cementing

Cementing in oil and gas well construction is the operation that places a Portland cement slurry in the annular space between the outside of a casing string and the borehole wall, or inside a casing string for plug purposes, where the slurry is pumped as a fluid, undergoes a thickening and setting reaction driven by cement hydration chemistry, and hardens into a durable low-permeability stone that provides zonal isolation between geological formations penetrated by the wellbore, mechanical support for the casing string, corrosion protection of the casing from formation fluids, and the hydraulic seal that AER and provincial regulators require as a condition of well license and as the primary engineered barrier protecting groundwater aquifers in the overburden above WCSB hydrocarbon reservoirs. Primary cementing, the most common cementing operation in WCSB wells, pumps the slurry down the inside of the newly run casing string using a cement head at surface, through the float collar and float shoe at the bottom of the casing, and back up the annulus between the casing OD and the open borehole wall until the slurry reaches the designed top-of-cement (TOC) elevation that provides the required isolation coverage above the producing interval; the TOC in WCSB Montney and Duvernay horizontal wells must cover the entire horizontal lateral plus the curve section and extend at minimum 100 m above the top of the perforated interval as required by AER Directive 009 to isolate the stimulated zone from overlying freshwater aquifer formations in the Belly River and Horseshoe Canyon formations. The cement slurry design for WCSB primary cementing balances competing engineering requirements: the slurry must remain pumpable (below 70 Bearden units of consistency on the API 10A Consistometer) throughout the displacement period that includes mix, pump, and placement time plus a 30-minute safety factor at bottom-hole circulating temperature; the slurry must develop sufficient compressive strength (minimum 3.45 MPa per AER Directive 009) within 24 hours of placement to allow the casing pressure test and the perforation or drilling operations that follow; the slurry density must be high enough to control formation pressure in overbalanced sections and low enough to prevent losing returns to weak formations in low-fracture-gradient zones, with WCSB slurry densities ranging from 1,440 kg/m3 (extended light slurry with microspheres or foam cement) to 2,160 kg/m3 (heavyweight slurry with iron oxide or barite in overpressured sections). Understanding primary cementing execution for WCSB casing strings (surface casing, intermediate casing, production casing), slurry design parameters (water-to-cement ratio, thickener, retarder, extender, dispersant, fluid loss additive), the wiper plug sequence that separates cement from displacement fluid and provides the bump pressure signal indicating the top plug has landed on the float collar, and the cement bond log interpretation used to verify annular cement coverage gives WCSB drilling engineers, company men, and AER-licensed well operators the cementing knowledge to design, execute, and verify wellbore integrity across the full casing program of a WCSB oil or gas well.

