INTERMEDIATE CASING DESIGN
IntroductionIntermediate casing is similar to surface casing in that its function is to permit the final depth objective of the well to be reached safely. When possible, the general procedure outlined for surface casing is also used for intermediate casing strings.
However, in some cases, the burst-design requirements in below figure are extremely expensive to meet, especially when the resulting high working pressure is in excess of the working pressure of the surface BOP stacks and choke manifolds for the available rigs. In this case, the operator may accept a slightly larger risk of loosing the well and select a less severe design load in which the burst limitation is equivalent to the BOP stack rating.
Casing Design Load
- The design load remains based on an underground blowout situation assumed to occur while a gas kick is circulated out. However, the acceptable mud loss from the casing is limited to the maximum amount that will cause the working pressure of the surface BOP stack and choke manifold to be reached.
- If the existing surface equipment is to be retained, it is pointless to design the casing to have a higher working pressure than the surface equipment.
- When the surface burst-pressure load is based on the working pressure of the surface equipment, Pmax, internal pressure at intermediate depths should be determined, as shown in in the below figure.
- Pi, in psi, is the injection pressure opposite the fractured zone:
The injection pressure is equal to the fracture pressure plus an assumed safety margin to account for a possible pressure drop within the hydraulic fracture.
- ρm and ρg are the densities of the mud and gas in pcf :
The density of the drilling mud is determined to be the maximum density anticipated while drilling to the depth of the next full-length casing string. This permits the calculation of the maximum intermediate pressures between the surface and the casing seat
- Dlc is the depth of the fractured zone in ft:
The depth of the fractured zone is determined from the fracture gradient vs. depth plot to be the depth of the weakest exposed formation
Intermediate Casing With a Liner
The burst design-load criteria for intermediate casing on which a drilling liner will be supported later must be based on the fracture gradient below the liner. The burst design considers the intermediate casing and liner as a unit. All other design criteria for the intermediate casing are identical to those previously presented.