Well Kill Using Wait and Weight Method (Balanced Method)

The “Wait and Weight” method is the method recommended, in some circumstances, for controlling an influx taken while drilling or circulating on bottom. When drillpipe (string) volume is greater than open hole volume, the influx will already be inside the casing before heavy mud reaches the open hole.

In this case the “Driller’s Method” can be a better solution as the danger of gas expansion is removed immediately while weighing up mud can take hours.

Advantages of “Wait and Weight” method

the annular pressure will usually be lower and the chance of formation breakdown is therefore reduced.

the hole and the wellhead equipment are subjected to high pressures for the shortest possible time since the influx is circulated out and the well is killed in one circulation.

Disadvantages of “Wait and Weight” method

considerable waiting time while weighing up mud can cause gas migration

if large increases in mud weight is required, this may be possible in stages only

This method involves one circulation. Kill mud is prepared and is pumped from surface to bit while following a prepared drillpipe pressure drop schedule. Once the kill mud enters the annulus, a constant drillpipe pressure is maintained until the heavy mud returns to surface.

Procedure

The procedure for the Wait and Weight method is as follows:

After the well has been secured and pressures have stabilised, complete kill sheet including kill graph

Bring pumps up to speed keeping casing pressure constant by manipulating the choke

When pump is up to kill speed the choke is manipulated to keep the drill pipe pressure at initial circulating pressure (ICP).

Pump kill mud down drill pipe keeping casing pressure constant and allowing drill pipe pressure to fall from ICP to final circulating pressure (FCP).

When kill mud reaches the bit the drill pipe pressure should be at FCP. Continue pumping kill mud keeping drill pipe pressure constant at FCP until the kick is circulated out and kill mud reaches surface.

Equations

KMW = (SIDPP / (0.052 * TVD))  + OMW

Trip margin may not be included in the calculation for kill mud weight. The major reason for this is to avoid any unnecessary additional wellbore pressure that could result in formation breakdown.

Calculate initial circulating pressure:

Calculate Final circulating pressure:

Calculate surface to bit strokes:

Calculate time to pump surface to bit: 

Where:

• KMW = Kill mud weight (ppg)
• SIDPP = Shut in Drillpipe pressure (psi)
• TVD = True vertical depth (ft)
• OMW = Original Mud Weight (ppg)
• ICP = Initial circulating pressure (psi)
• SCRP = slow circulating rate pressure (psi)
• FCP = Final circulating pressure (psi)