top of the cement (Fa)
— buoyant force acting on the casing shoe (F),u)
where:
Fa = (D — x)Wn, D — x, is the cemented interval or length of casing
below the top of cement, lbf.
Fbu — A0p0 A{ p,, lbf.
Po = GVo x + GPcm (D — i), psi.
Pi = GPxD, psi.
A{ = internal area of the pipe, in.2
A0
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DRILLING FLUID |
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CEMENT |
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^/ш/хТ |
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Fig. 2.27: Partially cemented casing string, external area of the pipe, in.2 pressure gradient of the fluid inside the pipe, psi/ft. pressure gradient of the fluid outside the pipe, psi/ft. pressure gradient of the cement slurry, psi/ft. |
Gr „
Hence:
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(2.180) |
Faw = (D-x) Wn — {A0 [GPo x + GVcm (D — .r)] — A, GPt D}