MINIMUM GAS FLOW VELOCITY
up a GAS WELL
The "Turner Critical Rate" is defined as the
minimum gas flow volume
needed to maintain steady state flow up a gas well. At lower gas rates, liquid collects in the wellbore, resulting in intermittent flow and potentially the need to employ artificial lift to remove the liquid.
The Turner Critical Rate formula is
commonly used as a guage in deciding what diameter tubing to install in a well.
The Turner Critical Rate formula was based on data from about 100 gas wells using standard production tubing, mostly having an ID of 2 inches or more. Converting to flow velocity,
the Turner Critical Rate volume through 2-inch ID tubing flows at greater than 20 feet per second. The Turner Critical Rate formula was updated by Coleman and Li, drawing data from lower pressure wells and indicating a critical rate of ~ 20% less.
The study of "critical gas flowrates" up gas wells by petroleum engineers has exclusively used standard tubing diameters, mostly 2-inch ID or
more. For analyzing smaller tubing diameters, other fields must be
The "Wallis Correlation" was developed by Dr. G.B. Wallis, more a generalist in the study of multi-phase flow than Turner, and very prominent in the field of cooling nuclear reactors. He authored seminal articles about "flooding velocity" (the minimum velocity of gas flow up a tube to prevent flooding, with varying rates of liquid fed through a porous section of the tube). As shown below, Wallis found that the
flooding velocity was highly dependent on tubing diameter, steadily
declining with reducing diameter.
For owners of gas wells producing under 20 Mcf per day, a publication
in 1989 by Dr. D.J. Reinmann regarding airlift pumps may be more
applicable when considering installation of an MCS. Reinmann studied
the efficiency of gas lift with tube diameters ranging from 3 mm to 19 mm (1 inch = 25.4 mm) and found that
efficiency increased at an increasing rate from 20 mm
down to 6 mm in air-water systems. Below 6 mm, no increases in liquid-lifting efficiency were found, where the flow regime became "capillary bubble flow" (sequential layers of gas and liquid,
see drawing below right).
Particularly interesting regarding the "minimum gas velocity" needed to lift water out of a well is that for
tubing diameters of 6 mm or less,
there is no minimum velocity. With the capillary bubble flow structure, the liquid will be suspended (not fall), even at zero velocity.
For comparison, we calculated the gas flow velocity inside the MCS production tubing in our 1,930-foot
pilot well (seven 7mm passageways MCS, see "Pilot Well" page). The flow velocity was 4.4 fps at the bottom and 11 fps at the top, both far below the gas velocity at Turner Critical Rate flowing up 2-inch ID tubing (>20 fps).
Therefore, by virtue of selecting the appropriate
tubing diameter of the individual MCS passageways,
the "minimum velocity of gas" can be "managed"
(i.e., designed/ specified into the "production tubing")... and the number of individual MCS passageways can be decided in order to accomodate the desired flow volume.
Theoretically, if the "minimum velocity" can be reduced from >20 fps (2-inch diameter tubing) to ~2 fps (MCS with 7mm diameter passageways), it implies that from the point in time when artificial lift would otherwise be initiated (when using standard 2-inch ID tubing), ~
90% of the remaining gas reserves can be produced at steady state using MCS production tubing... thus avoiding the cost and complexities of "artificial lift" (pumps, plungers, surfactants, etc.), and implying a high ultimate recovery.