Heavy Oil Rod Pumping: Problems and Fixes
Short answer: heavy oil fights a rod pump on the downstroke. Viscous drag slows the rods so much that they can't keep up with the pumping unit (rod float), it pushes the bottom of the string into compression, and it cuts how much fluid each stroke moves. The fixes are slower, matched strokes, a string designed to stay in tension, and control that reacts to the well rather than a fixed setting.
What makes heavy oil hard to pump?
Heavy oil is crude with high viscosity and high specific gravity: typically above 10 cP, and below 22.3° API by the API's classification. That viscosity drags on the rods and plunger on every stroke. 12
On a light-oil well the rods fall freely under their own weight on the downstroke. In heavy oil they can't. A patent on pumping viscous oil describes the problem directly: "there is a viscous drag on the sucker rod string which slows down its free fall by gravity on the downstroke." 2 The same drag works against the upstroke too: it slows the plunger "because of stretch in the rods," reduces the flow through the tubing and increases the power needed to lift the oil and the rods. Where the oil is highly viscous, the patent notes, "the depth of operation of the pump is limited." 2
Heavy-oil rod pumping is everyday work in Alberta and Saskatchewan, California, Venezuela and Colombia, and Oman, and the physics in the barrel is the same in all of them.
Rod float: when the rods can't keep up
Rod float happens when viscous drag makes the rod string fall more slowly than the pumping unit's downstroke, so the polished rod separates from the carrier bar. When they meet again, the shock loads the rods, the polished rod and the gearbox. 345
The pumping unit moves the carrier bar down at its own pace. If the rods can't follow, they hang behind, and the polished rod lifts off the carrier bar. One patent defines the term plainly: "Polished rod/carrier bar separation may be referred to as rod float," and notes it "may overload the gearbox and/or shock load the pumping unit." 4 Another attributes rod float "largely" to "excessive viscous and rod drag friction forces," and describes the mechanical stress on the rods when the carrier bar picks the polished rod up again. 3
Stop-start operation doesn't help either: a patent on variable-speed control notes that running a pump on a fixed cycle can let rod float occur and can result in excessive wear, or even catastrophic failure, of the pumping unit. 5
How to spot it: rod float shows in the rod load, which drops away on the downstroke as the rods separate, and in the motor, which works harder because it lifts the counterweights without the rod load against them. One patent notes that rod float "may be detected by higher motor torques." 4
Compression and buckling at the bottom of the string
The same viscous drag that slows the rods also resists the plunger's fall, which pushes the bottom of the rod string into compression on the downstroke. Past a critical load, the rods buckle into the tubing wall.
Heavy oil adds to the downstroke compression from pump friction and fluid flow through the traveling valve. Slender rods can't carry much compression before they bow into a helix, and every stroke then adds bending stress and rubs couplings against the tubing. That's why heavy-oil wells often fail in the bottom of the string. See the sucker rod buckling guide and sinker bars and rod guides for the options. On the dynamometer card, a downstroke load that runs below zero is the signature.
On the SRABS site, a modelled Alberta heavy-oil scenario, "The 900 cP drag," shows this pattern: "Viscous drag on the downstroke buckles the bottom string." See cases; the scenario is modelled, not a field record.
Sand and solids
Many heavy-oil wells also produce sand and solids, and they wear the pump. Solids entering the clearance between the plunger and the barrel are a primary cause of pump failure in sandy wells. 6
A short-course case study describes a well that needed a pump-replacement workover every 90 days on average, with sand and solids entering the barrel-plunger interface as a primary cause. 6 The fixes are pump designs and clearances chosen for solids, and running the well steadily rather than stopping and starting it.
Thermal wells: cyclic steam and SAGD
On thermal wells, conditions don't stay put. Cyclic steam stimulation and SAGD alternate between injecting steam and pumping, and the fluid's temperature, and with it its viscosity, changes over time. A pumping speed that suits the well one week is wrong the next. 3
A rod-float patent from a pumping-unit maker makes exactly this point: operations such as cyclic steam injection and SAGD keep changing well conditions, and wellhead temperatures shift over time, so a fixed speed setting stops working. 3 High temperatures also matter for the pump and rod materials themselves; specify them with the supplier for the well's temperature range.
