Gas well deliquification using PCP

[koma72]

Gents,
further to my message about the PCP trial to dewater a gas well, here are some more information:
Reservoir: Gas
Perf. Depth: 1714,5-1717,5 m
Static fluid level: 200m
Expected gas production @ 1200-1400m dynamic fluid level
Gas Prod.: 700scbm/d
WaterProd expected.: 50cbm/d
Pump depth 1680m
H2S: 0
CO2 0,1%
Installed PCP: NTZ350*200*ST25
Rotor: P6
Stator(elastomer) HNBR01

The PCP was installed twice and failed after couple of days operation due to parted rotor @ 150 rpm. The elastomer was found to have excessively swollen.
Does anybody have experience with this type of gas dewatering and can help us to analyse/discus the problem?

 

[koma72]

sorry the expected gas rate is 7000 scbm/d not 700 scbm/d

 

[JAFCO]

This is likely pushing the depths for PCP pumping, but the first thing that glares out at me is the pump setting depth. I would expect a high volume of gas to be entering your PCP, which would lead to premature failure.

I would first look at getting the pump below perfs. If not possible a gas anchor would be beneficial.

Also because of the depths, I would seriously consider a Beam Pumping system.

 

[lonster]

The depth limit of PC pumps will depend on the requirement for lift and volume and the confinement from the casing (or tubing if they are insertable) as they get unmangeable in terms of length at some point. There have been successful applications as deep as 3000 m but most of those are low to moderate rate applications. The required fluid rate and lift in this application are not unreasonable and I have seen similar successful dewatering applications.

One thing I would strongly agree with is the need to get the pump intake below the perforations to minimize the gas coming through the pump. Running the intake above the perfs will result in significant free gas inflow and will reduce the pump efficiency resulting in additional heat that must be dissipated by what will also be reduced mass flow do to the inefficient operation. If the pump begins to heat up then the torque will increase and in the absense of poor lubrication it can be quite severe. Was there any particular reason for landing the pump above the perfs?

With regard to your failure in these dewatering applications rod problems are quite common due to underestimating of the friction as well as harmonics that often occur due to stick and slip in the pump especially if it is not operating fast enough (my experience is that low speeds are bad here and that you want to be in the 150 to 250 RPm range) and if the rotor/stator fit is too tight due to swell or heat generation. You mention you had a rotor failure which is unusual as normally the rod string is the weak point. Was this rotor failure at the connection, near the top in the profile or elsewhere such as in a weld? If near the top it may be from improper space-out (either too high or too low, this can usually be determined in a post inspection of the rotor and stator). If in the connection or at a weld may be due to high torques due to what you indicate was excessive swelling. Elastomer swell in dewatering applications is usually not a problem unless (1) the wrong elastomer is chosen and the temperature are high enough to escalate the water swell or (2) you are seeing explosive decompression effects from the CO2 and CH4. A detailed pump inspection can quantify the extent of the swell and isolate whether it was due to the fluid or the gas. According to your information you ran a hydrogenated nitrile elastomer which is not the normal choice for dewatering or any PCP application for that matter except where there is H2S or high temperatures (above 100C). I don't have any specific experience with that particular elastomer to know how it would react to water but normally in these dewatering applications With you are best to stick to a low ACN nitrile to minimize the water swell.

If you want to provide answers to the questions above as well as some insight on your downhole temperature, casing size, rod string size and operating speeds I might be able to share some more specific experience.

Lonnie