THE WAIT AND WEIGHT METHOD
INTRODUCTION TO WAIT AND WEIGHT METHOD
The Wait and Weight is sometimes referred to as the Engineer's Method or the One Circulation Method. It does, at least in theory, kill the well in one circulation.
Once the well is Shut In and pressures stabilized, the Shut In Drill Pipe pressure is used to calculate the kill mud weight. When ready, Kill mud is pumped down the Drill Pipe. At commencement, enough Drill Pipe pressure must be held to circulate the mud, plus a reserve equivalent to the original Shut In Drill Pipe pressure. This total steadily decreases as the mud goes down to the bit, until with Kill mud at the Drilling Bit , the required pressure is simply that needed to pump Kill mud around the well.
The choke is adjusted to reduce Drill Pipe pressure while Kill mud is pumped down the Drill String. With kill mud at the bit, the static head of mud in the Drill Pipe balances formation pressure. For the remainder of the circulation, as the influx is pumped to the surface, followed by Drill Pipe contents and the kill mud, the Drill Pipe pressure is held at the final circulating pressure by choke adjustment.
ADVANTAGES OF WAIT AND WEIGHT METHOD
- Lowest wellbore pressures, and lowest surface pressures - this means less equipment stress.
- Minimum chokE circulating time - less chance of washing out the choke.
DISADVANTAGES OF WAIT AND WEIGHT METHOD
- Considerable waiting time (while weighting up) - gas migration.
- If large increases in mud weight required, this is difficult to do uniformly in one stage.
HOW TO KILL THE WELL BY WAIT AND WEIGHT METHOD (PROCEDURES)
1- Calculate Kill mud weight:
2- Initial Circulating Pressure:
3- Once the capacity of the Drill String is calculated, it is possible to draw a graph showing how Drill Pipe pressure varies as Kill mud is pumped down to the Drilling Bit
|Profile of Circulating and Annular Pressure Killing by Wait and Weight Method|
4- The choke is cracked open, the pump started to break circulation, and then brought up slowly to the Kill Rate, While that keep the casing pressure as near as possible to the SICP reading.
5- When the pump is up to the Kill Rate, the choke operator transfers to the Drill Pipe pressure gauge.
6- As the Kill mud proceeds down the Drill Pipe, the Drill Pipe pressure is allowed to drop steadily from the Initial Circulating Pressure to the Final Circulating Pressure, by choke adjustment. (through the table you have already done)
- Where the Kick is a small one, at or near the bottom of the hole, the Drill Pipe pressure tends to drop of its own accord as the kill mud moves down. Little or no choke adjustment is required.
- Only in cases of diffused gas Kicks with gas far up the annulus will significant choke adjustments be needed during this period.
7- After kill mud has reached the Drilling Bit, the Drill Pipe pressure is maintained at the Final Circulating Pressure, until the kill mud returns to surface.
Note: As with the Driller's Method, this Final Circulating pressure is held constant as long as pump rate is held constant at the selected value. If, for any reason, the pump rate is felt to be wrong, it can be changed using the same procedure described previously. However, pump rate changes should be avoided, where possible.
Changing Pump Rate :While the pump rate is adjusted, the casing pressure is held steady by adjusting the choke. Once the pump is stabilized at its new speed, the revised circulating pressure is ready from the Drill Pipe gauge. If a gas influx is very near to the surface, adjusting pump rate by holding a steady casing pressure may significantly increase the bottom hole pressure. This is due to the rapid expansion of gas near the surface. Alterations in pump rate are to be made early on!