Hi, I'm Mark with Kimray, where we help energy producers solve their largest control problems.
Today, we're going to follow the pipe on an oil and gas well from the
wellhead to a free water knockout.
This is a well head.
This well is being operated by an electric submersible pump, or ESP.
One of the identifying features of an ESP is the power cord – this flat ribbon that comes into the
wellhead which is providing electricity all the way down to the bottom of the
well to run a pump.
In this case, on this location, the bottom of the well is
somewhere around 7,000 feet deep.
Locations using ESPs are usually
producing very high volumes of fluid that a normal rod pump can't produce
The fluid comes up through the tubing goes down through the rubber hose into a
flow line system which takes the fluid from the well down to the central tank battery.
And that's where we'll go next.
This is a flow line coming from the ESP well
that we identified earlier. So, all the production fluid is coming through
this line into the header system.
As we follow the pipe we see the main
production header here and a test header below.
The valve to the test header is
closed and the valve the main production header is open.
This means the well fluid
is going into the main production header through this pipe and then into the free water knockout.
Free water knockouts are designed to help us remove excess free water.
Removing the water at this first stage of separation allows us to
build a smaller vessel downstream which reduces the cost of the producer and
also cuts down on the amount of fuel needed to heat the downstream vessels.
Here's the inlet line to the freewater knock out. All the oil, water and gas and
the flow line is coming into the vessel at this point
An inlet diverter causes the emulsion to break up in separation to begin.
A quieting baffle is located about halfway down the length of the vessel.
As flow comes through, the holes in the baffle slow down the motion inside the vessel.
The free water knockout will be about halfway full of water. Above that will be
oil emulsion and then the remaining volume filled by a blanket of gas.
Now we'll look at how these three elements – oil, gas and water – are separated and sent downstream.
First, we'll explain how water is dumped from the free water knockout.
This is a trunnion. Inside the vessel it's attached to a float arm and flow ball.
The float ball is set up to hold an interface,
where the oil and water emulsion meet inside the free water knockout.
This sight glass indicates where the water level is inside the vessel.
The producer has set a marker right here that indicates to him the normal level where water and interface meet.
As the water level rises, the float ball will go up
which will use this mechanical linkage to raise the lever on the mechanical
dump valve and send the water out of the knockout downstream to be processed further.
On this location, the producer has a separate water disposal system.
This is the oil emulsion outlet. The dimensions of this vessel allow us to
use a weight operated treater valve because there's more than a four foot
downcomer pipe. This producer has decided to use one of our spring-loaded
back pressure valves to control the oil outlet.
So, with proper spring range, this
valve is holding approximately 60 pounds of pressure on the whole system.
That translates back all the way to our flow line and header system, and then back to
the well, and all the way down to where the ESP is located.
This is a very important valve.
As the pump creates pressure against this spring the set point is reached.
In this case, about 60 pounds. The fluid will open the valve and
send the oil emulsion downstream to be processed further.
This line comes over
here and at this point the oil is not yet sellable. The next step in the
process is to send the emulsion to a heater treater.
So now that we've gotten the oil emulsion and the water out of the free water knockout, let's look at the gas.
The line off the top of the vessel is the gas outlet.
The producer has another spring loaded back pressure valve installed with a pressure setting
slightly higher than the oil outlet valve. The gas will now go to the gas sales system.
Typically we do not sell gas off of free water knockouts.
There's no mist extractor inside the vessel to knock out liquids entrained in the gas.
If the producer decides to sell the gas, he will have to install a scrubber downstream to remove these liquids.
Free water knockouts must be properly sized for the volume that's anticipated to flow through the vessel.
If you undersized the vessel you'll have carryover and cause problems for separation down the line.
One of the important influencers of free water knockout sizing is retention time.
As you see in this fluid sample, after it's been
agitated it takes a certain amount of time for the water to settle out of the
emulsion – that's called retention time.
Proper retention time allows free water
to settle out of the emulsion, resulting in higher quality emulsion leaving the vessel.
The controls on this vessel are set up to throttle. If the producer wants to meter
the fluids, snap controls may be used.
This is a three-phase free water knockout being used for test purposes on a set schedule.
The meter measures the amount of water being produced by this single well.
After it's metered, the water connects to the water disposal system. The emulsion from
this knockout is sent to a storage tank to be gauged. After comparing the records
the producer decides as a well needs to be reworked.
The larger vessel on the other end is designed to handle more fluid if necessary.
There is a high volume of fluid to be maintained and controlled and that's what a free water knockout
does – it allows a producer to control, monitor, and help dispose of the water
without having to handle it unnecessarily.
One last element of the
free water knockout we'll mention is the sacrificial anode. The rapid movement of
saltwater inside this vessel creates static electricity which causes
corrosion. Sacrificial anodes are installed in order to prevent corrosion
to the shell of the free water knockout. These rods extend into the vessel about
five feet and each one is surrounded by a block of magnesium or aluminum or
combination of the two materials.
These are called sacrificial anodes because
they are eaten up by the corrosive salt water and sacrificed instead of the
shell the vessel being eaten up.
Want to learn more about oilfield equipment?
Watch the next video in our Oil & Gas 101 series about rod pumps, two-phase
vertical separators and heater treaters.