casing string

A casing string is the full assembled length of steel pipe sections (joints) threaded together and run as a single continuous tubular string from the wellhead to the planned setting depth in the wellbore, where it is cemented in place to provide structural support, pressure containment, and zonal isolation for all drilling, completion, and production operations conducted in the hole sections below and through it. Each well requires multiple casing strings of progressively smaller diameter run in telescoping sequence from the largest (conductor or surface casing set at shallow depth) to the smallest (production casing or liner set near or at total depth), with each string protecting the hole section and formations above from the fluid pressures and formation instability challenges encountered when drilling the hole section below, and each string cemented with sufficient contact to the previous string or formation to provide hydraulic isolation between the formation intervals penetrated by that string and all shallower formations. In Western Canada Sedimentary Basin well construction, a standard Montney horizontal well uses four casing strings: conductor casing (30-inch or 20-inch OD, driven or jetted to 20 to 60 m to provide surface structural support and wellhead anchor); surface casing (13-3/8-inch or 10-3/4-inch OD, set at 300 to 600 m and cemented to surface to isolate all fresh water horizons as required by AER Directive 008); intermediate casing (9-5/8-inch or 7-inch OD, set at 1,500 to 2,500 m to isolate overpressured zones and provide the casing shoe fracture gradient needed to drill the high-pressure Montney with 1.45 to 1.55 sg oil-based mud); and production casing (5-1/2-inch or 4-1/2-inch OD, run to the Montney production horizon at 3,500 to 5,500 m TVD and cemented across the producing interval before perforation and hydraulic fracturing). Each casing string is designed by burst, collapse, and tension analysis using API 5C3 or ISO 10400 methodology: burst design accounts for the maximum internal pressure that could occur if reservoir gas fills the string to surface during a well control event or if hydraulic fracturing treats against the full string; collapse design accounts for external formation pressure acting on an empty or partially evacuated string during cementing or during production when reservoir depletion lowers internal pressure; and tension design accounts for the combined axial load of the hanging weight of the string below, thermal expansion or contraction forces, pressure-induced end-cap forces, and bending stresses in deviated well sections. Casing string design in WCSB sour gas and sour oil service requires material specification under NACE MR0175 (ISO 15156), limiting the maximum yield strength of the casing material to prevent sulfide stress cracking in H2S-containing environments; for WCSB Foothills wells with H2S partial pressures above 0.0003 MPa, standard P-110 or Q-125 high-strength casing must be replaced with ISO 11960 C-90 or T-95 sour service grades that provide acceptable yield strength below the threshold for hydrogen embrittlement. Casing string integrity throughout the well's producing life is monitored in Alberta under AER Directive 008 (casing inspection and integrity reporting) and Directive 020 (annular pressure monitoring), with multi-finger caliper surveys, electromagnetic inspection logs, and annular pressure measurements at the wellhead providing the primary indicators of casing corrosion, mechanical damage, or cement integrity deterioration that may require workover intervention to maintain regulatory compliance and well safety. Understanding casing string design methodology, the telescoping multi-string wellbore architecture, the burst-collapse-tension load case analysis, the WCSB-specific material requirements for sour service, and the regulatory integrity monitoring framework gives drilling engineers, casing design specialists, well integrity engineers, and completions engineers the technical foundation to design, run, cement, and maintain casing strings that safely contain wellbore pressures and provide reliable zonal isolation throughout the full lifecycle of every WCSB well.

