This well control method had its beginnings in the drilling industry and is widely used in that arena. Although it is not the predominant kill method in workover well control, it can be used. For a description of this method in a workover, see “Well Scenario: Wait-and-Weight Method” on page 3-15.
The name of the method is indicative of what happens—you wait until the fluid is weighted up to the correct density and then kill the well. Whether the fluid density should be increased is determined by the stable SITP reading. If the SITP does not bleed to 0 psi (after a check for trapped pressure), then the fluid density is
insufficient and must be weighted up. The density can become insufficient for the following reasons:
• Mismanagement of the fluid on the surface, resulting in light fluid being pumped downhole.
• Formation fluid contamination of the fluid in the tubing.
• Penetration of a zone of higher formation pressure, as when sidetracking or washing through sand plugs.
The wait-and-weight method, by design, is a one-circulation kill procedure. Kill fluid is pumped in while the influx is circulated out. If performed properly, it will require the least amount of “on choke” time. Furthermore, it should result in lower ultimate casing pressure than other circulating methods because the increase in hydrostatic pressure of the kill weight fluid in the annulus offsets the decrease in hydrostatic pressure caused by gas expanding in the annulus above it. A drawback to this method is the time required to weight up and condition the fluid before pumping begins. In the event of a gas influx, the time required to condition and weight up may allow gas migration to take place, requiring surface pressure monitoring and controlled bleeding of fluid until the actual well killing operation can begin. This controlled bleeding process will be addressed later in this lesson (see “Constant Tubing Pressure Method” on page 3-39).
Additionally, the WSS must generate a circulating “pressure schedule” and use it to monitor the tubing pressure while displacing the tubing string. Tubing pressure will gradually decrease as the tubing string is displaced to kill fluid—that is, filled with kill weight fluid. This decrease in tubing pressure is the result of kill fluid
hydrostatic pressure replacing the original underbalance shown on the tubing gauge.
Wait-and-Weight Procedure
Follow these steps to kill a well with the wait-and-wait method:
1 Calculate and increase the fluid weight to kill value (see Fig. 2-5).
2 Create a tubing pressure reduction schedule (see Fig. 3-5). Monitor the well for gas migration. Use the choke to maintain tubing pressure within 50–100 psi above the original SITP.
3 Line up the manifolds and the pump to circulate kill fluid down the tubing and take returns from the annulus.
4 Bring the pump to the predetermined kill speed (one of the speeds at which SCRPs were taken) while holding the annulus pressure constant with the choke. These steps will establish initial circulating pressure (ICP) on the tubing gauge. If the value observed on the gauge does not agree with the calculated value, use the observed value. Verify the observed reading by repeating the startup
procedure. If you obtain the same reading a second time, consider it valid. You will have to modify the tubing pressure schedule so it reflects the new value. Do
not change the pressure gauge reading (with the choke) to fit the pressure schedule. Change the pressure schedule to fit the observed gauge reading. This
can be done quickly without stopping circulation.
5 Circulate the kill fluid to the end of the tubing following the pressure schedule from ICP to final circulating pressure (FCP).
6 Once the kill fluid is in the annulus, hold the tubing pressure constant at FCP until the kill fluid returns to the surface.
7 Shut down the pumps and check for well flow. Close the choke and check for pressure on the tubing or casing gauge.
Fig. 3-4 shows the pressure profiles for the wait-and-weight method. Note the sloped portion of the pump pressure line between ICP and FCP. This portion pertains to the pressure reduction schedule you will create. Also note that FCP is reached when the kill fluid reaches the end of the tubing. At this point, continue
pumping, holding the tubing gauge (pump) pressure constant at the FCP value by manipulating the choke. Perhaps the most important thing to learn from the figure is that although two pressures are plotted, the pump pressure is the “process control.” Use the choke to control pump pressure, not the casing pressure, except for a short time during the pump-start procedure.
Figure 3-4 Pressure profile for wait-and-weight method Wait-and-Weight Calculations
1 Obtain accurate SITP and SICP readings.
2 Calculate tubing or work string volume in barrels or strokes (see Fig. 2-7). Record this number on the pressure reduction schedule (step 1 on the schedule in Fig. 3-5).
3 Calculate Kill Fluid Weight. (Use the appropriate equation depending on information available.)
Kill FW = (SITP ÷ TVD perfs ÷.052) + Tubing Fluid Weight Kill FW = Formation Pressure (psi) ÷ TVD (ft) ÷.052
Kill FW = [Safety Margin (psi) + Formation Pressure (psi)] ÷ TVD (ft) ÷.052
4 Determine Initial Circulating Pressure (ICP). Record this number on the pressure reduction schedule (see Fig. 3-5).
Observed ICP: Use the stabilized pump pressure reading after the pump-start procedure.
5 Calculate Final Circulating Pressure (FCP). FCP =
SCRP (psi) × Kill (or balance) Fluid Weight (ppg) ÷ Original Tubing Fluid Weight (ppg)
6 Create Tubing Pressure Reduction Schedule using steps 1 through 5 as shown on Fig. 3-5.
1 After calculating the total tubing or work string volume in barrels or strokes, as indicated in the wait-and-weight calculations above, record it on the pressure reduction schedule.
2 Determine and record the number of strokes or bbls from 0 to Total Strokes using the formula given in step 2.
3 Record the ICP that was predetermined in the wait-and-weight calculations above.
4 Record the FCP that was predetermined in the wait-and-weight calculations above.
5 Determine and record the circulating pressures from ICP to FCP using the formula in step 5.
Additional Considerations for the Wait-and-Weight Method
• Accuracy of readings, especially SITP, is crucial to the success of the kill. A false reading will lead to error in calculating kill weight fluid, which results in circulating the wrong fluid weight around the well.
• Because of the importance of accurate readings, the WSS should ensure the accuracy of the gauges beforehand and perform the trapped pressure checks described earlier in the lesson.
Well Scenario: Wait-and-Weight Method
The following scenario describes a potential use of the wait-and-weight method during workover operations. Fig. 3-6 shows a well shut in on a 10 bbl kick. The pore pressure of the producing zone is 5,200 psi, which is equivalent to 10 ppg at a vertical depth of 10,000 feet. The WSS decided to work the well over with 10.2 ppg brine, thus rendering the well dead during the workover. Through inadvertent dilution, the density of the workover fluid was reduced to 9.5 ppg and pumped into the well, creating an underbalance.
Figure 3-6 Well with 10 bbl kick
The wellbore hydrostatics and shut-in pressures for this example are as follows: • Annular hydrostatic pressure = 5,048 psi, yielding SICP of 152 psi
• Work string hydrostatic pressure = 4,940 psi, yielding SITP of 260 psi • Pump output = 0.070 bbl/stroke
At the beginning of the day, the driller took SCRPs at various pump rates and recorded the rates and observed pressures with a 10.2 ppg fluid in the well.
Following the kick, the WSS selected the rate of 2 bpm as the kill rate. When the well was brought on choke, the choke operator held the casing pressure constant at approximately 150 psi. At 2 bpm the pump pressure stabilized at 950 psi, which was the observed ICP. (Remember, there is 9.5 ppg fluid in the well, not 10.2 ppg fluid, so the ICP must be observed rather than calculated with the standard equation.) The FCP is a known value. It equals the SCRP previously taken before the fluid was diluted and the kick taken. Therefore, a pumping schedule (Fig. 3-7) can be
generated from the observed ICP and the known pressure at the selected kill speed of 2 bpm, which was 750 psi with 10.2 ppg fluid.
Figure 3-7 Circulating pump pressure schedule