Marsh Funnel

A Marsh funnel is a standardized cone-shaped vessel fitted with a small-bore orifice tube at its apex, used to measure the apparent viscosity of drilling fluid by timing how long it takes a fixed volume of mud to drain through the orifice. The device is named after Hallan Marsh, who designed it for the API in the 1930s. To use it, the operator covers the orifice with a thumb, fills the funnel to the 1,500-millilitre (1.5-quart) mark while holding a screen over the top to catch coarse solids, removes the thumb, and measures in seconds the time for exactly 946 millilitres (1 quart) to drain into a cup. Clean water at 21°C (70°F) drains in 26 seconds; this is the reference value. Drilling muds typically read between 35 and 85 seconds depending on mud weight and viscosity. The Marsh funnel viscosity (also called funnel viscosity or simply funnel seconds) is the fastest, cheapest, and most widely used field check on mud condition.

Key Takeaways

Marsh funnel viscosity is not a true viscosity measurement in the physical sense. True viscosity (dynamic viscosity in millipascal-seconds) requires knowing the shear rate applied to the fluid during the measurement, which the Marsh funnel does not control precisely. The funnel measures apparent viscosity at an average shear rate that decreases as the fluid drains and the head pressure drops. For Newtonian fluids (water, light oils), Marsh funnel viscosity converts approximately to dynamic viscosity. For non-Newtonian fluids (almost all drilling muds), the funnel viscosity is a comparative number that tells you whether the mud is thicker or thinner than normal but cannot be directly converted to cP without knowing the yield point and plastic viscosity from a more precise viscometer measurement.
The standard API Marsh funnel holds 1,500 millilitres above the orifice tube and discharges 946 millilitres. The orifice tube is 4.76 millimetres (3/16 inch) internal diameter and 51 millimetres (2 inches) long. The geometry is fixed so that all field measurements are comparable between different rigs and different locations. Non-standard funnels (different orifice sizes or volumes) exist for specific purposes (cement slurry rheology, cement API funnel), and a reading from one cannot be compared to a reading from another.
Marsh funnel is typically measured every 15 to 30 minutes at the flowline (mud returning from the borehole) during active drilling. A sudden increase in funnel seconds (thicker mud) at the flowline without any mud additions is a warning sign: the mud may be cutting (incorporating formation solids from the drill cuttings) or the mud properties have changed due to contamination. A sudden decrease (thinner mud) may indicate dilution by formation water influx or gas contamination that is breaking the mud structure. The driller and mud engineer use the trend of funnel readings over time rather than any single reading to diagnose mud problems.
The funnel reading must always be paired with mud weight (density) to be interpreted correctly. A funnel reading of 55 seconds on a 1.20 specific gravity (10.0 ppg) mud means something completely different from 55 seconds on a 1.50 specific gravity (12.5 ppg) mud. Mud weight is measured simultaneously on the same sample with a mud balance (also called a Baroid balance or pressurized balance). Together, funnel viscosity and mud weight are the two most basic field measurements of drilling fluid properties, reported on the mud log every few minutes during critical operations.
API Recommended Practice 13B-1 (for water-based fluids) and 13B-2 (for oil-based and synthetic-based fluids) standardize the Marsh funnel measurement procedure and temperature correction. Readings should be made at or near 25°C (77°F) because viscosity decreases with increasing temperature. If the mud temperature at the flowline is significantly different from 25°C (common in deep, hot wells where flowline mud can be 40 to 60°C), the standard correction charts apply a temperature factor to normalize the reading for comparison to target values.

How the Marsh Funnel Is Used on a Drilling Rig

On a drilling rig, mud testing happens continuously because drilling fluid is the first line of defence against wellbore instability, kicks, and stuck pipe. The Marsh funnel sits in a rack near the mud pits, and the mud engineer or derrickman tests the mud at the flowline (incoming from the well) and at the active pit (going back into the well) several times per tour.

The procedure takes about 90 seconds. The operator covers the funnel's orifice with a thumb, holds the funnel upright, and pours fresh mud through the screen at the top until the level reaches the 1,500-millilitre mark. A stop watch starts the moment the thumb is removed from the orifice. When exactly 946 millilitres have drained into the quart measuring cup, the watch is stopped. The recorded time is the Marsh funnel viscosity in seconds.

If the target range from the mud program is 42 to 55 seconds and the reading comes in at 68 seconds, the mud is too viscous. The mud engineer adds water to dilute the system, measures again in 15 minutes, and continues until the reading falls within range. If the reading comes in at 33 seconds, the mud is too thin, possibly because the solids content has dropped from dilution, and the engineer adds viscosifier (bentonite, attapulgite, or polymer depending on mud type) to restore the viscosity.

