We have seen how Oliasoft WellDesign (OWD) through the parallelisation feature, enables the user to quickly and efficiently establish a range of kill mud weights and pump rates that will kill the given blowout, without fracturing the reservoir. However, this is only half the job done – the engineer will have to investigate if the required pump rate and mud weight combination can in fact be delivered by the mud pumps on the rig that is planned/available to drill the relief well.
The mud pumps’ capabilities are represented by the so called “pump curves” which show the pressure and rate output for the specific mud pumps and the pump liners they are set up with. OWD display pump curves for a variety of mud pumps, which can be chosen in a drop down menu.
The plot below shows the result matrix from a parallelisation run on a relatively challenging gas blowout. As can be seen, several combinations accomplish the target of killing the blowout without fracturing the well.
Two options are shown in the pressure plots below. The 1.6 sg and 12000 lpm case is the preferred option, as it kills the well quicker and stays further below the fracture pressure compared to the 1.7 sg and 11000 lpm case.
Given the casing design for the relief well, the next step is to establish if this pumping schedule can be administered with the rig pumps available on the chosen/available relief well rig.
During the kill operation, the mud is pumped down the kill & choke lines into the annulus between the drill pipe and casing of the relief well. Seawater is often pumped at low rate down the inside of the drill pipe to measure the pressure at the intersection point. Having control of the intersection pressure is crucial in order to know when the well is killed and to avoid fracturing the well.
The figure below shows two rig pump setups. The curve to the left is for three rig pumps lined up to pump kill mud down the annulus, and one to pump seawater down the drill pipe. As can be seen, our kill case (1.6 sg @ 12000 lpm) requires a higher rate/pressure combination (HHP) compared to what three rig pumps can deliver. The curve to the right shows a case with all four rig pumps aligned down the annulus. This is not an optimal setup as this allows for no backup and no pressure control down the drill pipe. Besides, as can be seen from the plot, four pumps down the annulus are still not capable of delivering the required rate at the required pressure to kill the well.
Pumping kill mud also down the drill pipe
The menu below shows the setup in the user interface for a standard kill, where a 1.6 sg water based mud is pumped at 12000 lpm down the annulus. We now know this combination will kill the well, but we also know that 3 rig pumps (2200 HHP) will not be able to deliver the required rate at the required pressure.
What if the fourth pump can be used to pump kill mud down the drill pipe? In order to obtain control of the intersection pressure, a PWD (pressure while drilling) tool must be included in the drill string.
By ticking off the “Pumping kill mud also down drill pipe” box, the user has the option to distribute some of the kill mud down the inside of the drill pipe. By doing this, one can lower the rate and thus the pressure loss in the annulus. The rate and pressure required are automatically plotted on two pump curve plots, in this case, 9200 lpm and 2800 lpm down the annulus (9 5/8” casing) and inside 5” drill pipe respectively, as opposed to all 12000 lpm down the annulus.
As can be seen from the plots, we are now within what the pumps can deliver, both down the annulus and the drill pipe.
We have now designed a kill operation, where the blowout is stopped, the fracture pressure is not exceeded and the pumping operation can be performed by the rig pumps from a single drilling rig pumping down one relief well.
Take a closer look at the Oliasoft WellDesign Blowout & Kill simulator
