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
Wellbore Problems While Killing the Well
In many places, wellbore instability is one of major wellbore issues. If the drill string is kept in a static condition for a period of time, the pipe can get stuck easily. For this situation, the Driller’s method will give you a better chance to successfully kill the well and minimizing wellbore collapses and pack off than Wait and Weight.
For W&W method, kill weight mud must be prepared prior to circulation therefore the drill string is in static condition with no circulation for a while. There is high chance for wellbore to collapse and pack the drillstring.
Casing Shoe Pressure
Shoe will be exerted the maximum pressure when top of gas kick is at the casing shoe. Once the gas pass the shoe, the shoe pressure will remain constant. The W&W can reduce shoe pressure when the kill weight mud goes into the annulus before the top of gas arrives at shoe. If you have larger drillstring volume than annular volume, you will not be able to lower the shoe pressure using Wait and Weight method. However, if time to prepare the kill weight mud is very long, gas migration will increase shoe pressure. There will be a possibility that using W&W can create more shoe pressure due to gas migration while preparation of drilling mud.
Nowadays, oil-based drilling fluid is widely used for drilling operation. Gas will be soluble in oil based mud and it will not be able to detect at the bottom. Gas may expand when it moves almost to the surface and it is often above the shoe. Hence, W&W will not help reduce shoe pressure.
Capability of Fluid Mixing System
Around the world, there are a lot of drilling rigs which don’t have great capability to mix drilling fluid effectively, therefore, kill weight mud cannot not be mixed as quickly as the operation required for killing the well using W&W. The Driller’s Method will not have this issue because the circulation can be performed right away. Waiting for preparing kill weight mud for a long time can lead to increasing in shoe and surface pressure due to migration of gas.
Well Control Complications when Bit Nozzles Plugged
If the bit nozzles are plugged during the first circulation of Driller’s method, drill pipe pressure is allowed to increase temporary by maintaining casing pressure constant until the drill pipe pressure stabilizes and then the new circulating pressure. During the second circulation of Driller’s method, if the plugged nozzles are encountered, casing pressure must maintain until the kill mud to the bit and then change to hold drill pipe pressure shown on the gauge.
While killing the well using W&W method, if the bit nozzles are plugged, the drill pipe schedule must be recalculated as soon as possible. If the new pressure schedule is not properly determined, the well can be unintentionally underbalance resulting more serious in well control situation. The situation will be more complex, if the well is highly deviated with/without taper string because it is quite tricky to calculate.
Well Ballooning Issue
Well ballooning effect is a natural phenomenon occurring when formations take drilling mud when the pumps are on and the formations give the mud back when the pumps are off. When ballooning is observed, it must be treated as kick. If W&W is utilized to manage this issue at the beginning, the additional mud weight can increase complexity of wellbore ballooning situation. More mud weight can induce more mud losses and the situation will be worse. Since the Driller’s method does not require additional mud weight hence there is no increasing in wellbore pressure. Therefore, the ballooning situation will not become worse.
Hydrate in Deepwater
Deepwater condition is high-pressure and low-temperature conditions which are ideal case for hydrate. Therefore, there is a high chance of hydrate formation in choke/kill lines and BOP when gas influx is taken in a deepwater well. Driller’s method will minimize hydrate issue because the circulation is established as soon as possible. The mud is still warm and the hydrate issue can possibly be mitigated. Conversely, killing the well using wait and weight method requires longer time to shut in because the kill mud must be properly prepared prior to circulating. The static condition will make the mud cool and it is a favorable condition for hydrate formation due to decreasing in temperature of drilling fluid.
Time to Kill The Well
The Wait and Weight method requires only one circulation but the Driller’s method requires two circulations. In the real well control situation, you may need to circulate more than one circulation therefore W&W may just save a little bit rig time compared to the Driller’s method.
Hole Deviation and Tapered String
For the Wait and Weight method, the drill pipe schedule must be calculated. It is very simple to figure out the schedule if there is only one size of drill pipe and the wellbore is vertical. However, nowadays there is little chance that you will drill a simple well like that. The drill pipe pressure schedule becomes difficult and complex in complex wells with multiple size of pipe. Without computer program, it is very difficult to do hand calculations to determine the right schedule. This can lead into more problem while performing well control operation because the bottom hole pressure can be unintentionally over or under balance.
Driller’s method has more advantages than Wait and Weight method. It is a preferred way to kill the well for many operators. The calculation is simple and operation is easier for crew to follow on the rig. The Driller’s Method also can reduce operational issues which may happened in well control as wellbore collapse, hydrate, etc. This method will not shut in for a period of time therefore gas migration effect is minimal. When the complication is observed, controlling the well using Driller’s method will not need any additional calculations but if the W&W is used, the new drill pipe pressure schedule must be properly recalculated.
W&W can achieve lower casing shoe and surface pressure in some situations; however, it has more complexity in calculations and operation Due to gas migration the well is shut in, there are several cases when W&W will not lower shoe pressure. Additionally, W&W can give you higher shoe pressure due to incorrect drill pipe pressure schedule. If you take a kick in deepwater well, using W&W can increase chance of hydrating the BOP and choke line.
In our opinion, Driller’s method is better than Wait and Weight for well control.
What is your opinion?
Reference books: Well Control Books