Flow-Concentrating: Diverter and Basket Spinner Flowmeters, Low-Rate Production Logging, and WCSB Multiphase Allocation
Flow-concentrating describes a class of production logging spinner flowmeters in which most or all of the fluid moving in a wellbore is mechanically diverted so that it must pass directly over the spinner, rather than allowing fluid to slip past the impeller through the annular gap between the tool and the casing wall. A conventional continuous or fullbore spinner measures velocity by counting impeller rotations, but it only samples the fraction of flow that happens to strike the blades; in low-rate, large-diameter, or highly deviated wells a great deal of fluid bypasses the spinner entirely, and the tool reads below its threshold velocity and registers little or nothing. The flow-concentrating design solves this by forcing the total stream through a controlled aperture. Two principal hardware families exist. The basket flowmeter, also called a petal basket, uses a set of overlapping metal vanes or petals that fold closed while running in hole and open against the casing at the measurement station, funnelling fluid over the spinner; the petals do not seal completely, so a portion of flow still bypasses, but enough is captured to lift the reading above threshold. The diverter flowmeter, the more capable modern design, deploys a collapsible fabric cage that is run through the tubing collapsed, then opened below the tubing until an inflatable ring seals against the casing wall, forcing essentially the entire production stream through the diverter and over the impeller. By concentrating the flow, these tools dramatically improve the low-end velocity response and make it possible to quantify rates that a bare spinner cannot resolve. In Western Canadian Sedimentary Basin (WCSB) wells this matters most in mature, watered-out, or low-deliverability completions and in deviated wellbores where gravity segregation of oil, water, and gas makes the spinner-only velocity profile ambiguous. A diverter pass run together with fluid-density and water-holdup sensors lets the analyst allocate which perforations are contributing oil, which are making water, and at what rate, the core deliverable of a production logging tool (PLT) survey. The trade-off is mechanical: a diverter or basket is more complex, slower to operate because it requires stationary stations rather than continuous logging, and the inflatable seal or petals can fail to fully deploy in rough or scaled casing, so flow-concentrating runs are reserved for the low-rate and diagnostic situations where a conventional spinner has already proven inadequate.
Key Takeaways
- Diverting beats bypass at low rates: A bare spinner only samples the flow that strikes its blades, so in low-rate or large-diameter casing much of the stream slips past through the annular gap and the tool reads below threshold. A flow-concentrating tool forces most or all fluid over the spinner, lifting the velocity signal above its measurable minimum and turning an unreadable pass into a quantifiable one.
- Basket versus diverter hardware: The petal basket uses overlapping metal vanes that open against the casing but do not fully seal, so some bypass remains. The diverter flowmeter uses a collapsible fabric cage with an inflatable ring that seals against the casing wall, forcing essentially the whole stream over the impeller. The diverter gives better low-rate accuracy; the basket is the older, simpler design.
- Run stationary, not continuous: Because the diverter must be deployed and sealed at a fixed depth, flow-concentrating measurements are taken at stationary stations across and between perforation intervals, not as a continuous moving log. This makes the survey slower and more depth-by-depth deliberate than a standard continuous spinner pass, which is part of why it is reserved for diagnostic work.
- Pairs with holdup and density sensors: Concentrated velocity alone is not enough in multiphase WCSB wells. The diverter spinner is run alongside fluid-density, water-holdup, and temperature sensors so the analyst can convert velocity into phase-split rates, identifying which perforations make oil, which make water, and at what volume, the central output of a PLT allocation survey.
- Reserved for low-rate diagnostics: The added mechanical complexity, slower stationary operation, and risk that petals or the inflatable seal fail to deploy in scaled or rough casing mean flow-concentrating tools are not the default. They are deployed in mature, watered-out, or deviated wells where a conventional fullbore spinner has already failed to resolve the contributing intervals.
Why a Conventional Spinner Fails at Low Rate
A continuous spinner has a threshold velocity below which the impeller will not turn against bearing friction, often in the range of a few metres per minute of apparent fluid velocity. In a 139.7 mm casing string carrying a modest 10 m3 per day of fluid, the bulk velocity is so low that a fullbore spinner sits near or below threshold and the contribution profile becomes noise. By diverting the entire stream through a small aperture, a diverter flowmeter multiplies the local velocity across the impeller several-fold, pushing the reading well into the linear, calibratable range. This is the entire reason flow-concentrating tools exist: they trade operational speed for the ability to measure flows that would otherwise be invisible to spinner technology.
Multiphase Allocation in Deviated WCSB Wells
In deviated and near-horizontal WCSB completions, oil, water, and gas segregate by density along the low side and high side of the wellbore, and a centered spinner samples an unrepresentative slice of that segregated flow. A diverter that seals the casing and forces the mixed stream through one aperture homogenises the measurement point, giving a velocity tied to the true total rate. Combined with a capacitance or optical water-holdup sensor and a gradiomanometer density reading, the analyst resolves the per-interval oil and water rates needed to decide whether a watering-out Cardium or Viking interval should be isolated with a straddle packer or a cement squeeze.
Fast Facts
The petal basket flowmeter dominated low-rate production logging into the late 1980s before the inflatable diverter and fullbore spinner designs largely displaced it, mainly because the petals never sealed completely and the residual bypass left a stubborn uncertainty in the calculated rate. The modern inflatable diverter can raise effective spinner velocity by a factor of five or more over a bare tool in large casing, which is what allows a PLT to allocate flows of only a few cubic metres per day across individual perforation clusters that a continuous spinner reads as a flat, uninformative line.
Related Terms
Flow-concentrating is one design philosophy within the broader family of the Spinner Flowmeter, the rotating-impeller velocity sensor at the heart of most flow profiling. The concentrated velocity is only useful as part of a full Production Logging Tool string, where it combines with density and holdup channels to allocate flow per zone. Because these surveys so often diagnose unwanted water entry, they connect directly to Water Cut, the volumetric fraction of water in the produced stream that the diverter survey is frequently run to localise.
Real-World WCSB Scenario: Localising Water Entry in a Cardium Well Near Pembina
A mature Cardium oil well near Pembina, Alberta watered out from roughly 15 percent to 78 percent water cut over two years, but the operator could not tell from surface data which of five perforated intervals had broken through. A continuous spinner pass produced a flat, near-threshold response because total liquid rate had fallen to about 12 m3 per day in 139.7 mm casing. A diverter flowmeter run on a stationary-station survey, with water-holdup and density sensors, cost roughly CAD 110,000 including coiled-tubing conveyance and pinpointed a single basal interval producing nearly all of the water.
The operator set a through-tubing straddle packer to isolate that interval, cutting water cut back to about 40 percent and restoring economic oil rate. The flow-concentrating survey paid for itself within two months by deferring an otherwise imminent CAD 900,000 workover and abandonment decision.