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Hydraulic Oil Selection

Hydraulic Oil Selection

Hydraulic Oil Selection

I'm evaluating a non-return hydraulic system that will run vertically 15,000 ft down a natural gas well.  This system has a switching valve on the surface and it vents one line while pressurizing the other line to move a downhole hydraulic piston that is driving a piston pump.

At this depth in 1-inch tubing the two big issues are oil specific gravity and compressibility.  SG relates directly to the amount of pressure that I have to design the hydraulic cylinder barrel for.  Lighter oil means smaller wall thickness and I can either fit inside a smaller wellbore or can pump more water from the well.  This is pretty important, but if I change from the oil I'm using today (SG=0.85) to the lightest oil I've found (SG=0.802) then I can only carve out 1/16 inch of wall thickness, good, but not wonderful.

The bigger issue is compressibility.  The oil I'm using now has a compressibility of 0.4%/1000 psi.  This means that if I apply 3,000 psig to 400 gallons of fluid I need to make up (including the pump travel) 8 gallons.  At 3 gpm it takes nearly 3 minutes to stroke the pump each way, virtually all of it spent compressing the oil.

Does anyone know of an oil (I'm using ESSO Nuto-A10) that might be lighter and less compressible?


RE: Hydraulic Oil Selection


Try posting on www.noria.com.  You might get some input there.

RE: Hydraulic Oil Selection

I think your solution must be to get a larger pump.  You have to consider also the expansion of the tubing absorbing oil volume.  Tube expansion will increase as you make the wall thickness thinner.


RE: Hydraulic Oil Selection

As always, there are a number of trade-offs.  The pump I'm using can go to 12 gpm, but above 8 gpm friction losses in the top of the pipe take pressure too high (within 2 seconds of switching in 1-inch pipe at 8 gpm, pressure jumps to over 3,000 psig, when I drop to 4 gpm the pressure never exceeds the pressure required to move the piston).  Going to bigger pipe to avoid this friction problem increases the volume of oil that must be compressed.  I would far rather find a less compressible oil so that I don't have to pump so much of it.


RE: Hydraulic Oil Selection

Is it possible to locate the switching valve at the bottom of the hole and run Electrical signals to it for switching?

That way the oil would alrady be compressed at the pump and the cycle would not have the 3 minute pause you are experiencing.

Bud Trinkel, Fluid Power Consultant

RE: Hydraulic Oil Selection

Switching valves that are only accessible by a $400k pulling job have not been terribly successful in this industry.  The environment is 15,000 ft deep in the ground.  It is hot, high pressure, and really nasty.  I recently saw a pump with a downhole switching valve that ran 2 hours, they pulled it, fixed it, ran it back in and it failed in 3.5 hours.  The client has been happier.


RE: Hydraulic Oil Selection

"I recently saw a pump with a downhole switching valve that ran 2 hours, they pulled it, fixed it, ran it back in and it failed in 3.5 hours.  The client has been happier."

Was the operation improved??

By how much??

What Failed in that short time period??

Was the improvement great enough for you to try other valve designs??

It seems that if a Piston Pump can operate in that atmosphere a switching valve could be made that would also. Design one that works and patent it and charge big bucks for replacing the unsatisfactory arrangement presently in use.

Just a thought.

Bud Trinkel, Fluid Power Consultant

RE: Hydraulic Oil Selection

You need a synthetic oil to resist the high temperature down hole. Check out Anderol 500 by Tenneco. Used in High pressure compressors up to 5k psi. Wonderful stuff, not strictly a hydraulic oil ,but will perform.

Offshore Engineering&Design

RE: Hydraulic Oil Selection

Temp is an issue, but so is SG and Bulk Modulus.  Anderol 500 has a SG of 0.96 which adds over 1,000 psi to the required MAWP.  I haven't found the Bulk Modulus of Anderol yet.  

I might as well use water (which has a bulk modulus over twice as high as mineral oil so about half the compressibility and no degradation with increased temp).


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