Casing Seat Selection

Selecting casing setting depths for each casing string to be run in a well is often the most critical decision made in pre-planning; especially where abnormal pressures or weak, lost circulation zones, are expected.

The key to satisfactory casing seat selection is the assessment of pore pressure (formation fluid pressures) and fracture pressures throughout the well. Evidently, as the pore pressure in a formation being drilled approaches the fracture pressure at the last casing seat then a further string of casing is necessary.

Figure 1 illustrates this, with an idealised casing seat selection shown.

Figure 1 illustrates this, with an idealised casing seat selection shown.

Formation Pressure Oil Well
PRESSURE -►

Figure 1 - Casing Seat Selection

Casing is set at Depth 1, where pore pressure is and the fracture pressure is Fr Drilling continues to Depth 2, where the pore pressure P2 has risen to almost equal the fracture pressure (F1) at the first casing seat. Another casing string is therefore set at this depth, with fracture pressure (F2). Drilling can thus continue to Depth 3, where pore pressure (P3) is almost equal to the fracture pressure F2 at the previous casing seat.

Note that this example does not include any safety or trip margin, which would, in practice, be taken into account.

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