SETTING DEPTH
Selection of the number of casing strings and their respective setting depths is based on geological conditions and the protection of fresh-water aquifers. For example, in some areas, a casing seat is selected to cover severe lost circulation zones whereas in others, it may be determined by differential pipe sticking prob-
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8 9 10 11 12 13 14 15 16 17 18 19 20 Fig. 3.1: Typical pore pressure and fracture gradient data for different depths. |
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EQUIVALENT MUD SPECIFIC WEIGHT (ppg) |
lems or perhaps a decrease in formation pore pressure. In deep wells, primary consideration is either given to the control of abnormal pressure and its isolation from weak shallow zones or to the control of salt beds which will tend to flow plastically.
Selection of casing seats for the purpose of pressure control requires a knowledge of pore pressure and fracture gradient of the formation to be penetrated. Once this information is available, casing setting depth should be determined for the deepest string to be run in the well. Design of successive setting depths can be followed from the bottom string to the surface. A typical example is presented in Fig. 3.1 to illustrate the relationship between the pressure gradient, fracture gradient and depth.