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Design Criteria for Casing in Stimulated Wells

The maximal allowable temperature at which the pipe body or joint yields is usually determined by using the classical theory proposed by Holliday (1969). According to Holliday, the maximal allowable casing temperature is given by the following relationship:

^casing ^surrounding 3” (3-112)

where:

AT = ^ + ^2 (4.113)

I ь

ared — reduced yield stress due to temperature and internal pressure

= [(сгут-)2 — 0.75 <j,] -0.5(j,.

<jyT = yield stress corrected for temperature (hot yield stress).

<7, = (Barlow’s equation, corrected for pipe imperfections.)

ay] = joint yield stress (cold yield stress).

Values of elevated yield stress, cryT, in tension, for different steel grades are pre­sented in Table 4.8 (Goetzen, 1986). An iterative solution is necessary to deter­mine the value of allowable casing temperature because the yield strength of the casing material is a function of temperature.

Equation 4.113 is derived based on the following assumptions:

Table 4.8: Yield stress of different steel grades at elevated temperature. (After Goetzen, 1986; courtesy of ITE-TU Clausthal.)

API-steel

Hot yield strength, аУТ, in psi

Steel

grade

68°F

212°F

392°F

572°F

752° F

composition

H-40 (ST)

40,000

34,000

48,000

52,500

41,000

ST grade

J-K-55 (ST)

55,000

51,150

65,000

61,500

51,150

ST grade

C-75 (ST)

75,000

64,680

58,505

56,300

51,890

P 38 Mn6

C-75 (TR)

75,000

68,355

63,060

60,858

59,240

P 26 Cr Mo4

L-80 (ST)

80,000

68,945

62,475

59,975

55,125

P 28 Mn6

L-80 (TR)

80,000

72,910

67,325

64,827

63,210

P 26 Cr Mo4

N-80 (ST)

80,000

76,000

73,600

69,600

58,400

P 38 Mn6

C-95 (ST)

95,000

86,730

81,880

78,940

71,295

P 41 Mn V5

C-95 (TR)

95,000

88,641

85,700

83,790

78,940

P 34 Cr Mo4

P-105 (ST)

105,000

102,000

100,000

102,000

90,000

P 41 Mn V4

P-110 (ST)

111,425

92,460

89,230

84,672

75,850

P 41 Mn V5

P110 (TR)

111,425

100,400

93,640

91,435

88,055

P 34 Cr Mo4

ST = standard, TR = thermal resistance

1. Casing is an elastic-perfect plastic material which: does not strain harden, but flows plastically; exhibits Bauschinger’s Effect; has yield strength in compression.

2. The coupling is as strong in compression as the pipe body at elevated tem­perature.

3. The tensile coupling strength is unaffected by thermal axial compressive strain.

4. Biaxial stress effects result only from internal pressure. There is no external casing pressure present.

5. Casing is fully cemented and, therefore, no axial displacement of pipe is expected.

The design method presented here is different from the traditional elastic method discussed in Chapter 3 because the casing is assumed to deform plastically. How­ever, the successful application of plastic design requires the exclusion of creep rupture effects due to extremely high temperatures and it is, therefore, recom­mended that the design procedure should be limited to casing temperatures of 700 °F or less and that any increase in yield strength due to blue brittleness be neglected.

450

400

350

300

250

200

150

100

50

100 200 300 400

RADIUS (mm)

Fig. 4.33: Temperature distribution in a typical steam injection well. (After Sugiura and Farouq Ali, 1978.)

The exclusion of any effect of external pressure in the casing design may not be completely realistic for deep wells, i. e., below 5,000 ft. Thus, the design method is applicable to shallow wells where the casing is not subjected to collapse loads. The vast majority of the casing designs for thermal wells, however, have been based on this method and no serious casing failures have been reported (Goetzen, 1986).

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