cement bond log

A cement bond log (CBL) is an acoustic wireline logging tool run after primary cementing in a Western Canada Sedimentary Basin well to evaluate the quality and completeness of the cement sheath in the annular space between the casing and the borehole wall, measuring the amplitude of the acoustic casing arrival and the waveform character of the formation acoustic signal to assess what percentage of the casing circumference is bonded to solid set cement versus surrounded by free fluid or micro-annulus, providing the primary post-cementing quality assurance data required by AER Directive 009 and BCOGC Drilling and Production Regulation to verify that primary cementing has achieved the zonal isolation between hydrocarbon-bearing formations and shallow freshwater aquifers mandated by Alberta and British Columbia well licensing conditions. The acoustic measurement principle of the CBL is the energy transfer concept: when cement is bonded to the casing outer wall and to the formation, acoustic energy from the transmitter travels through the steel casing, into the cement, and into the formation, dissipating rapidly so that the casing arrival amplitude at the receiver (located 0.9 m or 1 ft from the transmitter in standard CBL tools) is low (typically 0 to 5 millivolts in a fully bonded cemented interval); when the annulus contains mud or free water instead of cement, acoustic energy is confined in the casing, reflects back and forth within the steel, and arrives at the receiver with high amplitude (25 to 50 millivolts for uncemented casing in typical WCSB conditions); partially bonded cement produces intermediate amplitude values that are converted to a bond index (BI = 1 minus the ratio of partially-bonded amplitude to free-pipe amplitude, ranging from 0 for no bond to 1 for full bond) that quantifies the fraction of circumferential bonding at each depth. The variable density log (VDL), recorded simultaneously with the CBL amplitude curve, displays the full acoustic waveform as a variable-density image in which dark bands represent high-amplitude wave cycles and light bands represent low-amplitude cycles; in a well-bonded interval, the VDL shows the formation compressional and shear wave arrivals as coherent stripes across the image, confirming energy transfer from the casing through the cement into the formation; in an uncemented interval, the VDL shows only the ringing casing arrival as a series of parallel stripes with no formation signal, confirming fluid-filled annulus regardless of the casing pipe geometry. In WCSB CBL interpretation, the critical distinction is between a genuine cement channel (a continuous annular path of free fluid connecting two formations, which constitutes a zonal isolation failure requiring squeeze cementing remediation) and a micro-annulus (a thin gap between the cement and the casing OD caused by pressure or temperature differential between the setting and measurement conditions, which does not represent a fluid communication path and can be confirmed by re-logging with applied casing pressure to close the gap and restore bonding signal). Understanding CBL operating principles, the bond index calculation and its interpretation thresholds, the VDL waveform analysis for formation signal confirmation, the micro-annulus versus channel distinction, the AER Directive 009 requirements for CBL logging and remediation criteria, and the limitations of CBL in heavyweight mud, gas-cut annuli, and eccentered casing conditions gives WCSB drilling engineers, well integrity specialists, cementing service engineers, and AER compliance managers the acoustic log interpretation and regulatory framework to verify primary cementing quality, identify isolation failures, and design squeeze remediation programs on every WCSB well where CBL is required or where production operations reveal potential zonal isolation concerns.