Fast Facts

Hallan Marsh developed the Marsh funnel while working for the Baroid Division of the National Lead Company in 1931. The design was adopted as an API standard in the 1940s and has not changed substantially since then. It remains the only piece of drilling fluid test equipment that requires no calibration, no electronics, no power, and no consumables: just a clean funnel, a stopwatch, and a quart cup. In areas of the world with unreliable power or limited infrastructure, the Marsh funnel is sometimes the only mud test performed. It costs approximately USD 20 to purchase and lasts indefinitely if cleaned after each use. The companion API viscometer (six-speed Fann VG meter), which measures true plastic viscosity and yield point, costs approximately USD 400 and requires calibration.

Marsh Funnel Versus Six-Speed Viscometer

For precision mud engineering, the Marsh funnel is supplemented (not replaced) by the six-speed rotational viscometer, commonly called the Fann meter or VG meter. The Fann meter drives a bob at six different rotational speeds (600, 300, 200, 100, 6, and 3 rpm) and measures the torque resisting rotation at each speed. From these six readings, the plastic viscosity (PV) and yield point (YP) are calculated.

Plastic viscosity (PV, in millipascal-seconds or cP) measures the frictional resistance to flow from solid particles. Yield point (YP, in Pascals or lbf/100ft²) measures the electrochemical force that causes the mud to gel when circulation is stopped. A mud with high YP has good hole-cleaning ability (it suspends drill cuttings when pumps stop) but high equivalent circulating density (ECD), which may fracture weak formations.

The Marsh funnel captures a combined signal of PV and YP but cannot separate them. A mud can have the same funnel viscosity with different PV/YP combinations that behave completely differently in the borehole. For routine monitoring between mud reports, the funnel is adequate. For troubleshooting a specific rheological problem (excessive ECD, poor hole cleaning, barite sag in weighted muds), the Fann meter readings are required to diagnose the cause and prescribe the right treatment.

The Marsh funnel is also called a mud funnel, flow cup, or funnel viscometer. The measurement it produces is called Marsh funnel viscosity, funnel viscosity, or funnel seconds. Related terms include drilling fluid (the engineered fluid circulated through the drill string and annulus during drilling; maintains wellbore pressure, cools the bit, transports cuttings to surface, and stabilizes the borehole walls; the Marsh funnel viscosity is the primary field check on its rheological condition), plastic viscosity (PV, the component of drilling fluid viscosity caused by mechanical friction between solid particles; measured by the six-speed viscometer; high PV indicates excessive solids in the mud), yield point (YP, the initial resistance to flow of a drilling fluid; related to the electrochemical interactions between particles; contributes to hole-cleaning ability and ECD; measured by the six-speed viscometer alongside PV), mud weight (the density of drilling fluid, measured in kilograms per litre or pounds per gallon; the primary control on bottomhole pressure; measured simultaneously with Marsh funnel viscosity using a mud balance), and equivalent circulating density (ECD, the effective density of drilling fluid at any point in the borehole when the mud is circulating; higher than the static mud weight due to annular friction; related to mud viscosity and affected by Marsh funnel changes).

How a Rising Marsh Funnel Reading Caught a Stuck Pipe Hazard on a Duvernay Well

A horizontal Duvernay well was being drilled from a multi-well pad in the Kaybob area of west-central Alberta. The mud program called for an oil-based mud (OBM) system at a Marsh funnel viscosity of 48 to 56 seconds throughout the horizontal section. The mud engineer performed flowline funnel readings every 20 minutes during the lateral.

At approximately 4,100 metres measured depth, the flowline funnel reading jumped from 54 seconds to 71 seconds over two consecutive readings, 40 minutes apart. There was no planned change to the mud system and no new mud additions in that window. The mud engineer reported the anomaly to the toolpusher and recommended circulating a complete bottoms-up before continuing to drill.

When the bottoms-up was circulated, the shale shaker screens rapidly loaded with fine, dark formation cuttings that were not being adequately lifted from the lateral by the viscous mud. Analysis of the cuttings showed they were Duvernay shale from behind the bit, not fresh cuttings from the current bit position. The formation solids from the shale had been incorporating into the mud system and driving the viscosity increase. The mud engineer treated the pit with an organophilic clay and solids-control additive to restore the OBM rheology.

Had drilling continued without addressing the high viscosity, the fine solids buildup would have continued to thicken the mud, increasing ECD above the fracture gradient of the Duvernay, potentially causing lost circulation and differential sticking. The Marsh funnel reading, acting as the earliest available indicator of mud property change, triggered the corrective action before a stuck pipe incident occurred. Estimated rig time and materials cost to free a differentially stuck string in that well section: CAD 800,000 to CAD 1.4 million depending on severity.