Intermediate Casing
Intermediate or protective casing is set at a depth between the surface and production casings. The main reason for setting intermediate casing is to case off the formations that prevent the well from being drilled to the total depth. Troublesome zones encountered include those with abnormal formation pressures, lost circulation, unstable shales and salt sections. When abnormal formation pressures are present in a deep section of the well, intermediate casing is set to protect formations below the surface casing from the pressures created by the drilling fluid specific weight required to balance the abnormal pore pressure. Similarly, when normal pore pressures are found below sections having abnormal pore pressure, an additional intermediate casing may be set to allow for the use of more economical, lower specific weight, drilling fluids in the subsequent sections. After a troublesome lost circulation, unstable shale or salt section is penetrated, intermediate casing is required to prevent well problems while drilling below these sections.
Intermediate casing varies in length from 7.000 ft to as much as 15.000 ft and from 7 in. to ll| in. in outside diameter. It is commonly cemented up to 1,000 ft from the casing shoe and hung onto the surface casing. Longer cement columns are sometimes necessary to prevent casing buckling.