Kickoff

When a velocity string is installed in a gas well with liquid loading problems, it is common for there to be a substantial amount of standing water in the well.  Given the relatively low reservoir pressure in a liquid-loaded well and the higher flow resistance through smaller diameter tubing, temporarily shutting in the well to increase wellbore pressure is often not sufficient to kick off the well.  Swabbing is often required to remove the water to initiate flow up the velocity string.

MCS production tubing would be even more difficult to kick off under these circumstances, given the even higher flow resistance through the smaller diameter internal MCS flow passageways.

An MCS Endpiece eliminates this problem, facilitating the evacuation of liquid in the column prior to flowing at steady state.  An "MCS Endpiece" is a gas collection device attached to the bottom end of the MCS production tubing (see "MCS Endpiece" page).   The following is an explanation of the MCS installation and kickoff process.

As MCS production tubing is lowered into a well loaded with water, the water enters the small MCS internal passageways, and its level equalizes with the water level of its surroundings (the tubing or casing annulus).  The MCS production tubing is lowered down the well until it touches bottom, and then it is lifted to where its bottom entrance is just above the perforations; once its entrance is at the correct depth, it is hung off at the wellhead using a conventional hanger.  Wellhead lines are installed to direct flow production into the separator.  Only the MCS production line is left open to atmosphere.  Initially, there is typically no flow up through the MCS production tubing.
 
Gas enters the well through the perforations in the casing, pressurizing the well.  Preferably, there is no packer, so that there is more casing volume to pressurize, lengthening the period where gas entering the wellbore feeds the MCS Endpiece.   Rising gas bubbles enter the MCS Endpiece, and its enclosed space reduces turbulence, facilitating gas phase coalescence and separation.  Gas then collects in the upper region of the MCS Endpiece, presenting 100% gas phase to the entrance of the MCS production tubing.
 
Liquid then drains down from the MCS internal passageways into the MCS Endpiece.  At the interface between the MCS production tubing entrance and the top of the MCS Endpiece, fluid is exchanged.  Buoyant gas slowly works its way up into  the MCS passageways, and the (heavier) liquid in the MCS passageways slowly falls down into the MCS Endpiece.  Gas in the MCS Endpiece is resupplied by gas
entering the casing through the perforations.  This process leads to
a reduction  of liquid in the MCS production tubing, reducing its
hydrostatic head.
 
Liquid continues to leak down from the MCS passageways into the MCS Endpiece until the hydrostatic head in the MCS is reduced sufficiently to initiate upward flow in the MCS production tubing.  Once upward flow is initiated in the MCS, slug flow forms in the MCS passageways and continues until the well is unloaded of liquid higher than the entrance to the MCS Endpiece.  Slug flow up small-diameter tubing (~7 mm) is highly efficient at low velocity, and the minimum critical velocity is estimated at ~2 feet per second (see “MCS Detailed Presentation” page). 

Once the well is unloaded with liquid, steady state flow becomes established in the MCS production tubing and will continue indefinitely.  And when flowing at steady state, the MCS Endpiece biases gas flow up through the MCS production tubing, increasing 
its capacity to lift liquid.
 
It is important that one or more of the MCS passageways has a diameter greater than 6 mm so that the gas can rise and slip past the liquid up the MCS passageways during kickoff.  If the well liquid is water and all of the MCS passageways are less than 6 mm, then the capillary bubble flow structure will form in the MCS passageways, preventing liquid from draining down the MCS production tubing and falling into the MCS Endpiece… and therefore there will be no kickoff.