• Telescoping casing string architecture in WCSB well design: Each successive casing string in a WCSB well must fit inside the previous string with sufficient clearance to pass the next bit size and all downhole tools required for the next hole section. Standard WCSB Montney well telescoping dimensions: 30-inch conductor inside 24-inch hole; 13-3/8-inch surface casing inside 17-1/2-inch hole inside conductor; 9-5/8-inch intermediate casing inside 12-1/4-inch hole inside surface casing; 5-1/2-inch production casing inside 8-1/2-inch hole inside intermediate casing. Each size reduction limits the maximum production tubing OD and pump size available to the reservoir, making the number of casing strings and their diameters a critical economic design variable.
• Burst, collapse, and tension safety factors for WCSB casing design: AER Directive 010 requires minimum safety factors of 1.0 on burst, 1.0 on collapse, and 1.6 on tension for all WCSB casing strings. Most WCSB operators apply higher internal design standards: 1.1 to 1.25 on burst (to account for uncertainty in maximum internal pressure during well control or fracturing), 1.0 to 1.1 on collapse (the Klever-Tamano biaxial collapse model is used for HPHT wells), and 1.6 to 1.8 on tension at the top joint connection. Biaxial loading effects (combined burst and tension) require de-rating the burst capacity at the top of long production casing strings by 5 to 15 percent using the API ellipse of plasticity correction.
• Sour service material selection for WCSB Foothills casing strings: WCSB Foothills wells producing from Devonian sour gas reservoirs with H2S concentrations of 5 to 40 mol% require casing strings in ISO 11960 C-90 (758 MPa minimum yield) or T-95 (655 to 758 MPa yield) grades with hardness below 22 HRC under NACE MR0175 to prevent sulfide stress cracking. Standard P-110 (758 MPa minimum yield) is acceptable in some sour service applications where H2S partial pressure is below 0.01 MPa, but Q-125 (862 MPa yield) is excluded from all sour service regardless of H2S concentration because its hardness exceeds the NACE threshold for SSC susceptibility.
• Cementing the casing string for zonal isolation: AER Directive 009 requires cement returns to surface on all surface casing strings and a minimum of 150 m of cemented overlap above the shoe on intermediate and production casing strings, with cement bond log verification required across freshwater protection zones and producing intervals. WCSB production casing strings in Montney and Duvernay horizontal wells are typically cemented with lead slurry at 1.60 to 1.80 sg (to fill the shoe track and lower annulus) and tail slurry at 1.85 to 1.95 sg across the producing interval, with the tail slurry compressive strength reaching 3,500 to 5,000 kPa within 24 hours of WOC (waiting on cement) to support hydraulic fracturing at 60 to 100 MPa treating pressure.
• Casing string running practices for WCSB horizontal wells: Running production casing strings to 4,000 to 5,500 m TD in WCSB Montney and Duvernay horizontal wells requires torque and drag analysis to confirm the string can be run to total depth without exceeding the connection tensile rating or generating excessive compressive loads in the lateral section. Centralizers are placed every 3 to 6 joints through the build section to reduce drag; casing rollers are used at maximum curvature; and the running speed is limited to 0.5 to 1.5 m/min in the horizontal section to control surge pressure on depleted zones near the lateral. Real-time weight indicator monitoring during running identifies ledges, tight spots, and casing bridges before they become stuck pipe situations.

Production Casing String Design for a WCSB Duvernay Horizontal Well

A west-central Alberta operator designing a Duvernay Formation horizontal well at 5,100 m TD required production casing capable of withstanding 75 MPa hydraulic fracturing treating pressure (burst requirement) and 42 MPa external formation pressure on an empty string during initial completion (collapse requirement). The design analysis for 139.7 mm production casing showed that P-110 at 23.07 kg/m (15.5 lb/ft) met the burst requirement (MIYP 74.4 MPa, SF = 74.4/75 = 0.99, marginally below the 1.0 minimum), requiring an upgrade to Q-125 at 23.07 kg/m (MIYP 85.4 MPa, SF = 1.14). Collapse analysis confirmed Q-125 at 23.07 kg/m provided collapse resistance of 63.1 MPa (SF = 63.1/42 = 1.50, exceeding the 1.0 minimum). Tension analysis at the top joint: hanging weight of the string (680 kN) plus pressure end-force (480 kN) plus thermal expansion force (210 kN) equaled 1,370 kN total, against Q-125 connection tensile rating of 2,850 kN (SF = 2.08, above 1.6 minimum). The final specification was Q-125 at 23.07 kg/m throughout the full 5,100 m string; material cost premium over P-110 at the same weight: $218,000. All 24 fracturing stages were completed without casing integrity events at treating pressures of 68 to 74 MPa.

Related Terms

Casing design is the engineering process of selecting the grade, weight, and connection type for each joint of the casing string by analyzing burst, collapse, and tension loads at every depth point in the wellbore, applying API 5C3 or ISO 10400 strength equations with the safety factors required by AER Directive 010 to ensure the string can safely contain wellbore pressures throughout drilling, completion, and production operations. Casing shoe is the bottom fitting of the casing string that guides it into the wellbore during running and incorporates the float valve that prevents cement u-tubing after primary cementing; the shoe depth defines the casing setting depth and the formation at the shoe is tested after drill-out to confirm the fracture gradient available for the next hole section. Primary cementing places the cement sheath in the annulus between the casing string OD and the borehole wall or previous casing ID, providing the zonal isolation that is the primary well integrity function of the casing string in addition to its structural pressure containment role. Liner is a partial casing string that does not extend to surface but is suspended from the previous casing string at its top using a liner hanger; liners are used in WCSB well designs to reduce the telescoping size step between casing strings and allow a larger production casing OD to reach the reservoir than would be possible with a full production casing string hanging from surface through all the intermediate strings. Casing spool is the wellhead component that supports and seals each casing string at the surface, providing the bowl that receives the casing hanger and the side outlet ports that allow annular pressure monitoring required for regulatory compliance under AER Directive 020 throughout the producing life of the WCSB well.