Bottom Log Interval: Identifying the Deepest Formation on the Wireline Log Suite and Its Role in Well TD Decisions
The bottom log interval (BLI) is the deepest formation interval captured on the wireline log suite of a wellbore — the section between the lowest logged depth (which may stop at the top of the next casing string, at the planned total depth, or at the formation evaluation target) and the total depth of the open-hole section — a formation evaluation boundary that determines whether the geological objective has been fully characterized, whether the casing seat has been placed below the lowermost zone of interest, and whether the logs provide sufficient data to support a well completion and TD decision without additional runs or re-logging. The bottom log interval is a concept most relevant to WCSB formation evaluation programs where multiple stacked pay zones exist above and below the primary target: a Cardium-focused well may have a geological curiosity at the base of the Viking, a Devonian reef well may wish to confirm the Beaverhill Lake is penetrated below the Leduc, and a Montney horizontal pilot may log through the Doig and into the Montney C below the target Montney A to confirm structural dip and stratigraphic thickness changes that affect the lateral landing program. In production logging, the bottom log interval has a more specific and immediately consequential meaning: it is the deepest perforated zone in the well, from which reservoir fluid contributions are evaluated to determine whether the lowermost interval is producing fluid at an economic rate or is suffering from bottom-water influx, scale plugging, or depletion that would justify a workover to shift production uphole. The bottom log interval in a pressure transient analysis (PTA) context defines the total productive interval contributing to the flowing wellbore pressure, and misidentifying the base of the perforated interval by even 2-3 m causes systematic error in the calculation of net pay thickness (h) used in the Darcy flow equation, altering the permeability-thickness (kh) product estimate and consequently the injectivity or productivity index of the well. Accurate identification of the bottom log interval requires that the wireline tool string reach its planned logging depth without mechanical obstruction — a common challenge in WCSB horizontal wells where tool conveyance by wireline gravity is impossible beyond approximately 60-70° inclination and coiled tubing or tractor conveyance must be used to reach the bottom of the horizontal section — and that the logging run speed be sufficiently controlled that the bottommost tool reaches and records data at the planned base of the logged interval before the tool string is pulled back toward surface.
Key Takeaways
- Well TD decisions based on bottom log interval penetration of the target formation: A TD decision — whether to stop drilling at the current depth or continue to a deeper target — is typically contingent on the bottom log interval confirming penetration of the geological marker that defines the planned TD. In WCSB exploration wells, the critical marker is often a dense Devonian carbonate (Ireton shale base, Beaverhill Lake top, or Leduc base) confirmed by the gamma ray spike or density response on the logging run. If the BLI log shows the marker at shallower depth than prognosed (the formation is high structurally), TD may be revised upward and casing set with reserve hole for future deepening; if the marker is below prognosis (structure is low), the operator may choose to drill ahead before logging to ensure full penetration of the pay reservoir. Confirming the BLI extends below the pay by at least one tool length (typically 3-5 m) is required to ensure the lowermost measurement point is reading formation and not borehole-bottom disturbance artifacts.
- Bottom log interval in cased-hole production logging for inflow allocation: A production log (PLT — production logging tool) measures fluid flow rate, velocity, and phase fractions at successive depth stations through the perforated production interval, typically using a spinner flowmeter, gradiomanometer, and capacitance or holdup measurement. The bottom log interval in a PLT survey is the lowest perforation or completion interval, and the measurement at this station defines the fluid contribution from the bottom zone relative to all zones above — a critical input to the well's inflow performance relationship (IPR). If the PLT shows zero velocity and high water holdup at the bottom log interval, the bottom zone is contributing mostly water and may be a candidate for mechanical isolation (bridge plug below the lowermost perforations) to improve the produced GOR or water cut of the overlying pay zones. Conversely, a high-velocity, low-holdup reading at the bottom interval confirms the bottom zone is the highest-productivity zone in the well, which might suggest increasing perforations there or installing a downhole flow control valve to balance production across all intervals.
- Wireline conveyance limitations and coiled tubing solutions for deep bottom log intervals in horizontal wells: Wireline logging tools are deployed by gravity in near-vertical wellbores, limiting logging reach in deviated wells beyond approximately 60-70° inclination without assisted conveyance. In a Montney horizontal well with a 600 m curve section and 3,000 m horizontal lateral, the wireline tool string cannot reach the toe by gravity alone — it will stop at approximately 200-400 m into the lateral due to friction on the low side of the borehole. Coiled tubing conveyance (pushing the wireline tool string from the heel to the toe using a coiled tubing work string with the logging sonde attached below) or a wireline tractor (an electro-mechanical device in the wireline string that drives itself along the borehole wall) extends the bottom log interval coverage to the planned toe depth at 5,400 m measured depth. The additional cost of tractor conveyance (approximately CAD 25,000-50,000 per run) must be weighed against the value of the log data from the toe, which is often the most critical section of the horizontal for completion design if the toe zone has different natural fracture density or rock strength than the intermediate lateral.
