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Arty121 (Chemical) (OP)
22 Oct 08 0:30
Dear experts,

I would like to ask about Electrical Submersible Pump (ESP) operation in the offshore industry.

My understanding is described below.

The normal operation of ESP is that the pump will separate gas and liquid, then liquid will be pumped up through the tubing.

Gas will be separated into the casing and therefore a casing vent is required for this casing gas.

I was told that we have to keep the liquid level over the ESP by venting casing gas as much as possible (to lower the casing gas pressure) but I'm still not quite sure.

So, my question is

"Do we have to lower the casing gas as much as possible to enhance the ESP production? If so, what is the particular reason(s) for doing that?"
BigInch (Petroleum)
22 Oct 08 1:44
You have to keep the pressure (liquid or gas pressure) downstream of the pump as low as possible, so the pump can pump into that resistance at maximum possible flowrate and thereby allow the flow to seep from the formation into the hole.

Venting gas pressure happens first, since gas rises to the top of the oil column.  If you vent all the gas out, then you'll get oil.  If you vent all the oil out, then maybe you'll get water.

"Make everything as simple as possible, but not simpler." - Albert Einstein (1879-1955)
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http://virtualpipeline.spaces.live.com/

zdas04 (Mechanical)
22 Oct 08 21:24
An ESP will not separate gas and liquids.  The turbulence within the pump is so high and the centrifugal traverse is so short that gas bubbles (not disolved gas or phase changes) will just get smaller as the pressure increases through the pump.  If you have too much gas then you'll either overheat the pump from lack of lubrication or overheat the motor from lack of cooling.

The Net Positive Suction Head required (NPSH-r) for an ESP is a function of several pump parameters, but you can say for certain that the higher the capacity of the pump (in volume per unit time), the more pressure you need at the pump inlet (the higher NPSH-r).  A mid-range pump will require about a hundred meters or so of head above the pump inlet.  As casing pressure increases, the apparent head increases (3 bar increase in casing pressure raises the NPSH available by 30 meters), but the actual liquid level in the casing will tend to go down as this "extra" pressure forces liquid back into the formation.  In moderately extreme cases, the casing pressure can push all the liquid back into the formation and introduce a very large quantity of gas into the pump which won't run for long.

In general, it is best not to rely on "artificial" head created by casing pressure and to keep casing pressure as low as practical.

 

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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Arty121 (Chemical) (OP)
27 Oct 08 1:04
Thanks for your valued replies.

Art
chemicalengineerthai (Chemical)
3 Dec 08 9:00
1) To prevent pump run dry
2) To gain pump efficiency
3) To improve NPSHa
 
d23 (Petroleum)
15 Feb 09 20:54
Arty121

I believe there may be a little confusion about submersible pumps.  In reality there are pumps all over the world operating successively under packers without venting.  With today's technology under ideal conditions you could produce two-phase flows with gas-liquid ratios above 80%.  Usually we do not have ideal laboratory conditions, but 50 to 60 percent usually is not a problem.  It all goes back to velocity of fluid inside the pump.  Small casings with small pumps will be limited to lower flows while larger casings with larger pumps allow higher flows.  

If you vent a well using a gas separator you can approach 90 percent efficiency under low flow conditions.  Increasing the volume and velocity will lower the separator efficiency.  Submersible separators will be vortex or rotary types where the rotary is a true centrifuge that works quite well.  Rotary separators are a little more efficient, but a little less reliable.

In any case I would suggest you talk to several ESP manufactures to get the facts about a specific application.  Pump behavior under specific operating conditions is very predictable.

D23  
 

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