concentric

Concentric in oilfield engineering refers to two or more cylindrical objects sharing the same central axis, with the most common applications in Western Canada Sedimentary Basin well design being concentric casing strings (multiple progressively smaller-diameter casing strings nested inside each other to isolate progressively deeper formations), concentric tubing-in-tubing completions (used in WCSB thermal wells for steam injection and fluid return in the same wellbore), and concentric annuli (the ring-shaped flow spaces between adjacent casing strings or between the outermost casing and the borehole wall, through which cement slurry is pumped during primary cementing and through which formation fluids or injection fluids flow during production and injection operations). In WCSB well construction, the concentric casing design is the fundamental architectural principle that allows a single wellbore to safely penetrate multiple geological formations with different pore pressures, fracture gradients, and fluid types without hydraulic communication between formations; each concentric casing string is set and cemented before the next, progressively smaller string is drilled and run, so that by the time the production casing is set at total depth, the borehole is protected by a series of nested steel cylinders each isolated from adjacent formations and from each other by cement in the annular space between them. In a typical WCSB deep gas well (3,000 to 4,500 m total depth targeting Devonian Nisku or Beaverhill Lake carbonates), the concentric casing program consists of: conductor pipe (406 to 508 mm OD, 20 to 50 m, driven or drilled to protect the surface from borehole collapse during drilling); surface casing (244 to 340 mm OD, 300 to 600 m, cemented to surface to isolate fresh groundwater above the base of groundwater protection as required by AER Directive 008); intermediate casing (178 to 244 mm OD, 1,500 to 2,500 m, cemented to protect against overpressured zones or salt formations in the Devonian Elk Point Group); and production casing (114 to 178 mm OD, to total depth, providing the pressure vessel for completions and production operations). The annular space between each concentric casing string and the next-larger string (or the open borehole for the outermost string) is the cement sheath that provides zonal isolation, structural support for the casing, and corrosion protection from formation fluids; AER Directive 009 specifies minimum annular clearance, cement volume, and cement quality requirements for each concentric casing string in WCSB wells to ensure the well can withstand the mechanical, thermal, and pressure loads imposed during drilling, completions, and production.

• Concentric casing design and annular clearance calculations for WCSB deep well programs: The selection of casing OD and weight for each concentric string in a WCSB well program is governed by three constraints: the mechanical properties required (burst, collapse, and tensile ratings must exceed the maximum anticipated loads with a safety factor of 1.1 to 1.25), the annular clearance between concentric strings (minimum 12.7 mm radial clearance for cement circulation, but typically 19 to 32 mm for adequate cement job quality), and the drift diameter of the outer string (the production casing must fit inside the intermediate casing with enough clearance for the running tool). For a WCSB Devonian target requiring 114.3 mm (4.5 inch) production casing, the intermediate casing must have an inner drift diameter of at least 121 mm; a 177.8 mm (7 inch) intermediate casing with 12.65 mm wall (38 kg/m, L80 grade) has a drift diameter of 152.1 mm, providing 30 mm radial clearance for the production casing running operation. The bit size for drilling the production hole must be selected to fit through the intermediate casing drift diameter (maximum 149.2 mm for this example) while providing enough annular clearance for cement circulation above the production casing shoe.
• Concentric tubing-in-tubing completions for WCSB SAGD and CSS thermal recovery wells: In WCSB in-situ oil sands thermal recovery operations (SAGD at Athabasca, CSS at Cold Lake), concentric tubing-in-tubing completions are used in CSS injection-production wells to simultaneously inject steam down the inner tubing string and produce hot oil and condensate up the concentric annulus between inner and outer tubing strings, allowing both functions to occur in the same wellbore casing without cross-contamination. The inner tubing string (typically 60 to 73 mm OD) carries high-pressure steam from surface (8 to 12 MPa, 295 to 310 degrees Celsius) to the perforated interval in the Clearwater or McMurray Formation; the outer tubing string (114 to 127 mm OD) provides the production conduit through which heated oil (80 to 200 cP at 200 degrees Celsius versus 500,000 cP at reservoir temperature of 12 degrees Celsius) flows back to surface through the concentric annulus under the drive pressure of the steam injection. Thermal expansion differentials between the inner and outer concentric tubing strings at CSS operating temperatures of 200 to 260 degrees Celsius (inner string 10 to 15 cm longer than cold length per 100 m of steam zone) require expansion joints or polished-bore receptacles in the tubing hanger design to prevent compressive buckling of the inner string.
• Concentric annulus hydraulics and equivalent circulating density in WCSB well control operations: The hydraulic behavior of fluid flowing in the concentric annulus between the drill string and the casing or borehole wall governs the equivalent circulating density (ECD) experienced by the formation during drilling; ECD is the sum of static mud density and the annular friction pressure expressed as an equivalent density at any given depth. Annular friction pressure in a concentric annulus with narrow gap (drill string OD close to borehole or casing ID) is higher than in a wide-gap annulus at the same flow rate because the hydraulic diameter (4 x flow area / wetted perimeter) is reduced; in a WCSB horizontal well with 127 mm drill pipe in a 152 mm borehole (12.5 mm radial gap), annular friction pressure at 25 L/s is 1.5 to 3.0 kPa/m, adding 0.06 to 0.12 SG to the static mud density ECD and creating significant narrow-margin drilling risk if the static mud density is already near the fracture gradient. WCSB Montney horizontal well concentric annulus design requires careful hydraulic modeling (Drillbench, WellPlan, or CAPP-recommended simulators) to confirm that the ECD at maximum pump rate during drilling does not exceed the fracture gradient at the weakest exposed formation, particularly in the transition from vertical to horizontal where the annular geometry changes and cuttings transport velocity requirements shift.
• Concentric casing annulus monitoring for WCSB sustained casing pressure detection and well integrity: Each concentric casing annulus in a WCSB multi-string well (A annulus between tubing and production casing, B annulus between production casing and intermediate casing, C annulus between intermediate casing and surface casing) is a distinct pressure monitoring zone that provides diagnostic information about well integrity; sustained pressure in any annulus after bleed-down indicates fluid migration through the cement sheath or a casing integrity failure at that specific annulus level. AER Directive 020 requires WCSB operators to monitor all accessible annuli annually and to report sustained casing pressure (SCP) exceeding defined thresholds: surface casing annulus (A annulus for shallow wells without intermediate string) SCP above 690 kPa, or production casing annulus SCP above 1,380 kPa, requires notification and a risk assessment of whether the sustained pressure indicates H2S or sour gas migration that poses a surface safety hazard. The concentric architecture of the casing string means that SCP in the B or C annuli (outer annuli) is generally less immediately hazardous than A annulus SCP because the gas or fluid must have already migrated through the inner cement sheath and the inner casing material before reaching the outer annuli, providing additional containment barriers between the migrating fluid and the surface environment.
• Concentric centralizer placement for uniform cement distribution in WCSB casing cementing programs: Achieving a uniform concentric cement sheath requires that the casing be positioned as close to the center of the borehole as possible during cement pumping; a casing that is lying eccentrically on the low side of the borehole (standoff below 50 percent) allows cement to channel preferentially through the wide-gap side of the annulus while the narrow-gap side retains an uncemented mud channel. AER Directive 009 requires minimum 67 percent standoff (casing centered within 67 percent of the theoretical maximum from borehole center) at the top of the cement column and at casing connections in environmentally sensitive zones; centralizers (bow-spring or rigid) are placed at intervals of 10 to 20 m in horizontal sections and 20 to 40 m in vertical sections to maintain concentric positioning. In WCSB Montney horizontal completions where the production casing must remain concentric within a 152 mm borehole over 2,000 m of horizontal lateral, rigid centralizers are preferred over bow-spring because they maintain consistent standoff regardless of the compressive load from casing weight resting on the low side of the borehole, ensuring the concentric cement annulus quality needed for hydraulic fracture isolation between stages.

