The two modes of gas behavior in the wellbore were explained in an earlier lesson:
gas expansion, where gas is free to expand normally (as it does in constant
shut-in or blocked wellbore and does not or cannot expand. For a review, see “Gas Behavior in the Wellbore” on page 2-41.
The WSS should know these characteristics of gas migration:
• It can occur when the well is shut in with gas present (either when the well is shut in intentionally or the flow path is mechanically plugged or blocked). • It is indicated by a uniform increase in SICP and SITP.
• If uncontrolled, it increases pressures everywhere in the wellbore.
• If ignored, these increased pressures can cause formation damage and loss of whole fluid into the perfs.
• It occurs rapidly in the clear workover fluids typically used (research indicates speeds of 4,000-6,000 feet per hour).
There are two recognized methods of dealing with migration, or “allowing expansion,” so to speak: the constant tubing pressure method and the volumetric
method (also called the “stairstep” method). These methods are used to control gas
migration when it is not possible to circulate or bullhead the well. They can be used temporarily while operations are ongoing to get in a position where the well can be circulated or bullheaded. (Unplug the tubing, fix the pumps, shift a sliding sleeve, or create a flow path in some way.)
Constant Tubing Pressure Method
The constant tubing pressure method is the simpler of the two methods; its name describes what is done. It is based on the following assumptions:
1 There is communication between the tubing and the choke located on the annulus.
2 The tubing pressure can be read.
This procedure can be used to control gas migration while mixing kill fluid or making other preparations for a circulating kill procedure.
Procedure for Constant Tubing Pressure Bleed Method
1 Allow SITP to increase by a safety margin of 50-100 psi (which prevents further influx due to overbleeding with choke). This is called the lower limit.
3 Using the choke, bleed the annulus until the tubing pressure drops down to the lower limit. Remember the time lag (see Important Note below).
4 Repeat steps 2 and 3, keeping the tubing pressure between the lower and upper limits as long as desired or until another procedure is implemented.
Important Note: There is a time lag between opening the choke and seeing the pressure drop on the tubing gauge. The pressure “signal” must travel down the annulus and up the tubing to the tubing gauge, which is thousands of feet away. The recommended procedure is to open the choke until the desired drop is seen on the casing gauge, then close the choke and wait until the change appears on the tubing gauge.
In the long term, the casing gauge reading will not stay constant the way the tubing gauge does (the WSS should not use the choke to make it so!). With successive bleed cycles, the gas is continually expanding as it rises up the annulus. If you do not have a full understanding of why the casing pressure must increase in this case, review “Gas Behavior in the Wellbore” on page 2-41.
Volumetric Method
This procedure accomplishes the same objective as the constant tubing pressure method in allowing gas expansion, but it uses a different process control. This method is used when there is no tubing communication. Since tubing pressure cannot be read, the process must be controlled with the casing pressure and the volume of fluid bled from the annulus. There must be a calibrated tank on the rig located downstream of the choke capable of reading in as small as 1/2 bbl
increments. (For a review of tank calibration, see “Fluid Tank Volumes” on page 2-24.)
Procedure for Volumetric Method
1 Select a safety margin and a range.
Recommended margin: 100 psi; range: 100 psi.
2 Calculate hydrostatic pressure (Hp) per bbl fluid in the upper annulus.
Hp per bbl (psi/bbl) = Fluid Gradient (psi/ft) ÷ Annular Capacity Factor (bbl/ft)
3 Calculate volume to bleed each cycle.
4 Construct casing pressure vs. volume to bleed schedule. Fig. 3-22 illustrates the volumetric calculations and construction of the volume to bleed (stairstep) schedule.
5 Allow SICP to increase by margin without bleeding.
6 Allow SICP to increase by range without bleeding.
7 Maintaining SICP, bleed small volumes of fluid into tank until calculated volume in step 3 is bled. Repeat steps 6 and 7 until gas is at surface or another procedure implemented.
Important Note: During the procedure, it is critical to hold SICP constant while bleeding fluid. The stairstep schedule is shown in Fig. 3-22. The bleeding is done on the flat part of the stairstep—that is, the SICP is not to increase or decrease. The choke should not be opened more to speed up the bleeding process (which lowers SICP below the line) or another kick will result. Patience is required: the bleed for the first stairstep may take several hours (depending on well depth and type of wellbore fluid).
The question often arises, “How long should this procedure be carried on?”
Remember that the goal is to control gas migration and allow expansion. If the gas influx reaches the top of the well during the stairstep schedule, the procedure is over: gas migration has been controlled. (This is evidenced by the sound of gas flowing across the choke and a stable SICP when the well is closed in.)
Do not open the choke at this point and bleed gas off the well. This will reduce
bottomhole pressure and most likely result in additional influx. It will then be necessary to create yet another pressure schedule and repeat this rather time- consuming procedure. Removing gas from the top of a well (at constant BHP) requires lubricate-and-bleed procedures, which are explained in the next section.