Surface Casing String
The surface casing string is often subjected to abnormal pressures due to a kick arising from the deepest section of the hole. If a kick occurs and the shut-in casing pressure plus the drilling fluid hydrostatic pressure exceeds the fracture resistance pressure of the formation at the casing shoe, fract uring or an underground blowout
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4(a) |
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4(c) |
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1 |
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4(b) |
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-2^ 16 J |
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20"-l 13}Р |
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30“ |
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20"J |
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20"-1 |
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20"J |
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20 ■ |
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13s-1 |
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135 J |
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15 |
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13S |
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ioJ-1 |
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_13S^ |
со
О 20
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8i 6 |
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Т и 7§J S |
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25 30 35 40 45 50 |
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5" u 7"J |
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r-i |
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T-> |
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T |
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4" OR 4.5" J |
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5"J |
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 Ds; the maximal kick-imposed pressure at this point can be calculated using the following relationship:
Pk = GPfDt-GPi(D,- Ds) (3.2)
where:
Pk — kick-imposed pressure at depth Ds, psi.
Ds — setting depth for surface casing, ft.
Di = setting depth for intermediate casing, ft.
GPf — formation fluid gradient at depth D,, psi/ft.
Assume also that formation fluid enters the hole from the next casing setting
depth, Д. Expressing the kick-imposed pressure of the drilling fluid in terms of formation fluid gradient and a safety margin. 5.1/. Eq. 3.2 becomes:
Pk = (GPf + SM)D, — GPf(D, — Ds) (3.3)
Where рДД is the kick-imposed pressure gradient at the seat of the surface casing and must be lower than the fracture resistance pressure at this deptli to contain the kick.
Now, assume that the surface casing is set to a depth of 1.500 ft and SM. in terms of equivalent mud specific weight, is 0.5 lb/gal. The kick-imposed pressure gradient can be calculated as follows:
Pk = f 0.052 x 0.5 (— _10° 1+8.9 x 0.052
1,500 V V L 500
= 0.6552 psi/ft
The fracture gradient at 1,500 ft is 0.65 psi/ft (12.49 lb/gal). Clearly, the kick — imposed pressure is greater than the strength of the rock and. therefore, a deeper depth must be chosen. This trial-and-error process continues until the fracture gradient exceeds the kick-imposed pressure gradient. Values for different setting depths and their corresponding kick-imposed fracture and pressure gradients are presented below:
Table 3.1: Fracture and kick-imposed pressure gradients vs depth.
Depth Kick-imposed Fracture pressure (ft) pressure gradient gradient
____________ (psi/ft)__________ (psi/ft)
1,500 0.6552 0.65
2.0 0.61 0.66
At a depth of 2,000 ft the fracture resistance pressure exceeds the kick-imposed pressure and so 2,000 ft could be selected as a surface casing setting depth. However, as most fresh-water aquifers occur between 2.000 and 5,000 ft the setting depth for surface casing should be within this range to satisfy the dual requirements of prevention of underground blowouts and the protection of fresh-water aquifers.
20 in CONDUCTOR PIPE
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350* |
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11100* |
SURFACE CASING
13.375 in INTERMEDIATE CASING
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Л. |
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14000* |
9.625 in LINER
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19000* |
PRODUCTION CASING
Fig. 3.5 : Casing program for a typical 19.000-ft deep well.