downstroke, Oil & Gas Glossary

The downstroke is the portion of a downhole pump's cycle during which the rods are moving downward and the pump barrel is being filled with fluid below the plunger. It is one half of the reciprocating action of a sucker rod pump, the most common form of artificial lift on Western Canadian Sedimentary Basin oil wells, and it is best understood as the mirror image of the upstroke. A rod pump has two ball-and-seat check valves. The standing valve sits fixed at the bottom of the pump barrel and controls fluid entry from the wellbore; the traveling valve rides at the top of the plunger and moves up and down with the rod string. During the upstroke, the plunger is lifted, the traveling valve closes, and the column of fluid above the plunger is carried toward surface while the standing valve opens and draws a fresh charge of well fluid into the barrel. The downstroke reverses this. As the rods and plunger travel down, the fluid trapped in the barrel below the plunger is compressed, which forces the standing valve to snap shut so nothing is pushed back into the formation, and simultaneously forces the traveling valve to open so the plunger can descend through the fluid that now transfers above it. No fluid is lifted on the downstroke itself; instead the plunger repositions itself at the bottom of the barrel through the fluid charge, setting up the next upstroke that will actually raise that fluid. Across a full cycle, then, the standing valve and traveling valve alternate: on the upstroke the traveling valve is closed and the standing valve open, and on the downstroke the traveling valve is open and the standing valve closed. The sucker rod pump is driven from surface by a pumpjack, whose walking beam converts the rotation of a prime mover into the up-and-down motion transmitted down the rod string to the plunger. The downstroke carries its own engineering concerns. Because the rods are in compression-prone conditions as they push the plunger down through fluid, slow or incomplete downstroke behaviour signals problems: gas trapped in the barrel that will not compress properly causes gas interference and incomplete pump fillage; a heavily paraffined or sand-cut pump adds drag; and rod buckling or a sticking traveling valve shows up in the shape of the downhole pump card, the load-versus-position plot that analysts read to diagnose pump condition. On WCSB wells producing from the Cardium, Viking, Mannville and heavy oil Clearwater and McMurray intervals, understanding what happens on the downstroke, valve action, fluid transfer and fillage, is central to keeping a rod-pumped well running efficiently and to interpreting the surface and downhole dynamometer data used to optimize it.

Key Takeaways

Rods descend, barrel fills below plunger: The downstroke is the half of the pump cycle when the rod string and plunger move downward and fluid transfers into the space above the plunger. It is the complement of the upstroke, and together the two make up one full reciprocating stroke of a sucker rod pump.
Standing valve shuts, traveling valve opens: As the plunger descends it compresses the fluid below it, snapping the standing valve closed so nothing is forced back into the formation, while the pressure opens the traveling valve so the plunger can pass down through the fluid. This is the exact opposite valve state from the upstroke, when the traveling valve is closed and the standing valve open.
No lift on the downstroke itself: Fluid is actually raised toward surface only on the upstroke; the downstroke repositions the plunger and transfers the barrel charge above it to set up the next lifting stroke. Recognizing that lift happens on one stroke and fluid transfer on the other is the key to reading a dynamometer card correctly.
Downstroke shape reveals pump problems: Gas that will not compress, paraffin drag, sand cutting or a sticking traveling valve all distort the downstroke portion of the pump card. Incomplete fillage from gas interference is one of the most common WCSB rod-pump issues and shows up as a rounded, foreshortened downstroke on the load-position plot.
Driven by the pumpjack beam: The up-and-down motion comes from a surface pumpjack whose walking beam converts a prime mover's rotation into reciprocating rod motion. Stroke length and speed, set at surface in strokes per minute and stroke inches, govern how much fluid each downstroke-upstroke cycle can move against the well's inflow.

Valve Action Through One Full Pump Cycle

The elegance of the rod pump is in its two automatic check valves working in opposition. On the upstroke the plunger rises, the traveling valve closes to carry the fluid column upward, and the falling pressure below the plunger lets the standing valve open and pull a new charge into the barrel. On the downstroke the plunger falls, the rising pressure below it slams the standing valve shut so the charge cannot escape back to the formation, and that same pressure lifts the traveling valve so the plunger descends through the fluid, transferring the charge to the space above it. Each valve is a simple ball on a seat, opened and closed by nothing more than the pressure differential the stroke creates, which is why the pump can run for months unattended when conditions are clean.

Gas Interference and Incomplete Fillage on the Downstroke

The most common thing that goes wrong on the downstroke in WCSB wells is gas interference. If free gas enters the barrel with the liquid, it compresses on the downstroke instead of the standing valve seating cleanly, so the traveling valve opens late and the pump does not fully fill. The result is pump fillage below 100 percent, wasted rod cycles, and a rounded, incomplete downstroke on the pump card. Operators counter it with gas separators or gas anchors below the pump, pump-off controllers that slow or stop the unit when fillage drops, and stroke adjustments. Reading the downstroke on a dynamometer card is how a well analyst distinguishes gas interference from a worn traveling valve or a sanded pump.

Fast Facts

The counterweights on a pumpjack are sized specifically to balance the work done on the two strokes, because the load on the upstroke, when the pump lifts a fluid column plus the rod weight, is far heavier than on the downstroke, when the rods fall largely under their own weight. Well-tuned counterbalance lets the prime mover deliver nearly even torque through the whole cycle, cutting energy use and gearbox wear, and a beam pump that is out of balance shows it as a lopsided current draw between its upstroke and downstroke.

The downstroke is one half of the cycle of a sucker rod pump, the reciprocating downhole pump that lifts oil on the great majority of WCSB pumping wells, and it is the direct counterpart of the upstroke, on which the fluid column is actually raised. The reciprocating motion is supplied by a surface pumpjack, whose beam and counterweights drive the rod string. The behaviour of the valves and fluid on the downstroke is read from a dynamometer card, the load-versus-position plot used to diagnose pump condition and set optimum stroke speed.

Real-World WCSB Scenario: Gas Interference on a Viking Rod Pump

An operator pumping a Viking oil well near Provost notices production has fallen off while the pumpjack keeps stroking at 6 strokes per minute. A surface dynamometer survey shows a pump card with a badly rounded, shortened downstroke and calculated fillage of only about 55 percent, the signature of gas interference: free gas is compressing in the barrel on the downstroke instead of the standing valve seating and the pump filling with liquid. The wasted strokes are burning power and wearing rods for little oil, and a rod-string workover to change the pump would run roughly CAD 60,000 to CAD 90,000 if the problem were mechanical.

Rather than pull the well, the operator installs a downhole gas anchor to separate free gas below the pump intake and fits a pump-off controller that idles the unit when fillage drops. Fillage climbs back toward 90 percent, the downstroke on the card sharpens, and per-cycle liquid recovery improves. The low-cost fix restores production and defers the expensive workover, showing why correctly reading the downstroke is worth the effort.