In this chapter we will cover free water knockouts, including:
The term "free water" refers to the water that is not entrained in the emulsion coming from the wellhead.
The specific gravity of free water is heavier than the rest of
the fluid in the emulsion and drops to the bottom of the vessel easily.
A free water knockout is usually the first vessel that the well stream enters.
Typically, you will have additional separation vessels downstream that will further separate the emulsion.
There is not an economical use for the water at this point in the production process.
There is no benefit to using energy to
treat or separate this fluid in another vessel especially when there are high volumes of water.
Removing water at this stage in production allows producers to
build smaller vessels downstream to reduce the cost and cut down on the
amount of fuel needed to heat the downstream vessels.
After it is removed from the FWKO, the water is either recycled or sent to a water disposal system.
Free water knockouts are mainly found in the upstream sector of the energy industry.
Different production fields have their own criteria as to when to use a free water knockout.
There are multiple sizes of these vessels and knowing the fluid volume and the fluid properties is
important to getting the correct vessel on location.
Shale Plays produce large volumes of water. The Mississippi lime play is an example of a high volume
water producer some wells in the area can have a 1% oil cut meaning they get
one barrel of oil for every 100 barrels of water produced.
Mechanical separation is the most common design in free water knockouts.
When the oil gas and water entered the inlet of the free water knockout, mechanical separation inside
the vessel helps separate the emulsion.
Inlet diverters caused the emulsion to break up and separation to begin.
It reduces the disturbance from the inlet
fluids and keeps them from creating a channel through the vessel.
A quieting baffle is located about halfway down the length of the vessel.
As the flow comes through, the holes in the baffle slow down the motion inside the vessel.
In addition to mechanical separation, retention time, or the amount of time a
fluid stays in the vessel, is important in the natural separation of the elements.
In a 2-phase free water knock out, free water is separated from the emulsion.
The free water leaves the vessel through the
dump valve and the oil gas emulsion exits through the connection near the top of the vessel.
In the 3-phase design, the free water, emulsion, and gas
are all separated from each other in the same vessel. In this 3-phase free
water knock out the free water comes out of the right side dump valve,
the oil emulsion leaves through the dump valve on the left side, and the gas goes out at the top of the vessel.
How can we tell?
First, look at the sight glasses.
The one on the right is clear, indicating water, while the one on the left contains dark oil.
Also, notice that the dump valve on the right is larger. Because the majority
of the fluid coming out of the free water knockout will be water, the water
side will have a larger dump valve than the one for the oil.
2-phase knockouts are more economical to build and maintain but must be used with
additional vessels for further separation. 3-phase is more expensive
to build and maintain but can work in conjunction with other equipment or independently.
Working pressure is the maximum pressure the vessel can
withstand, which for free water knockouts is typically under 250 psi.
Operating pressure is the pressure at which the vessel has been adjusted to for the
process to be completed and the fluid produced to be moved to the next destination.
Free water knockouts are typically
operated at lower pressures due to the use of hollow floats which can collapse
if over pressured. The controls used on the vessel can be mechanical pneumatic
or a combination of both.
Producers will use pneumatic controls on the water outlet in order to get more
accurate measurements if the fluid is being metered.
Please note the inlet and outlet connections and the products used on this 3-phase knockout.
A trunnion and weighted flow are used to control the free water dump valve and another
trunnion and float are used to control the emulsion dump valve. The gas goes out
through the top of the vessel and through a back pressure valve.
This concludes this chapter. Thanks for taking this training.