How to Spot Gas Interference on a Dynamometer Card
Gas interference robs a rod pump of displacement by filling part of the barrel with compressible gas instead of liquid, and severe cases can gas-lock the pump entirely. On the dynamometer card it produces an incomplete-fill signature that looks like fluid pound but with a distinctive rounded corner. This page is a reading guide for identifying gas interference on a card, distinguishing it from liquid pound, and confirming it against the well's pressures. Use it when a well is underproducing and you need to know whether gas, rather than low inflow, is the culprit.
Spot Gas Interference on a Card in one line: To spot gas interference on a dynamometer card, look for load that holds high on the downstroke and then releases gradually in a rounded, curved transition rather than dropping in a sharp step. That soft curve is compressed gas in the barrel expanding smoothly as the plunger descends, instead of a liquid surface stopping it hard. The rounded release distinguishes gas from fluid pound, and a card that flattens into a thin loop with little load change signals severe gas locking.
Find the Rounded Load Release
The defining feature of a gas-interference card is a soft, rounded downstroke transition. When free gas enters the pump and is trapped in the barrel, the plunger compresses it on the downstroke and the gas expands on the way down, so the load changes gradually rather than abruptly. On the card, load holds high into the downstroke and then eases off in a curve, without the sharp step that a liquid surface produces. That rounded corner is the signature to look for.
As gas interference worsens, the card loses load range and collapses toward a thin, low loop. In severe gas locking the pump moves gas back and forth without transferring meaningful fluid load, so the card shrinks into a narrow band with little difference between upstroke and downstroke load. A card that has flattened this way on a well that used to make a full parallelogram is a strong indicator that gas has taken over the barrel, a condition described as gas locking when it stalls the pump entirely.
Compare against the well's healthy card. Placing a full-pump card next to the suspect card makes the rounded, shrunken release obvious against the clean, sharp corners of a full card. A stored card history is what lets you see the transition round off and the loop shrink over time as gas increases, so treat the card as a trended diagnostic rather than a one-off snapshot.
Distinguish Gas From Fluid Pound
Gas interference and fluid pound both stem from an incompletely filled pump, and their cards differ only in the downstroke transition. Gas gives a rounded, gradual release because compressible gas cushions the plunger. Fluid pound gives a sharp, sudden step down because the plunger strikes a solid liquid surface. Read the corner: curved means gas, stepped means liquid pound. This one distinction drives which remedy you pursue.
The remedies diverge sharply, which is why the reading matters. Fluid pound is about the pump outrunning liquid inflow, so slowing or cycling the pump lets the barrel refill with liquid. Gas interference is about free gas entering the pump, so the fixes involve gas handling: a downhole gas separator, lowering the intake below the perforations, or changing operating pressures to keep gas in solution or vent it up the casing. Slowing a gas-interfering pump helps less than slowing a liquid-pounding one, so misreading the card wastes effort.
Use supporting behavior when the corner is ambiguous. Gas interference often tracks with casing pressure and gas rate and can appear or worsen when casing pressure rises, whereas fluid pound tracks with pump-off as the well drains. Reading the card alongside the well's pressures usually resolves borderline cases, and keeping the definitions of gas interference and fluid pound in view helps you match the shape to the condition.
Confirm Against Casing Pressure and Act
Casing pressure is the corroborating evidence for gas interference. On a rod-pumped well, gas separated in the annulus builds casing pressure, and rising casing pressure that coincides with rounded, shrinking cards supports a gas diagnosis. If the casing is being blown down or vented and the cards improve, that further confirms gas was the problem. Reading the card and the casing pressure together is more reliable than the card alone, and it ties into broader casing pressure trend analysis.
Once confirmed, the response targets the gas. Managing casing pressure so separated gas can leave the annulus, ensuring the pump intake sits where it draws liquid rather than gas, and using downhole gas separation are the usual levers, all of which are well-plan decisions for the production engineer rather than surface tuning. The card tells you gas is the issue; the fix is a downhole and operating-pressure question that qualified personnel own.
Verify any change on the card. After improving gas handling, the downstroke release should sharpen and the loop should regain load range as the barrel fills with liquid instead of gas. Watching the rounded signature give way to a fuller, sharper card on the trend confirms the remedy worked. A card that stays rounded and thin after gas-handling changes suggests the gas source or the intake placement still needs attention.
Common Mistakes
The most common mistake is treating a rounded incomplete-fill card as fluid pound and responding by slowing the pump, which does little for a gas problem. Read the downstroke corner - rounded means gas - before choosing a remedy. The shape difference is subtle but it points to entirely different fixes.
The second is diagnosing gas from the card alone without checking casing pressure, when the two together are far more reliable. The third is expecting surface tuning to solve gas interference; the real levers are downhole gas separation, intake placement, and casing pressure management, which belong to the well plan and to qualified personnel, not to a controller setpoint.
Frequently Asked Questions
What does gas interference look like on a dyno card?
It shows as load held high into the downstroke followed by a rounded, gradual release rather than a sharp drop, because compressed gas in the barrel expands smoothly as the plunger descends. As gas worsens, the card shrinks toward a thin, low loop with little load change, and in severe cases the pump gas-locks. The rounded corner is the key feature that separates gas from the sharp step of fluid pound.
How do I tell gas interference from fluid pound?
Look at the shape of the downstroke load release. Gas interference gives a rounded, gradual curve because compressible gas cushions the plunger; fluid pound gives a sharp, sudden step down because the plunger strikes a solid liquid surface. Curved means gas, stepped means liquid pound. Confirm gas by checking whether casing pressure and gas rate track with the card, since gas interference often worsens as casing pressure rises.
How do I reduce gas interference on a rod pump?
The fixes target the gas rather than the pump speed: managing casing pressure so separated gas can leave the annulus, placing the pump intake where it draws liquid rather than gas, and using a downhole gas separator. These are well-plan and operating-pressure decisions for qualified production personnel. Confirm any change on the card, where an improved barrel fill shows up as a sharper downstroke release and a fuller loop.
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