How to pump heavy oil: the fixes
Pump heavy oil slowly and smoothly, keep the rod string in tension, and control the well on what the rods and motor are actually doing rather than a fixed setting. 35
| Problem | What helps | Limits |
|---|---|---|
| Rod float | Slower downstroke and fewer strokes per minute; a variable-speed drive 35 | A fixed slow setting gives up production and goes stale as the well changes 3 |
| Changing viscosity (thermal wells) | Speed control that reacts to rod load or motor torque 34 | Needs reliable load or torque data |
| Low fillage | Match pump rate to inflow instead of chasing it | Requires knowing the fluid level; see how to shoot a fluid level |
| Compression and buckling | String design: sinker bars, guides; or a pump that keeps the string in tension | Weight and wear trade-offs; see sinker bars |
| Sand and solids | Pump designs and clearances chosen for solids; steady operation 6 | Specify with the pump supplier |
Speed control is the common thread. The older practice was to install a variable-frequency drive and slow the motor by hand, which works until the well changes. 3 Patents since have described controllers that vary pumping speed automatically, including to adapt to changes in oil viscosity 5, and that detect rod float from rod load and adjust motor speed with minimum loss of production. 3 For the categories of control and their trade-offs, see pump-off controller vs. AI well control.
Where SRABS fits
Heavy oil is one of the problems SRABS is built around. The solutions page puts it in one line: "Heavy oil pumps starving. Fillage is low. SPM keeps climbing. Rods are paying for it." And the fix, in the site's words: "The anti-buckling pump takes viscous drag off the rods, and Brain slows the stroke to match inflow instead of chasing it."
For the full mechanism, see how it works. To estimate what fewer failures would mean on your fleet, run the ROI calculator. Its default 100-well scenario is modelled, not a field result.
The bottom line
In heavy oil, the downstroke is the problem. Viscous drag makes the rods float, pushes the bottom of the string into compression and cuts what each stroke delivers. Slow, matched strokes, a string kept in tension and control that follows the well are what keep heavy-oil wells running.
If your heavy-oil wells are parting rods at the bottom or fighting rod float, test the fix on one well.
FAQ
Why is heavy oil hard to pump with a rod pump?
Viscous drag slows the rods' fall on the downstroke, stretches them on the upstroke, cuts the flow per stroke and raises the power needed. It also pushes the bottom of the rod string into compression.
What is rod float?
Rod float is when the rod string falls more slowly than the pumping unit's downstroke, so the polished rod separates from the carrier bar. When they meet again, the shock loads the rods, polished rod and gearbox.
How do you stop rod float?
Slow the downstroke so the rods can keep up, usually with a variable-speed drive. Because heavy-oil wells change, especially thermal wells, speed control that reacts to rod load or motor torque works better than a fixed slow setting.
Does heavy oil cause rod buckling?
It can. The viscous drag that resists the plunger's fall pushes the bottom of the rod string into compression on the downstroke, and slender rods buckle into the tubing wall.
What is heavy oil?
Crude oil with high viscosity, typically above 10 cP, and a gravity below 22.3° API by the API's classification.
How do thermal wells affect rod pumping?
Cyclic steam and SAGD keep changing temperature and viscosity, so a pumping speed that suits the well one week may be wrong the next. Control that adjusts to the rods' actual behaviour copes better than a fixed setting.
Sources
- SLB Energy Glossary, "heavy oil." https://glossary.slb.com/en/terms/h/heavy_oil
- US Patent 3,986,552, "Pumping system for high viscosity oil" (Thick Oil Extractor Service, 1976). https://patents.google.com/patent/US3986552
- US Patent 7,547,196, "Method for mitigating rod float in rod pumped wells" (Lufkin Industries, 2009). https://patents.google.com/patent/US7547196
- US Patent 10,094,371, "Methods and apparatus to determine operating parameters of a pumping unit for use with wells" (Bristol Inc., 2018). https://patents.google.com/patent/US10094371
- US Patent 5,251,696, "Method and apparatus for variable speed control of oil well pumping units" (1993). https://patents.google.com/patent/US5251696
- Carson, R., Hudson, K., Narasimhan, R., "Specialty Rod Pump Reduces Workover Frequency and Associated OPEX Costs in Austin Chalk Well," Southwestern Petroleum Short Course, paper 2024041. https://www.swpshortcourse.org/paper/2024041-specialty-rod-pump-reduces-workover-frequency-and-associated-opex-costs-austin-chalk