Design Factors Collapse Corrected Casing under axial load has an effective yield 15

Design Factors Collapse Corrected Example Compute the corrected collapse-pressure rating for 20”, K55, 133lb/ft for In service conditions where the axial tension will be 1,000,000 lbf. Also compute the minimum external pressure required for a failure if the internal pressure will be 1000 psig. Area of the steel is 38.632”2

Design Factors Collapse Corrected Sub into equation 15

Design Factors Collapse Corrected D/t = 31.496 Collapse will be in transition mode

Design Factors Collapse Corrected For Y = 36,385 F1 = 2.941 F2 = .0446 F3 = 645.1 F4 = 2.101 F5 = .0319 This is the corrected pressure differential, so for an internal pressure 1000 psig, the collapse pressure will be 1267 + 1000 = 2267 psi

Design Factors Collapse Corrected The boundaries for the failure regions also change with the change of yield. Upper limit for Yield – strength collapse

Design Factors Collapse Corrected The Upper limit for Plastic Collapse range The Lower limit for Elastic Collapse range Transition range is between these limits

Design Factors Bending In directional wells, the effect of wellbore curvature and vertical deviation angle on axial stress in the casing and couplings must be considered in the casing design. When a casing is forced to bend, the axial tension on the convex side of the bend can increase greatly. On the other hand, in relatively straight sections of hole with a significant vertical deviation angle, the axial stress caused by the weight of the pipe is reduced.

Design Factors Bending The maximum increase in axial stress, sb, on the convex side of the pipe is given by Crandall and Dahl (1995) sb = ± 0.5EdnK. In oil field units, where dogleg severity, K, is expressed as the change in angle in degrees per 100 ft; E (lbf/in2) = stress/strain is elastic modulus. sb = ± 218dnK. In terms of an equivalent axial force Fab = ± 218dnKAs. In terms of weight per foot of pipe Fab = ± 64dnKwp.

Design Factors Bending Example: Determine the maximum axial stress for 7.625-in, 39-lbf/ft, N-80 casing if the casing is subjected to a 400,000 lbf axial tension load in a portion of a directional wellbore having a dogleg severity of 40/100ft. Assuming uniform contact between the casing and the borehole wall. Note that, the definition for the pipe body yield strength:

Design Factors Bending Solution: Nominal API pipe body yield strength for this casing is 895,000 lbf, and the ID is 6.625. The cross sectional area of steel in the pipe body is A = p/4(7.6252 – 6.6252) = 11.192 in2. The axial stress without bending: 400,000/11.192 = 35,740 psi The additional axial stress on the convex side of the pipe due to the bending sb = ± 218dnK = 218(7.625)(4) = 6,649 psi The total axial stress = 35,740 + 6,649 = 42,389 psi

Designing Casing Program Design Selection Casing strings most met all the future conditions in the well. Drilling -- Completions -- Production Designing Casing Program Start with the smallest string, usually the production string What size is required Completion Frac job Production Lift methods

Design Selection Intermediate casing strings Number depends on hole conditions. The last of the intermediate string must be large enough so that the hole out from under it is large enough for the production casing that is needed. The next string up the hole, another intermediate or surface must be large enough to drill the proper size hole.

Design Selection Basically all wells will have a surface string and production string. The number of intermediate strings depend on the well conditions Formations that cause problems when drilling Under of over pressured zones Frac gradient Lost circulation zones Most of the conditions can be anticipated before drilling the well

Design Pipe Selection Combination Strings For a more efficient design use multiple weights and grades. Differing conditions in the wellbore require different specs. Some cases a liner will be more economical

Design Pipe Selection The string must be designed not to fail while running and during operations. The three failure modes, Tension, Burst and Collapse

Design Pipe Selection Example Intermediate String Tension, the top joint must be able to support the weight of the string. Joint strength Burst, the burst rating of the job joint will control the pressure during well control. Collapse, there will be greater pressure outside the bottom of the string while cementing. The casing tables can be used for Joint Strength and Burst, but collapse must be calculated for the load.