Concentric Casing Annulus Pressure Communication Identifying Cement Failure in WCSB Intermediate String

A northwest Alberta deep gas well (3,800 m TD, 15 percent H2S in the Devonian Nisku) showed zero B annulus pressure (between 177.8 mm production casing and 244.5 mm intermediate casing) at initial completion, but annual monitoring found B annulus pressure building to 4,800 kPa over 14 months after first production. The pressure buildup rate was 280 kPa per month and the gas composition from B annulus bleed-down matched the Nisku formation gas (15 percent H2S, confirmed by H2S detector at wellhead), indicating migration through the production casing cement sheath rather than through the casing itself (casing pressure test to 28 MPa showed no leaks). An acoustic cement bond log run through the production casing confirmed a 22 m unbonded interval at 3,420 to 3,442 m (identified as the Nisku perforated interval) where the CBL amplitude was above 35 mV (free pipe standard), corresponding to the primary cement job region where a gas cut was noted during cement displacement. A cement squeeze job targeting the unbonded interval restored B annulus pressure to zero, confirmed by 90-day monitoring post-squeeze with zero build-up, satisfying AER Directive 020 remediation requirements.

Related Terms

Casing is the steel tubular that forms each concentric string in a WCSB well; the sequence of progressively smaller concentric casing strings from conductor through production casing provides the mechanical and hydraulic isolation framework required by AER Directive 008 and 009 for WCSB well construction. Annulus is the concentric ring-shaped space between adjacent casing strings or between casing and borehole wall; each annulus in a WCSB multi-string well is either cemented for zonal isolation or monitored as a pressure indicator of cement integrity under AER Directive 020 sustained casing pressure requirements. Centralizer maintains concentric casing positioning in the borehole during WCSB primary cementing; bow-spring and rigid centralizers placed at 10 to 40 m intervals ensure minimum 67 percent standoff for uniform cement distribution around the casing circumference. SAGD and CSS thermal recovery wells in WCSB Athabasca and Cold Lake use concentric tubing-in-tubing completions to simultaneously inject steam through the inner string and produce heated oil through the concentric outer annulus without requiring separate wellbores for injection and production functions. Sustained casing pressure (SCP) detected in concentric well annuli indicates cement or casing integrity failure; AER Directive 020 requires annual monitoring of all accessible WCSB casing annuli and reporting of sustained pressure above threshold values in each concentric annular zone.