casing burst pressure

Casing burst pressure is the internal pressure at which a casing string fails by radial splitting or ballooning outward, representing the upper pressure limit that the casing wall can contain before yielding or rupturing, and it is the primary design constraint governing casing grade and weight selection for all intervals where wellbore pressure significantly exceeds annular pressure in Western Canada Sedimentary Basin well construction. The burst rating of a casing string is calculated from the Barlow formula, which for thin-walled pipe states that internal yield pressure equals 0.875 times (two times minimum yield strength times wall thickness divided by outside diameter), where the 0.875 factor is the API 5C3 manufacturing tolerance applied to account for permissible wall thickness under-run of 12.5 percent; for thick-walled casing at diameter-to-wall-thickness ratios below 15, the more accurate Lame equation or von Mises yield criterion is applied instead of the simplified Barlow formula. In WCSB drilling programs, burst loading governs casing design in three primary scenarios: the gas-filled string scenario during a kick or blowout where reservoir gas displaces drilling mud inside the casing and the internal pressure profile exceeds the formation fracture pressure at the casing shoe; the stimulation scenario where hydraulic fracturing applies surface treating pressures of 60 to 100 MPa to the production casing during multi-stage Montney and Duvernay fracturing operations; and the annular pressure buildup scenario where thermal expansion of fluids trapped in a sealed annulus (particularly in SAGD steam injection wells where steam temperatures reach 200 to 240 degrees Celsius) generates internal pressure in the inner casing string of the annular pair. The industry-standard WCSB casing design workflow applies a safety factor of 1.0 to 1.25 on burst (ratio of casing burst rating to maximum anticipated internal pressure), with the higher safety factor applied for production casing strings where hydraulic fracturing will impose the highest pressure loading, and the lower safety factor applied for surface casing strings where the maximum anticipated burst load is a full gas column to surface during worst-case kick containment. API 5C3 and the successor ISO 10400 standard tabulate the minimum internal yield pressure (MIYP) for all standard casing grades and weights, with WCSB production casing in Montney and Duvernay plays commonly specified in P-110 grade (minimum yield 758 MPa) or Q-125 grade (minimum yield 862 MPa) at 139.7 mm (5-1/2 inch) or 177.8 mm (7 inch) outside diameter to achieve burst ratings of 55 to 90 MPa required for high-pressure hydraulic fracturing operations. Biaxial loading corrections are applied to the burst rating when the casing string is also under significant axial tension, because combined tensile and internal pressure loading reduces the effective burst capacity below the uniaxial MIYP value; the API 5C3 biaxial correction uses the ellipse of plasticity (von Mises yield envelope) to determine the de-rated burst pressure at the maximum anticipated axial tension in the string, which is critical for WCSB Foothills wells where long production casing strings in deep sour gas wells carry axial loads of 1 to 3 MN while simultaneously being exposed to hydraulic fracturing pressure. Collapse resistance and burst resistance trade off against each other in casing design because both depend on wall thickness and grade, and heavier wall casing that improves burst rating also improves collapse rating; however, for WCSB shallow gas and coal bed methane wells where low formation pressures mean burst loading is minimal, lighter-weight lower-grade casing (J-55, K-55) is specified to minimize material cost while still satisfying collapse and tension requirements. Understanding casing burst pressure calculation methodology, the Barlow and Lame burst equations, API 5C3 safety factors, biaxial loading corrections, and the specific WCSB burst load cases driven by kick tolerance, hydraulic fracturing, and thermal annular pressure buildup gives drilling engineers, casing design specialists, and well integrity engineers the technical basis to select casing grades and weights that provide adequate burst resistance throughout the design life of every WCSB well.

