Squeezing salt well


When a hole is drilled through a salt formation, and subsequently casing is set, the salt will gradually move and make contact with the casing. The following paragraphs describe the loading on the casing, and then the capacity of the casing to withstand such loading.

Casing loading

The plastic behaviour of salt formations may result in casing opposite this formation being subjected to the full formation overburden gradient. The value of the overburden gradient will vary with depth but may be as high as 1 psi/ft (22.6 kPa/m). As such, the collapse loads, whether designing for full or partial evacuation, will be extremely high.

This phenomenon is time dependent such that during the drilling phase increased external loading due to moving salt may be minor, while in the production phase, however, salt loading might play a greater role.

Another important factor is that the loading that results from the moving salt is not uniformly applied. If the hole is washed-out unevenly and the casing is not perfectly cemented, the salt will reach the casing at different times. This will cause one side of the casing to be exposed to the full overburden gradient, while the other side is completely unsupported. This type of point loading, resulting in high shear stresses, can cause casing failure at much lower loads than when applied uniformly.

It is also possible that, even if the casing does not immediately collapse, it may start to bend into the wash-out opposite the moving salt. The resulting increase in axial stress on one side of the casing may lead to a reduced collapse pressure or the casing may fail due to bending stresses alone.

Casing load bearing capacity

Studies have shown that to withstand this type of non-uniform loading, the diameter/thickness (d/t) ratio of the casing would have to be less than 4. The thick-walled casing often used to withstand the high (uniform) collapse pressures have d/t ratios between 9 and 11. At present, it is seen as impractical and uneconomical to design the casing to withstand these forces.

Instead, non-uniform loading effects are reduced by operational practices such as minimising hole enlargement during drilling, and ensuring that the casing is cemented over the entire salt interval in order to distribute the load uniformly over the casing circumference.

Following this approach, the casing is designed to withstand a concentrically uniform external pressure - equivalent to the overburden pressure at the depth of the salt formation - in the same manner as for fluid pressure.


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