• CBL bond index calculation and WCSB isolation acceptance thresholds: The bond index (BI) is calculated at each depth as: BI = 1 minus (CBL amplitude at depth / free-pipe amplitude), where the free-pipe amplitude is determined from a section of uncemented casing above the cement top or from the manufacturer's free-pipe amplitude chart for the specific casing weight and grade. WCSB operators and AER inspection use BI thresholds of: BI above 0.8 as adequate bonding; BI 0.5 to 0.8 as marginal bonding requiring engineering review; BI below 0.5 as poor bonding requiring squeeze consideration. However, the BI threshold for regulatory compliance is formation-specific: AER Directive 009 requires a minimum average BI of 0.8 over 3 m intervals opposite all flow zones and a minimum BI of 0.6 for any single depth, with any continuous channel (BI below 0.2 over more than 5 m) triggering a mandatory squeeze remediation assessment.
• Variable density log waveform interpretation for WCSB formation signal confirmation: The VDL is the most reliable CBL quality indicator because it cannot be faked by cement that is bonded only to the casing (casing bond without formation bond, sometimes called free cement): in a casing-bonded-only interval, the CBL amplitude may be low (suggesting bonding) but the VDL shows no coherent formation arrival because the cement is not bonded to the borehole wall and does not transmit acoustic energy to the formation. Genuine zonal isolation requires both low CBL amplitude (casing bond confirmed) and visible coherent formation compressional arrival on the VDL (formation bond confirmed). WCSB petrophysicists trained in CBL interpretation use the VDL formation arrival time (which matches the formation sonic compressional traveltime log from the same well) as the formation bond confirmation marker before accepting a CBL section as fully isolating.
• Micro-annulus identification and confirmation in WCSB CBL programs: A micro-annulus is a thin circumferential gap (typically 0.05 to 0.5 mm) between the casing outer surface and the cement inner face, created when cement sets at elevated downhole temperature and pressure and then cools and contracts on well completion, or when casing is pressured to a test pressure above the value at which cement set and then depressured. A micro-annulus produces high CBL amplitude (mimicking an uncemented annulus) but differs from a genuine channel in that: (1) it does not extend continuously to the surface or between formations (channels are continuous; micro-annuli are discontinuous); (2) pressurizing the casing to 3 to 7 MPa above the CBL logging pressure expands the casing, closes the gap, and restores low CBL amplitude and coherent VDL formation signal; (3) it is uniformly distributed around the casing circumference (visible as a systematic amplitude increase over all azimuths on sector CBL tools), whereas channels are localized to one azimuthal sector. WCSB operators performing CBL on production casing routinely log at 3.5 MPa applied casing pressure to pre-close any micro-annulus and obtain a true bond index measurement.
• CBL limitations in WCSB heavyweight mud and gas-cut annulus conditions: CBL accuracy is affected by the acoustic properties of the fluid inside the casing: heavyweight OBM with density above 1.7 g/cc attenuates the transmitted acoustic pulse and reduces the free-pipe amplitude baseline, causing the bond index to be overestimated (a fluid-filled annulus may read as partially bonded); gas-cut mud inside the casing with greater than 2% free gas volume significantly increases acoustic attenuation, reducing the transmitted signal and producing falsely low CBL amplitude that mimics bonded cement. WCSB CBL programs on wells drilled with OBM require a correction for mud acoustic impedance using the casing-fluid acoustic coupling factor; wells with significant gas shows require a pre-CBL kill pill to displace gas-cut mud with dead mud before logging to ensure valid free-pipe amplitude measurements. These corrections are documented in the CBL job report submitted to the AER with the post-cementing compliance package.
• Sector CBL and ultrasonic imaging tools for WCSB advanced cement evaluation: Standard single-receiver CBL tools provide a circumferential average bond index that cannot distinguish between full-circumference poor bonding (cement channel) and partial-circumference channeling (one quadrant unbonded). Multi-arm sector bond log tools (4 to 8 azimuthal sectors) identify the circumferential location of channels, distinguishing a 25% circumferential channel from 100% partial bond. Ultrasonic pulse-echo tools (Schlumberger USIT, Halliburton CAST-V) measure acoustic impedance at 0.9 to 2 degree azimuthal resolution around the full casing circumference at 5 to 10 cm depth resolution, generating a map of cement, free fluid, and gas-filled annular content that is the definitive cement evaluation tool for WCSB wells with zonal isolation risk adjacent to shallow freshwater aquifers or WCSB SAGD steam chambers where channeling could cause steam breakthrough to surface.

CBL Channel Identification Leading to Squeeze Remediation on a WCSB Cardium Well

A west-central Alberta Cardium production well showed sustained casing pressure of 520 kPa on the intermediate casing annulus within 6 weeks of completing primary cementing and perforating the Cardium sand at 2,140 m. Gas composition confirmed Cardium gas source, indicating communication from the perforated zone to the intermediate casing annulus. A CBL/VDL run identified a continuous low-bond-index interval (BI averaging 0.18) from 2,080 to 2,145 m with no coherent formation arrival on the VDL, confirming a fluid channel in the annulus rather than a micro-annulus (casing pressure test at 4 MPa during re-log did not improve BI, ruling out micro-annulus). A sector CBL tool re-run confirmed the channel was localized to a 75-degree azimuthal sector on the high side of the deviated wellbore (8 degrees inclination), consistent with mud channeling on the casing high side during primary cementing displacement. A remedial squeeze job using 0.8 m3 of microcement (d50 less than 6 microns) pumped through perforations at 2,095 to 2,100 m at 210 kPa squeeze pressure sealed the channel; the post-squeeze CBL showed BI above 0.85 across the previously channeled interval and the sustained casing pressure dropped to zero within 15 days of the squeeze job.

Related Terms

Oil well cement is the material whose placement quality the CBL evaluates; the CBL result reflects the combined performance of the cement slurry design, the displacement execution, and the wellbore conditions during cementing, making CBL interpretation the definitive quality indicator for the entire primary cementing program on a WCSB well. Squeeze cementing is the remedial operation triggered by CBL identification of cement channels or inadequate bond index opposite flow zones; microcement (d50 less than 6 microns) is the preferred squeeze material for WCSB channel remediation because its ultra-fine particle size penetrates micro-channel widths of 0.1 to 0.5 mm that cannot be entered by conventional Class G cement. Zonal isolation is the primary engineering objective that the CBL verifies, confirming that the cement sheath provides a hydraulic seal preventing fluid communication between the WCSB hydrocarbon-bearing formations being produced and the shallow freshwater aquifers and surface above the producing zones that must be protected under AER Directive 009 and Alberta Water Act requirements. Micro-annulus is the thin casing-to-cement gap that produces false poor-bond CBL readings but does not represent a zonal isolation failure; distinguishing micro-annulus from genuine channel by pressurized re-logging at 3.5 MPa casing pressure prevents unnecessary and costly squeeze remediation on WCSB wells where the cement sheath is actually intact but has contracted slightly from the downhole conditions at which it set. Primary cementing quality is the upstream determinant of CBL results; mud removal efficiency during displacement, cement slurry fluid loss control, and anti-gas-migration additive performance during the transition state are the three controllable factors that determine whether the WCSB CBL will show adequate bond index and coherent VDL formation arrivals or require remedial squeeze cementing at additional cost and rig time.