COLLAPSING STRENGTH OF STEEL CASING The question of strength of casing to resist collapse is of vital importance to oil operators contemplating deep wells, because it must be remembered that after the well is completed it will some times stand nearly empty and therefore the casing will be sub jected to the maximum pressure of the column of water outside it. The use of casing which is too light is false economy.
When a test of the water shut-off is made at a well, it is desir able to bail the well down as far as possible without collapsing the casing, and the accompanying table showing the strength of various sizes and weights of casing is presented as a rough guide.
The list is based on a great number of actual tests conducted by Prof. Reid T. Stewart (Vol. 27, Transactions American Society of Mechanical Engineers), and the figures for depth of water exert ing pressure enough to collapse casing is the limit beyond which safe operations should not extend. A prudent operator would stop far short of the limit by applying a safety factor of two, or possibly one and three-fourths. In this list no account is taken
of such underground forces as are sometimes reported to have cut off and carried casing to one side and out of reach of the drilling tools. It should be remembered that when any force is suddenly applied it is much more destructive than when gradually applied, and if a well were suddenly emptied of its fluid content, as in the case of a gas blow-out, the casing might collapse under a water pressure that it would have supported if the well were emptied slowly by a bailer or pump. The depth of water is of course sometimes much less than the depth of the well, and it is always important to note the level of the fluid which stands outside of the casing. Care of casing before it is used is of vital importance. Tests show that when the casing is slightly flattened, or out of round, collapse occurs most easily. In handling casing it is fer quently allowed to fall and become bruised or dented, thereby inviting collapse when it is later used in a well.