TERMINAL DE BUS 2

Prior to implementing the Wait & Weight Method, the relevant sections of the Kick Sheet as covered in Paragraph 3 should be completed.

The Wait & Weight Method accomplishes the kill operation in one complete circulation. It requires weight up of the mud after the well is shut in, followed by circulation with the kill weight mud. So several calculations are necessary prior to initiating circulation. These are as follows:

(a) Determine the circulation rate

The upper limit for the circulation rate is generally set by the maximum rate that baryte can be mixed into the mud. The following formula can be used to estimate the maximum possible circulation rate:

Baryte delivery rate (lb/min)

Maximum circulation rate = (bbl/min)

Baryte required to weight up mud (lb/bbl)

A limiting factor, particularly in the case of oil mud, may be the rate at which viscosity can be built in the mud. This, and associated problems of building mud weight are discussed in Chapter 1 in ‘Use of the Mud System’.

Having established the maximum possible circulation rate, the actual circulation rate will be determined on the basis of several factors. These factors are detailed in Chapter␣1 in ‘Drills and SCRs’. The chosen SCR and the relevant pumping data should be recorded in the kick sheet.

(b) Calculate the initial circulating pressure

The initial drillpipe circulating pressure, Pic, should be calculated in order to estimate the circulating pressure that will be required to maintain constant bottom hole pressure at the start of the circulation.

The initial circulating pressure recorded after the pump has been brought up to speed␣should be the sum of the shut-in drillpipe pressure and the SCR pressure at the chosen rate:

Pic = Pdp + Pscr

where Pic = Initial circulating pressure (psi)

Pdp = Stabilised shut-in drillpipe pressure (psi) Pscr = Circulating pressure at SCR with MW1 (psi)

(c) Calculate the final circulating pressure

As the drillpipe is displaced with kill weight mud, the standpipe pressure must be reduced to take into account the increased hydrostatic pressure of the mud in the pipe. The standpipe pressure must also compensate for the additional friction pressure in the drillpipe and across the bit as the kill weight mud displaces the original mud.

Once the drillpipe has been completely displaced to kill weight mud, the static drillpipe pressure required to balance the kick zone will be zero. At this stage therefore the␣␣circulating pressure can be estimated by determining the SCR pressure for the kill weight mud.

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March 1995

The final circulating pressure can be estimated as follows: MW2

Pfc = Pscr (at MW1) x MW1

where Pfc = Final circulating pressure (psi).

(d) Determine the displacement times and the cumulative pump strokes At all times during circulation, it is important to know the position of the influx in the wellbore, as well as the volume of hole that has been circulated to kill weight mud. The key points during the circulation are as follows:

• When the kill weight mud reaches the bit.

• When the top of the influx is circulated to the casing shoe or openhole weak point. • When the influx is circulated to the choke.

Before circulation is started, the estimated circulating time and the corresponding total pump strokes to each point should be calculated.

Volume to be displaced (bbl)

Pumping time to reach point = (min)

of interest Pump rate (bbl/min)

Volume to be displaced (bbl)

Total strokes to reach point = (stk)

of interest Pump output per stroke (bbl/stk)

(e) Plot standpipe pressure schedule

To ensure that the standpipe pressure is adjusted correctly as the kill weight mud is circulated down the drillpipe, a plot should be made of the required standpipe pressure (See Figure 6.1b).

The initial circulating pressure should be plotted corresponding to zero strokes. The final circulating pressure should be plotted corresponding to total strokes equivalent to complete displacement of the drillpipe. The two points on the graph can be joined up with a straight line to produce the standpipe pressure schedule. (Note: for high angle or horizontal wells, the graph is not a straight line. See Paragraph 6.)

In practice standpipe pressure is most easily controlled by reducing the pressure in small steps, rather than continuously.

(f) Procedure for the displacement of the kick

1 Bring the pump up to kill speed

• Line up the pump to the drillpipe and route returns through the choke manifold to the mud gas separator.

• Zero the stroke counter on the choke panel.

• Open the remote operated choke at the same time as the pump is started on the hole. Consider stroking the drillstring up at this point.

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• Maintain the choke pressure equal to the original shut-in casing pressure as the pump is slowly brought up to speed. This may take 1/2 to 1 minute.

• Once the pump is up to speed record the initial circulating pressure.

If the actual initial circulating pressure is considerably different from the calculated value, stop the pump, shut in the well and investigate the cause.

If the actual initial circulating pressure is equal to, or reasonably close to the calculated value, continue the displacement and adjust the standpipe pressure schedule accordingly. Any marginal difference between the actual and calculated initial circulating pressure is most likely to be due to the fact that the SCR pressure used to calculate the initial circulating pressure was inaccurate. The actual SCR pressure, and hence the corrected final circulating pressure, Pfc, can be determined from the initial circulating pressure as follows:

P

= P

– P

The standpipe pressure schedule can therefore be corrected to take into account the adjusted circulating pressures.

2 Circulate the influx from the well maintaining constant bottom hole

pressure

As the drillpipe is displaced with kill weight mud, the standpipe pressure should be stepped down according to the standpipe pressure schedule. (The standpipe pressure will have a natural tendency to drop as the kill weight mud is displaced down the drillpipe.)

Once the drillpipe has been displaced to kill weight mud, the drillpipe pressure should be maintained at the final circulating pressure for the rest of the circulation.

The pit gain, drillpipe pressure, choke pressure and all other relevant information should␣be recorded during displacement using the Well Control Operations Log (See␣Figure 4.5). These will help to determine the down hole condition during all stages of the kill operation.

As the influx is displaced up the hole, the drillpipe pressure will tend to drop as the influx expands. (This expansion will not occur if the influx is water or oil.) This effect will be especially marked if the influx contains a significant quantity of gas. The choke should therefore be adjusted to compensate for this. For example, if the drillpipe pressure drops by 70 psi below that required, the choke pressure should be increased by approximately 70 psi. The pressure on the drillpipe will increase after a lag time which can typically be 2 seconds per 300m of drillstring depth. This technique will be most effective at the early stages of displacement; and less so at later stages of the displacement, if the well contains a significant proportion of gas.

When the influx reaches the choke, the choke pressure will start to decrease due to the differences in density and viscosity between the influx and the mud. If the influx contains significant quantities of gas, the drop in choke pressure may be quite substantial, and the choke will have to be closed down quickly.

As the influx is circulated from the well and mud is circulated to the choke, the choke pressure will begin to rise rapidly. The choke should therefore be opened to allow the choke pressure to drop sufficiently to re-establish the final circulating pressure on the drillpipe, and hence maintain constant bottom hole pressure.

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Once the hole has been circulated to kill weight mud, the pump should be stopped, the well shut-in, and the casing and drillpipe checked for pressure. There should be no pressure on either the casing or the drillpipe. However, if there is still some pressure on the casing, circulation should be restarted to clear the contaminated mud from the annulus. Once the well has been completely killed, a flowcheck on the choke line return should be carried out before the rams are opened. If this flowcheck indicates no flow, the rams should be opened and a further flowcheck on the annulus carried out.

In line with Company policy, a further complete hole circulation should be carried out prior to continuing operations. A suitable overbalance can be added to the mud at this␣stage.