• Burst load cases in WCSB casing design: The governing burst load case for WCSB surface casing is typically the full gas column scenario (gas to surface in the casing with formation fluid pressure at the shoe balanced by annular fluid), generating burst pressures of 5 to 20 MPa at surface. For WCSB production casing in Montney and Duvernay horizontal wells, the governing burst case is hydraulic fracturing at maximum treating pressure (60 to 100 MPa surface), which requires P-110 or Q-125 grade casing with burst ratings of 65 to 95 MPa at the specified wall thickness. Both cases require a safety factor of at least 1.0 on burst (AER Directive 010 requires minimum 1.0, most operators use 1.1 to 1.25).
• Barlow versus Lame burst equations: The Barlow formula (MIYP = 0.875 x 2Yt/D) is specified in API 5C3 for standard thin-wall casing design and is adequate for casing with D/t ratios above 15. For thick-walled casing (D/t below 15) used in HP/HT WCSB Foothills wells, the Lame equation (internal hoop stress at the inner wall = P x (r-o squared plus r-i squared)/(r-o squared minus r-i squared)) gives a more accurate burst rating that is 3 to 8 percent higher than Barlow, reducing the tendency to over-specify heavy casing in deep wells where weight is a critical constraint.
• Biaxial de-rating for combined burst and tension: WCSB production casing strings in horizontal Montney wells are under axial tension from the weight of the string suspended below the surface casing shoe; at maximum tension, the effective burst capacity (de-rated MIYP) is reduced by 5 to 15 percent from the uniaxial MIYP. Casing design software applies the API 5C3 ellipse of plasticity correction automatically, but manual designs must explicitly include this de-rating to avoid under-designing the casing burst resistance at depths where the string is near its tensile capacity.
• Sour service and burst rating considerations: WCSB Foothills sour gas wells with H2S partial pressures above 0.0003 MPa require Sour Service (SS) or ISO 11960 H-grade casing under NACE MR0175, which limits the maximum yield strength of the casing material to 758 MPa (P-110 equivalent) to prevent sulfide stress cracking. This yield strength limit also limits the achievable burst rating, requiring larger diameter or heavier wall casing to meet burst requirements in the deepest Foothills wells where high yield strength grades would otherwise be preferred to reduce string weight.
• Thermal annular pressure and burst in SAGD wells: In WCSB SAGD and CSS steam injection wells (Cold Lake, Peace River, Athabasca), steam injection at 200 to 240 degrees Celsius heats the fluid trapped in the cemented annulus between the inner and outer casing strings, generating annular pressure buildup that loads the inner casing in burst. SAGD production casing burst ratings must account for both the maximum steam injection pressure (8 to 12 MPa) and the superimposed thermal annular pressure (2 to 8 MPa additional), often requiring 13-3/8 inch J-55 or K-55 surface casing of 68 to 72 lb/ft weight to maintain burst safety factors above 1.0 throughout the steam injection cycle.

Burst-Driven Casing Upgrade on a WCSB Duvernay Horizontal Well

A WCSB operator designing a Duvernay Formation horizontal well in west-central Alberta initially specified 139.7 mm (5-1/2 inch) N-80 production casing at 20.09 kg/m (13.5 lb/ft) wall thickness, which had an API 5C3 minimum internal yield pressure of 52.4 MPa. The hydraulic fracture design for 18 stages at 80-metre spacing required a maximum treating pressure of 62 MPa at surface, giving a burst safety factor of only 0.85, well below the operator's minimum of 1.10. The completions engineer evaluated three upgrade options: increasing wall thickness to 23.07 kg/m (15.5 lb/ft) in N-80 (MIYP 62.0 MPa, SF = 1.00, still insufficient), upgrading to P-110 grade at 20.09 kg/m (MIYP 64.8 MPa, SF = 1.04, marginally insufficient), or upgrading to P-110 at 23.07 kg/m (MIYP 74.4 MPa, SF = 1.20, acceptable). The final specification was P-110 at 23.07 kg/m, adding $185,000 to casing material cost but providing the required burst safety factor with 4 MPa margin above the maximum treating pressure. Post-completion, all 18 stages were stimulated without pressure barrier issues at treating pressures of 58 to 61 MPa.

Related Terms

Casing collapse pressure is the external pressure at which casing fails by inward buckling or ovalization, the complementary design limit to burst pressure; in WCSB casing design, collapse governs string selection for intervals with high pore pressure depletion or deep setting depths where hydrostatic mud pressure exceeds formation pore pressure. Casing design integrates burst, collapse, and tension load cases across the full wellbore depth profile to select the optimal casing grade and weight for each string, applying safety factors from AER Directive 010 and company design standards to ensure casing integrity throughout drilling, completion, and production operations. Minimum internal yield pressure is the API 5C3 tabulated burst rating for each casing grade and weight combination, calculated from the Barlow formula with the 0.875 manufacturing tolerance factor and used as the allowable burst capacity in WCSB casing design against all internal pressure load cases. Hydraulic fracturing is the primary source of maximum burst loading on WCSB Montney and Duvernay production casing, with surface treating pressures of 60 to 100 MPa requiring P-110 or Q-125 grade casing selected specifically to maintain burst safety factors above 1.10 throughout the multi-stage fracture stimulation program. Biaxial loading correction reduces the effective burst rating of production casing under axial tension, requiring WCSB drilling engineers to apply the API 5C3 ellipse of plasticity de-rating to the minimum internal yield pressure at depths where both maximum internal pressure and maximum axial tension are simultaneously anticipated.