cement plug

A cement plug is a column of set Portland cement placed at a specific depth inside a wellbore (either inside the casing or in open hole below the casing shoe) for the purpose of permanently or temporarily sealing a zone, providing a sidetrack kickoff platform, or isolating a lost-circulation interval, and in Western Canada Sedimentary Basin drilling and abandonment operations, cement plugs are governed by AER Directive 020 (well abandonment) and AER Directive 009 (drilling operations) which specify minimum plug length, compressive strength, and placement verification requirements that must be met before an abandonment is certified or a wellbore reuse operation can proceed. The fundamental distinction between a cement plug and primary cementing is mechanical isolation intent: primary cementing fills the casing-to-formation annulus to achieve long-term zonal isolation for the producing life of the well, while a cement plug is a discrete column of cement placed inside the open hole or casing bore for a specific point-in-time wellbore operation, after which the plug may be drilled out (temporary plug for lost-circulation isolation or kickoff platform) or left permanently in place (abandonment plug). In WCSB abandonment programs, AER Directive 020 requires a minimum 30 m continuous cement plug across each zone requiring isolation (typically each hydrocarbon zone, each freshwater aquifer, and the surface casing seat depth), placed through tubing or drill pipe by the balanced plug method or the two-plug method, with a minimum compressive strength of 3.45 MPa at 24 hours verified by Ultrasonic Cement Analyzer test at the planned bottomhole temperature before the job. The two principal WCSB cement plug placement methods each have specific equipment requirements and depth limitations: the balanced plug method (most common for WCSB open-hole abandonment plugs) pumps cement down the drill pipe or workover tubing to the target depth, then slowly pulls the tubing out of the cement while maintaining a balanced condition (equal fluid column height inside and outside the tubing) to avoid U-tube movement that causes the cement column to shift before it sets, with pull-out rate of 5 to 10 m/min required to prevent cement from being swabbed into the tubing bore on the way out; the two-plug cement plug method (used for WCSB casing abandonment plugs where a positive bottom seal is needed below the plug) uses a wiper plug ahead of the cement and a wiper plug behind the cement to create a bounded cement column that is mechanically displaced to depth without free mixing with the borehole fluid, providing better cement column integrity at the cost of requiring a cement head and plug-launching equipment on the workover unit. Understanding cement plug design parameters (length, slurry density, thickening time excess over pull-out time, compressive strength development at plug placement temperature), the balanced plug versus two-plug method selection criteria, the verification methods (pressure test, tagging with drill pipe weight indicator, sonic plug evaluation log) that confirm plug integrity, and the AER Directive 020 depth and continuity requirements for WCSB abandonment certification gives WCSB drilling engineers, abandonment engineers, and well service crews the design and execution framework to place cement plugs that meet regulatory standards for the full range of WCSB abandonment programs from shallow Belly River gas wells to deep Devonian carbonate oil wells.

