There are a number of areas in horizontal well casing design that may differ from a standard well. These mostly relate to the casing loading, although there can be instances where the capacity of the casing is purposely reduced.
Casing loading
Fpr the horizontal section itself, the stability of the formation must be determined in order to assess whether the casing in this section has to withstand the full overburden pressure.
In the case that the casing will be subject to formation loading, computer program should be used to determine the casing behaviour.
For short radius build-up sections, the bending stresses can be significant. For particularly high build rates, localised bending stress concentrations can occur near casing couplings due to the difference in outer diameter of the casing body and the coupling. The magnitude of these bending stresses, and their effect on the collapse and burst capacities of the casing, can be determined from the computer program. Note that all casing that has to pass through these high doglegs must be designed to withstand the bending stresses generated.
Due to the high contact forces between the casing and the borehole wall in highly deviated sections of the well, dynamic drag and torque loads will be high. Drag loads may be such that once the casing string passes a given depth, the total axial force required to pull the string upwards exceeds the axial capacity of the pipe.
Optimum cementation of the casing in the horizontal section is obtained by rotating the casing/liner. Thus consideration must be given to the torque required to achieve this rotation, and the ability of the selected couplings to withstand this torque. Selection of couplings should be based on the maximum anticipated torque.
Drag and torque loads can be simulated using computer programs. Computer programs can be used to quantify the ability to run the next work string or casing if high doglegs are to be considered, taking into account bending stiffness.
Casing load bearing capacity
Liners set in horizontal sections are often pre-drilled or slotted to avoid the need for complicated perforating operations. When running these liners, the reduction in axial capacity due to the holes should be considered. This is best achieved by calculating the stress concentration factor that results from the presence of the hole and comparing the resulting stress with the casing material yield stress.