Page 165 - Geothermal Energy Systems Exploration, Development, and Utilization

3.5 Planning a Well 141
• collapse pressure,
• burst pressure,
• tensile load of casing.

This has to be done for the pipe body as well as for the thread-connection.
By preselecting a certain wall thickness (which are also ‘‘standardized’’ for
each size) the yield strength of the pipe material is the most important material
parameter for the calculation of load and pressure capacities. Additionally, the
tensile strength is used to calculate the connection strength.
In these calculations the different temperatures are also to be considered, because
temperature has an influence on the material’s strength; it is reduced by elevated
temperatures, causing a reduction of the pressure capacities of the pipe as well as
of load capacities of casing pipe body and connection (Figure 3.18).
Always several load cases have to be calculated for each casing string, for example,
• Calculation of different pressure situations for external pressure:
– inside (partially) empty, outside mud;
– inside mud, outside (partially) cement;
– squeezing formations (salt, shales).
• Calculation of different pressure situations for internal pressure, for example:
– influx (brine, gas, etc.);
– stimulation (Hydrofrac).
• Calculation of different (load) stress situations, for example:
– casing string in mud (inside and outside mud);
– casing string while cementation (inside cement, outside mud);
– casing string at end of cementation (inside mud, outside cement);
– casing string under additional stress (e.g., additional tension to compensate
thermal elongation during production).

800
Temperature influence on yield strength for various material grades
700
600
Rp [N/mm] 500

400

300
P110 C 95 L 80 C 75-2 C 75-1 J/K 55
200
0 25 50 75 100 125 150 175 200 225 250 275 300 325
T [°C]

Figure 3.18 Yield strength of various casing material grades
versus temperature. (By A. Sperber.)