• Balanced plug method design for WCSB open-hole abandonment programs: The balanced plug method for a WCSB abandonment plug requires calculating the tubing volume (from surface to plug bottom depth), the open-hole volume over the 30 m plug interval, and the annular volume between the tubing and open-hole over the pull-out distance, then designing the cement volume such that after displacing cement to depth and before pulling tubing, the cement column is exactly balanced (equal height inside and outside the tubing string). For a WCSB Belly River well at 480 m total depth requiring a 30 m plug from 450 to 480 m, with 60.3 mm OD tubing in 152 mm open hole, the calculation requires 0.11 m3 cement for plug volume plus an overflush of 0.09 m3 to ensure the annular portion of the plug is fully filled, with pull-out rate not exceeding 8 m/min to avoid swabbing. Thickening time must exceed pull-out time plus 60 minutes safety margin at the 18 degrees C bottomhole temperature, typically achieved with a 1 to 2 weight percent CaCl2 accelerated Class G slurry for shallow WCSB abandonment programs.
• Cement plug length and placement depth requirements under AER Directive 020: AER Directive 020 specifies that each abandonment plug must be a minimum of 30 m continuous cement column placed to cover the zone being isolated, with the plug bottom at least 5 m below the zone top and the plug top at least 5 m above the zone bottom perforation or open-hole exposure. For a WCSB Horseshoe Canyon coalbed methane well with perforations at 410 to 430 m, the plug must be placed from at minimum 405 m to 435 m (30 m across the perforations with 5 m overrun each side); AER requires a pressure test at 7 MPa or a cement bond log across the plug to verify continuity after WOC. Wells in WCSB thermal EOR programs (SAGD, CSS) require temperature-resistant cement plug formulations with 35 weight percent silica flour addition above 110 degrees C to prevent strength retrogression during cyclic steam injection that could reach 250 to 300 degrees C adjacent to the abandonment plug.
• Cement plug as sidetrack kickoff platform in WCSB fish-and-sidetrack operations: When a WCSB fish (stuck pipe, lost BHA, or junk in hole) cannot be retrieved, the standard solution is to set a cement plug above the fish top, wait on cement (WOC) until the plug achieves at least 14 MPa compressive strength, then tag the plug with a mill or kick-off bit and drill a sidetrack around the fish. The plug must be hard enough not to creep or deflect under the weight on bit and side force of directional drilling tools; plug density of 1.90 g/cc (neat Class G) and minimum 14 MPa strength at the time of drill-out is the WCSB standard. Thixotropic cement additives (sodium silicate at 0.5 weight percent BWOC) are sometimes added to kickoff plugs to prevent plug fallback in near-vertical wells where the freshly placed cement could slump before setting, ensuring a stable flat plug top for the mill to engage. The plug length above the fish top must be at least 20 m to provide adequate kickoff platform for a 3 to 6 degrees/30 m build rate directional assembly.
• Cement plug verification methods accepted by AER Directive 020: AER Directive 020 accepts three methods of cement plug verification for WCSB abandonment certification: (1) pressure test of the casing above the plug to 7 MPa for 30 minutes with no visible leak, confirming the plug seals against wellbore pressure (most common for casing abandonment plugs); (2) tagging the plug with drill pipe or tubing at calculated depth with at least 20 kN overpull confirmed on the weight indicator, confirming the plug is solid at the correct depth (most common for open-hole abandonment plugs); (3) acoustic cement evaluation log across the plug confirming continuous low-amplitude response consistent with bonded cement (used for wells where pressure test or tag is impractical). For WCSB wells with abnormally high formation pressures adjacent to the abandonment plug (gas wells with shut-in casing pressure above 7 MPa), AER may require the pressure test to be conducted at a higher pressure equal to the maximum anticipated shut-in wellhead pressure plus a 15% safety factor.
• Lost-circulation cement plug design for WCSB drilling operations: When severe lost circulation is encountered during WCSB drilling (total returns loss to a thief zone, typically a vuggy Devonian carbonate or a Triassic sandstone with natural fractures), a cement plug is placed into the open-hole loss zone, allowed to set, then drilled out with the intent of forming a filter cake or bridging the fractures with set cement before resuming drilling. The lost-circulation cement plug design differs from abandonment design: compressive strength requirement is lower (3.5 MPa is sufficient to withstand drill-out), but the placement requires a low-density, low-fluid-loss slurry to avoid inducing further lost circulation during the plug placement itself. Fly ash extended cement at 1.65 to 1.75 g/cc with 50 mL/30 min fluid loss additive is the typical WCSB lost-circulation plug formulation, placed by balanced plug method with a short WOC of 4 to 6 hours before drill-out, allowing rapid resumption of drilling operations with minimal rig downtime.

Cement Plug Failure Due to Swabbing During Balanced Plug Pull-Out on a WCSB Abandonment

A WCSB Peace River area Bluesky oil well scheduled for regulatory abandonment required a 30 m open-hole cement plug from 720 to 750 m in a 152 mm open-hole section. The wellbore abandonment crew pumped a balanced plug using 50.8 mm OD coil tubing at 10 m/min pull-out rate rather than the specified 6 m/min. At 10 m/min, the upward movement of the coil tubing created a swabbing effect that pulled approximately 0.4 m3 of the freshly placed cement column upward into the coil tubing bore before the cement had developed gel strength sufficient to resist the swab pressure. The crew observed that cement returned to surface through the coil tubing during pull-out, but did not recognize the significance. When they tagged the plug with the coil tubing on the completion well service unit weight indicator, the plug topped out at 695 m rather than the designed 720 m, and the tag weight was only 12 kN, indicating the plug top was soft and partially contaminated. AER required a re-squeeze of the interval, adding 18 hours of rig time and $64,000 to the abandonment cost. The operator revised their coil tubing balanced plug procedure to specify a maximum pull-out rate of 6 m/min and added a cement volume reconciliation check (confirm that total cement returns to surface during pull-out do not exceed 10% of the designed plug volume).

Related Terms

Balanced plug method is the standard WCSB technique for placing open-hole abandonment cement plugs, relying on equal fluid column height inside and outside the tubing string at the moment of pull-out to prevent U-tube movement that shifts the freshly placed cement column before it gels, with maximum pull-out rate of 6 to 8 m/min critical to preventing swabbing of cement into the tubing bore. Well abandonment in Alberta is governed by AER Directive 020, which specifies the minimum cement plug length (30 m), strength (3.45 MPa), and verification methods (pressure test, tag, acoustic log) for each isolation zone in WCSB wells from surface to total depth, with plug design and placement records submitted to the AER as part of the abandonment certification package. Cement head is the surface equipment used for the two-plug cement plug method, containing the wiper plugs that bound the cement column ahead and behind for placement into a cased wellbore; the two-plug method provides better cement column integrity than the balanced plug method but requires cementing unit equipment and a cement head sized to the workover tubing or casing string. Sidetrack drilling from a cement kickoff plug requires the plug to achieve minimum 14 MPa compressive strength before drill-out, with the plug length above the fish top of at least 20 m to provide a stable platform for the kick-off bit or mill to initiate the directional departure from the original WCSB wellbore. Lost circulation during WCSB drilling operations in vuggy Devonian carbonates or fractured Triassic sandstones is the primary operational trigger for drilling cement plug placement, using fly ash extended slurries at 1.65 to 1.75 g/cc with low fluid loss to bridge the fracture or vug network without inducing further circulation loss during the plug placement operation itself.