127 I
2
2,-16"11
_
20"-
20"-
~o ~_I
2 0"-
,3~',3~'-
20 x
25
4(~)
4(b) 20"-
13J
lO
15
4(o)
3 30"-"
7" u 7~?J 6~"-
5" u
30
7"
35-
.J 7'
40-
7" I
45-
4" OR 4.5"-
50
Fig. 3.4: Typical casing program for different depths. can occur. The setting depth for surface casing should, therefore, be selected so as to contain a kick-imposed pressure. Another factor that may influence the selection of surface casing setting depth is the protection of fresh-water aquifers. Drilling fluids can contaminate freshwater aquifers and to prevent this from occurring the casing seat must be below the aquifer. Aquifers usually occur in the range of 2,000 - 5,000 ft. The relationship between the kick-imposed pressure and depth can be obtained using the data in Fig. 3.1. Consider an arbitrary casing seat at depth D,; the maximal kick-imposed pressure at this point can be cakulated using the following relationship: Pk - GpjDi - Gp1(Di - Ds)
(a.2)
where" Pk
=
Ds = Di Gpj
= =
kick-imposed pressure at depth Ds, psi. setting depth for surface casing, ft. setting depth for intermediate casing, ft. formation fluid gradient at depth Di, psi/ft.
Assume also that formation fluid enters the hole from the next casing setting