Driller’s method and wait and weight method (engineer’s method) are widely used to circulate wellbore influx while maintaining bottom hole pressure constant. There are a lot of opinions regarding which method is the best for well control operation therefore this article will discuss about pros and cons of both methods.
The driller’s method requires two circulations to kill the well. The first circulation is to circulate influx out of the well with original mud weight. The second circulation is to kill the well with kill weight fluid. During the first circulation, the bottom hole pressure remains constant due to maintain drill pipe pressure constant while circulating. For the second circulation, in order to maintain constant bottom hole pressure, casing pressure is held constant while circulating kill mud to the bit. Once the kill mud passes the bit, the drill pipe pressure will be held constant until the kill weight mud is on surface and there is no sign of influx in the annulus.
Wait and Weight Method
The Wait and Weight method requires only one circulation. The influx will be circulated out while the kill weight mud is displaced into the well simultaneously. While pumping the kill fluid from surface to the bit, drill pipe pressure schedule must be strictly followed. After that the drill pipe pressure is maintained constant until the kill mud returns back to surface. Some people call the Wait and Weight method as “Engineer’s Method” because there are more calculations compared to the Driller’s method.
Comparison between Driller’s Method and Wait and Weight Method
This topic will cover about pressure profile both drill pipe and casing pressure while kill a well with wait and weight method.
Drill Pipe Pressure
Firstly, we will take a look at the drillpipe pressure. When kill weight mud is displaced in the well, drill pipe pressure drops as per the drill pipe schedule. Once kill weight mud is to the bit, drill pipe pressure is maintained until the well is killed. A pressure profile will look like the following chart.
This example demonstrates how to determine drill pipe pressure schedule for wait and weight method.
Current mud weight = 9.5 ppg
Pump output = 0.1 bbl/stroke
Well depth = 9000’MD/9000’TVD
Drill string capacity = 0.0178 bbl/ft
Surface line volume = 15 bbl.
Shut in casing pressure = 700 psi
Shut in drill pipe pressure = 500 psi
ICP = 1600 psi at 30 spm as kill rate
Please follow steps below to determine the drill pipe pressure schedule (step down chart).
There are some well control formulas that you need to know for wait and weight method.
1. Kill Weight Mud
KWM = OMW + [SIDPP ÷ (0.052 x TVD)]
KWM is kill weight mud in ppg.
OMW is original mud weight in ppg.
SIDPP is shut in drill pipe pressure in psi.
TVD is true vertical depth of the well in ft.
Note: Read more about it>> Kill Weight Mud
Kill Weight Mud or Kill Drilling Fluid Density is the mud weight required to balance formation pressure. The kill weight mud may be pumped into the well at different time depending on kill methods (Driller’s method, Wait and Weight, Bull head, etc).
How to determine kill weight mud?
With the following equation, you can determine this figure.
Kill Weight Mud (KWM) = Current Mud Weight + (SIDPP ÷ 0.052 ÷ Well TVD)