- Bottom log interval quality control: short-circuiting, bridging, and depth accuracy at the base of the logged section: The wireline log at the bottom of the open-hole section is the most vulnerable to data quality issues: bridging (borehole wall collapse that stops the tool string before reaching TD), tool string short-circuiting caused by fluid influx from the formation at high differential pressure, and depth uncertainty from cable stretch at maximum depth. If the tool string bridges out 100 m above planned logging TD, the bottom log interval will be missing the interval between actual tool stop depth and formation TD — which may include the target formation casing seat depth or the oil-water contact that was the objective of the logging run. Depth accuracy at the base of the logged section is also poorer than at shallower depths because cable stretch is maximum at deepest depth: a 3,000 m wireline cable under tension stretches approximately 8-15 m depending on wire diameter and tension, and this stretch must be accounted for by comparing the log depth to a casing collar locator (CCL) correlation against known collar depths when the drilled depth is precisely known from the drilling report.
- Bottom log interval in dual-completion and straddle-packed well scenarios: In WCSB wells producing from two separate formations through a dual-completion string (two separate tubing strings, each perforated in one formation), the bottom log interval corresponds to the lower formation perforation interval — typically the deeper and higher-pressure zone on which the bottom string is set. Formation evaluation logs run before completion must confirm the BLI covers the entire lower zone reservoir section with adequate resolution for net pay cutoff determination, because the perforations on the lower string cannot be adjusted after casing is cemented without a re-entry workover costing CAD 150,000-400,000 in a typical WCSB Devonian dual completion. A 2 m error in identifying the base of the lower perforation zone from the bottom log interval gamma ray can shift the net pay calculation by 1-3 m and change the computed productive interval by 10-20%, directly affecting the economic justification for the second string of tubing in the dual completion design.
Bottom Log Interval Identification in a Devonian Reef Well: Confirming Beaverhill Lake Penetration
A Swan Hills Devonian reef exploration well drills through the Leduc Formation reef body (primary target, carbonate reservoir, 3,120-3,250 m) into the underlying Beaverhill Lake Formation (secondary target, dolomitic mudstone, 3,250-3,310 m). The wireline program calls for logging from TD (3,310 m) to the intermediate casing shoe at 2,200 m. The logging run stops at 3,295 m due to a partial bridge at the base of the Beaverhill Lake — 15 m above planned TD. The bottom log interval is the Beaverhill Lake interval from 3,250 to 3,295 m. The density-neutron-resistivity logs across this 45 m BLI confirm low porosity (4-6%) and high resistivity (greater than 50 ohm-m), consistent with tight dolomitic mudstone with residual oil staining — the Beaverhill Lake is not an economic target at this location. The Leduc reef above (3,120-3,250 m) shows 14-18% porosity and resistivity 200-800 ohm-m: an economic reservoir. Casing is set at 3,260 m (10 m below the BLI stop depth), confirmed as below the Leduc base by the BLI resistivity curve, and the well is completed with perforations only in the Leduc interval. The missed 15 m of Beaverhill Lake below the bridge is considered non-critical given the tight log character across the full BLI.
Fast Facts
The Alberta Energy Regulator's well completion and casing requirements (principally AER Directives 008 and 009) effectively mandate that the bottom log interval of any logging run intended to support a casing seat depth decision must confirm penetration of the specified formation marker to at least one full tool length below the marker — a requirement that developed from several 1970s-era WCSB wells where casing was set above the planned seat formation because the logging run stopped short of the marker, leading to wellbore integrity problems during subsequent drilling and completion operations. The directive language uses "confirmed log penetration" as the criterion, which requires physical evidence from the gamma ray and/or density log that the marker formation is present below the logged section, not merely extrapolated from regional maps.
Related Terms
The wireline log suite that establishes the bottom log interval's formation properties — density, neutron, resistivity, and gamma ray — is described collectively under wireline log, which covers the sensor physics, tool conveyance methods, and combination logging strings used in WCSB open-hole formation evaluation programs from shallow Cretaceous to deep Devonian targets. The production logging tools used to characterize the bottom log interval's contribution to wellbore inflow in cased-hole production analysis are described under production logging, where spinner flowmeter, temperature gradient, and gradiomanometer tools are covered alongside the PLT interpretation workflows used to allocate zonal contributions in multi-zone WCSB completions. The casing seat selection process that uses the bottom log interval to confirm formation penetration for regulatory compliance is described under casing, where AER requirements for casing seat depth verification are outlined alongside the string design criteria for surface, intermediate, and production casing in typical WCSB well programs.