Casing for Intermediate Section of the Well
The principle behind the selection of the intermediate casing seat is to first control the formation pressure with drilling fluid hydrostatic pressure without fracturing the shallow formations. Then, once these depths have been established, the differential pressure along the length of the pipe section is checked in order to prevent the pipe from sticking while drilling or running casing.
From Fig. 3.2 the formation pressure gradient at 19,000 ft is 0.907 psi/ft (equivalent mud specific weight = 17.45 lb/gal). To control this pressure, the wellbore pressure gradient must be greater than 0.907 psi/ft. When determining the actual wellbore pressure gradient consideration is given to: trip margins for controlling swab pressure, the equivalent increase in drilling fluid specific weight due to the surge pressure associated with the running of the casing and a safety margin. Generally a factor between 0.025 and 0.045 psi/ft (0.48 to 0.9 lb/gal of equivalent drilling mud specific weight) can be used to take into account the effects of swab and surge and provide a safety factor (Adams, 1985). Thus, the pressure gradient required to control the formation pressure at the bottom of the hole would be 0.907 + 0.025 = 0.932 psi/ft (17.95 lb/gal). At the same time, formations having fracture gradients less than 0.932 psi/ft must also be protected. Introducing a safety factor of 0.025 psi/ft, the new fracture gradient becomes
0. 932 + 0.025 = 0.957 psi/ft (18.5 lb/gal). The depth at which this fracture gradient is encountered is 14,050 ft. Hence, as a starting point the intermediate casing seat should be placed at this depth.
The next step is to check for the likelihood of pipe-sticking. When running casing, pipe sticking is most likely to occur in transition zones between normal pressure and abnormal pressure. The maximum differential pressures at which the casing can be run without severe pipe sticking problems are: 2.000 — 2,300 psi for a normally pressured zone and 3,000 — 3,300 psi for an abnormally pressured zone (Adams, 1985). Thus, if the differential pressure in the minimal pore pressure zone is greater than the arbitrary (2,000 — 2,300 psi) limit, the intermediate casing setting depth needs to be changed.
From Fig. 3.2, it is clear that a drilling mud specific weight of 16.85 lb/gal (16.35 + 0.5) would be necessary to drill to a depth of 14,050 ft. The normal pressure zone, 8.9 lb/gal, ends at 9,150 ft where the differential pressure is:
9,150(16.85 — 8.9) x 0.052 = 3.783 psi
This value exceeds the earlier limit. The maximum depth to which the formation can be drilled and cased without encountering pipe sticking problems can be computed as follows:
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(3-1) |
Ap = Dn{"im — 7j) x 0.052
EQUIVALENT MUD SPECIFIC WEIGHT (ppg) 9 10 11 12 13 14 15 16 17 18 19
ion of casing seats based on the pore pressure and
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8 9 10 11 12 13 14 15 16 17 18 19 20 Fig. 3.3 : Selection of setting depths for different casings in a 19.000-ft well, where: |
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EQUIVALENT MUD SPECIFIC WEIGHT (ppg) |
Ap ~ arbitrary limit of differential pressure, psi.
7m = specific weight of new drilling fluid, lb/gal.
7; = specific weight of formation fluid, lb/gal.
Dn = depth where normal pressure zone ends, ft.
0. 052 = conversion factor from lb/gal to psi/ft.
Given a differential pressure limit of 2.000 psi. the value for the new mud specific weight becomes 13.1 lb/gal (0.681 psi/ft gradient). Now the depth at which the new drilling fluid gradient becomes the same as the formation fluid gradient, is 11,350 ft. For an additional safety margin in the drilling operation, 11,100 ft is selected as the setting depth for this pipe.
The setting depth for casing below the intermediate casing is selected on the basis of the fracture gradient at 11,100 ft. Hence, the maximal drilling fluid pressure gradient that can be used to control formation pressure safely, without creating fractures at a depth of 11,100 ft. must be determined.
From Fig. 3.3, the fracture gradient at 11,100 ft is 0.902 psi/ft (or 17.35 lb/gal equivalent drilling mud weight). Once again, a safety margin of 0.025 psi/ft which takes into account the swab and surge pressures and provides a safety factor is used. This yields a final value for the fracture gradient of 0.877 psi/ft and a mud specific weight of 16.85 lb/gal, respectively. The maximal depth that can be drilled safely with the 16.85 lb/gal drilling fluid is 14,050 ft. Thus, 14,000 ft (or 350 joints) is chosen as the setting depth for the next casing string. Inasofar as this string does not reach the final target depth, the possibility of setting a liner between 11,100 ft and 14,000 ft should be considered.
The final selection of the liner setting depth should satisfy the following criteria:
1. Avoid fracturing below the liner setting depth.
2. Avoid differential pipe sticking problems for both the liner and the section below the liner.
3. Minimize the large hole section in which the liner is to be set and thereby reduce the pipe costs.
As was shown in Fig. 3.2, the mud weight that can be used to drill safely to the final depth is 17.95 lb/gal (gradient of 0.93 psi/ft). This value is lower than the fracture gradient at the liner setting depth.
Differential pressures between 11.100 ft and 14.000 ft and between 14.000 ft and
19,0 ft are 821 psi and 451 psi. respectively. These values are within the prescribed limits.
Thus, the final setting depths for intermediate casing string, drilling liner and production casing string of 11,100 ft, 14,000 ft, and 19,000 ft, respectively, are presented in Fig. 3.3. These setting depths also minimize the length of the large hole sections.