Minimium NPSH for Sucker Rod Pump

[zdas04]

I'm operating in a Coalbed Methane field where the reservoir pressure on most wells has been drawn down to under 90 psig. The flowing bottom hole pressure is around 20 psig. On many wells we have been using rod pumps to lift the water. We are frequently seeing pumps gas lock. None of the anti-gas-lock widgets has been effective.

I've always heard that rod pumps need 75-100 ft of water above the pump to overcome the slippage past the plunger. Since 100 ft of head would be well above our bottom hole pressure it looks like the rod pumps have passed the end of their useful life in this field.

My question is: Does anyone know of an objective calculation for minimum head above a rod pump?

Thanks

 

[KernOily]

Did you post this question over in the Petroleum production forum? Did you talk to your pump vendor yet? I am not a rod pump expert. I do know though that the reservoirs in this area (Calif. heavy oil) have BHPs of nearly zero, not including static head, and they rod-pump the snot out of these zones so there must be something you can do. Thanks!
Pete

 

[zdas04]

Isn't this the Petroleum Production forum?

So far I've only been able to talk to salesmen at the pump shops and haven't completly believed the responses. Still trying.

In the heavy oil, you're pumping the economic product, in CBM you're pumping the water to get to the saleable product. That difference really changes the economics of where you measure your bottom-hole pressure.

 

[KernOily]

Yeah, I said that BEFORE I looked to see which forum I was in... duuuuhhhhhhhh!

Did you look in any of the standard references, like the API monograph on rod pumping, or the SPE Handbook? Thanks!
Pete

 

[zdas04]

Haven't got to the API monograph yet, but have scrubbed the SPE (including the handbook [assuming you mean Bradley's lightwieght tome] and their new e-Library). I'm finding some good information on slippage, bypassed liquid, and unswept volume, but nothing that pulls all this together. Hope I don't have to try to derive the relationships myself.

David

 

[katyJackson]

In low pressure situations isn't the usual practice to set the pump intake below the perforations and keep the wells pumped off? That way you could set the pump intake, say, 200' below the perfs, have 200 ft. NPSH, and still have only casing head plus gas column pressure on the perfs.

 

[zdas04]

Putting the pump in rat hole is the common solution for low pressure, but our producing formation sits right on top of another productive strata (that typically has different ownership) and we have zero rat hole.

 

[ArtLift]

KatyJackson is right - normally, you'd put the pump in the rathole. I don't think NPSH is quite the way to think of it. If you pulled slowly & the water was degassed, you could pull the liquid level down to within an inch of the pump intake.

From my experience (and 20 psi is getting close to the normal operating limits -esp. for oilfield):

1) Put the pump as low as possible & look at extending a dip tube to the base of the perfs. You don't mention how much rathole you have. A dip tube will allow some of the gas to escape around the pump. But a dip tube will not "suck" water. Your pump intake setting depth will still determine the minimum flowing bottom hole pressure -- so get it as low as possible.
2) Watch your compression ratio on the pump. You want as little dead space as possible. Dead space is the space that the plunger doesn' sweep. You can have your pump manufacturer estimate your dead space.
3) Pay careful attention to the space out. I don't know how deep you are, but CBM usually is relatively shallow. Set the pump up so that it's initially "tapping down" on the standing valve & then pull up slightly. Check it again after it's pumping for a few hours & the rods are warmed up. You need to eliminate as much dead space as possible (to increase the compression ratio & help the pump process the gas).

If all that fails, then you're getting close to the limit. For your application, the "gas buster" valves probably won't help too much.