• Primary cementing execution sequence for WCSB production casing strings: A WCSB Montney horizontal production casing cementation follows a fixed sequence: (1) circulate bottoms-up at least one full annular volume to condition the mud and confirm full returns before cementing; (2) pick up the cement head at surface and load the bottom wiper plug into the launching chamber; (3) mix and pump the pre-flush spacer (150 to 300 L of weighted surfactant spacer) ahead of the cement to water-wet the casing OD and remove OBM filter cake from the borehole wall; (4) pump the designed cement volume (typically 1.2 to 1.5 times the calculated annular volume to account for washouts confirmed by caliper log); (5) launch the top plug behind the cement and pump displacement fluid (typically fresh water or synthetic-base fluid) to drive the top plug down the casing until it bumps the float collar; (6) confirm bump pressure (typically 3.5 to 7.0 MPa above final displacement pressure) indicating the top plug has seated; (7) bleed off pressure and verify the float holds (no back-flow through the float equipment confirms cement is in place). The entire displacement sequence is monitored in real time from the cement unit data acquisition system, with cement density, pump rate, and cumulative slurry volume plotted against theoretical displacement to confirm that the designed TOC has been achieved.
• Cement slurry thickening time design for WCSB deep high-temperature cementing: Thickening time is the most critical cement slurry design parameter in WCSB deep Montney and Duvernay horizontal wells where bottom-hole circulating temperatures of 90 to 130 degrees C dramatically accelerate cement hydration relative to surface mixing conditions. The API 10A schedule 8 or 9 Consistometer test simulates the temperature and pressure profile experienced by the cement slurry during placement; the slurry must remain below 70 Bc (Bearden units of consistency) throughout the simulated placement time and transition from 70 Bc to 100 Bc (effectively set) in less than 30 minutes to prevent premature bridging in the annulus. WCSB cementing engineers use retarder concentrations of 0.3 to 1.2% BWOC (by weight of cement) for deep wells, with the retarder dosage calibrated to provide a thickening time of 3 to 5 hours at BHCT, giving a minimum 1-hour safety margin over the estimated pumping time for the WCSB production casing cementation job.
• Cement top-of-cement verification by cement bond log in WCSB horizontal wells: The cement bond log (CBL) is a wireline acoustic tool that measures the amplitude of the casing-borne compressional wave to assess cement coverage in the annulus; well-bonded cement (full circumferential coverage) attenuates the casing wave by 80 to 95% relative to free pipe amplitude, while partial bond or microannulus produces intermediate attenuation. In WCSB horizontal production casing programs, the CBL is run within 24 to 48 hours of cementing while the cement is still gaining strength; AER Directive 009 requires CBL coverage from the perforated interval to at least 100 m above the top of the shallowest perforation cluster to confirm zonal isolation. WCSB operators record the AER-required bond index (BI) from the CBL, where BI = (free pipe amplitude minus bonded pipe amplitude) / free pipe amplitude; AER requires BI greater than 0.8 over the critical isolation interval, indicating at least 80% of free pipe wave attenuation by cement, before the well is approved for perforation and stimulation.
• Remedial cementing for WCSB wells with incomplete primary cement coverage: When the CBL confirms bond index below 0.8 over the required isolation interval in a WCSB well, AER Directive 009 requires remedial cementing to restore isolation before the well is perforated. The most common WCSB remedial cementing method is the cement squeeze: a packer is set above and below the un-cemented interval, perforations are shot through the casing into the annulus (or existing micro-channels are accessed), and low-volume microcement slurry (particle size below 15 microns to penetrate micro-channels) is squeezed into the annular gap at 3.5 to 7 MPa above formation fracture pressure. After a 12-hour wait-on-cement period, a second CBL verifies that the bond index has improved to the AER minimum; if the squeeze result is insufficient, a second squeeze is performed or a bridge plug and perforation isolation program is designed to mechanically separate the un-cemented zone from the producing interval.
• Foam cement applications in WCSB low-fracture-gradient horizontal sections: Foam cement is used in WCSB horizontal well primary cementing when the formation fracture gradient in the lateral or curve section is less than 0.019 MPa/m (equivalent to approximately 1,940 kg/m3 equivalent circulating density), requiring a cement slurry lighter than standard Class G slurry at 1,898 kg/m3. Foam cement is created by injecting nitrogen gas into the cement slurry at the blending unit, producing a stable foam with density of 1,150 to 1,650 kg/m3 depending on the nitrogen injection fraction; the foam is maintained as a stable dispersion by a foaming surfactant (0.5 to 1.5% BWOC) that prevents bubble coalescence during placement. WCSB foam cement jobs require specialized equipment (nitrogen pumping unit, foam generator, high-pressure mixing manifold) and more complex quality control (foam generator pressure ratio monitoring to confirm consistent foam quality during displacement) than standard Class G cementing, but provide the only viable cement density for WCSB horizontal programs where fractured or depleted formations in the lateral would be lost to if standard-weight cement were used.

Primary Cement Job Failure Requiring Remedial Squeeze on WCSB Duvernay Horizontal

A west-central Alberta Duvernay horizontal production casing cementation was designed to place a top of cement 150 m above the top perforation cluster at 3,900 m TVD, using a 1,920 kg/m3 Class G + silica flour + retarder slurry. The displacement was completed with a confirmed bump at 5.2 MPa, indicating the top plug had seated at the float collar. CBL run 36 hours post-cement showed bond index of 0.91 in the vertical section from 0 to 1,800 m and 0.86 in the curve section from 1,800 to 2,400 m, but bond index dropped to 0.53 to 0.68 over a 220 m interval of the lateral from 4,100 to 4,320 m MD, below the AER minimum of 0.80. AER was notified and a microcement squeeze was designed targeting the 220 m interval. A 0.45 m3 microcement slurry (particle size d95 = 9 microns, density 1,780 kg/m3) was squeezed at 4.8 MPa above formation breakdown through 6 perforation clusters at 50 m spacing across the un-bonded interval. A second CBL run 48 hours post-squeeze showed bond index of 0.83 to 0.91 over the previously un-bonded zone, meeting AER minimum requirements. The well was approved for perforation 6 days after the primary job, 4 days later than planned due to the remedial operation at an additional cost of $125,000.

Related Terms

Primary cementing is the initial casing cementing operation described in this entry; the slurry is pumped down the casing, through the float shoe, and back up the annulus to the designed top-of-cement elevation, providing zonal isolation that must be verified by CBL before AER approves perforation in WCSB wells. Cement slurry is the engineered fluid mixture of Portland cement, water, and chemical additives pumped during the cementing operation; slurry design determines density, thickening time, compressive strength development, and fluid loss that collectively control whether the WCSB primary cement job achieves the AER-required bond index. Cement bond log (CBL) is the wireline acoustic tool that verifies cement coverage after primary cementing; AER Directive 009 requires CBL confirmation of bond index above 0.80 over the critical isolation interval before a WCSB well can be perforated and stimulated. Cement squeeze is the remedial cementing technique used when primary cementing fails to achieve the required bond index; microcement slurry is forced through perforations or casing micro-channels into the un-bonded annular interval to restore zonal isolation in WCSB wells. Zonal isolation is the primary engineering objective of cementing in WCSB wells; hydraulic separation of the producing formation from overlying freshwater aquifers, adjacent geological formations, and the annular space between casing strings is required by AER Directive 009 and Directive 051 as a condition of well license and operation in Alberta.