Annulus Gas Pressure

[Luiz001]

Dear Fellows,

In our field we have some wells that produce gas in the annulus, causing a pressure (142,23 psi)on formation, and consequently reducing oil production, up till now we just left the annulus valve venting this gas to atmosphere to reduce annulus pressure, but gas is gold nowadays and to stop this wastefulness I have an idea to install a small compressor unit (20 pcm) to take out this gas from annulus and discharge on flow line. What do you fellows think about ? The prototype is being installed today!!!

 

[dcasto]

A lot of producers in the US even pull vacuums on the caseing to help oil production. Sliding vane, screw, or even recips are used. I've seen vacuums up to 2)" Hg.

 

[Luiz001]

Thanks dcasto

 

[KMBush]

I am assuming this is on a well with a pumping unit. Am I correct? If so there are several methods to reduce the pressure on the annulus whithout losing the gas to atmosphere. As dcasto listed above there are means of compression that will move this gas and put it down the flowline. Most of these methods involve a seperate compressor which may either run off the produced gas or electricity. I might suggest a Beam Gas Compressor (BGC). We have used them in our field off and on over the years. They use the pumping unit walking beam as the prime mover and consist of one large double acting compression cylider. They custom build all units and have a rental/trial program that you can used to try the unit out before purchasing it. You can find more info at

http://www.beamgascompressor.com

One note to dcasto's comment of pulling a vacuum above. You should be careful because some valves will allow air to be sucked in when they are placed on a vacuum. And we all know that hydrocarbon gas and oxygen do not mix well under compression (aka BOOM). Leaky flanges and fittings can also allow oxygen to be pulled in. Let me know if you have any questions.

 

[zdas04]

KMBush,
You sound pretty knowledgeable, I've been looking for a documented case of an explosion related to vacuum operations for the last 15 years. I've never found a single one. A few years ago I did find an explosion that was initially called a vacuum-related problem, but after looking at it, the investigation root-cause was found to be improper start-up purge of the compressor piping (like most explosions at compressor stations). When I do the arithmetic I simply can't find a way to ingest enough air to get into the explosive range and still maintain a vacuum (i.e., the LEL of Methane in air is 4%, so for a compressor designed to move 100 MCF/d you would have to bring in 2.5 MMCF/d of air, I think you'd run out of hp first). Do you have a source for your "AKA BOOM" comment?

Ingesting air can create an increased corrosion risk on non-CP, non-pigged lines, but I can't find a real explosion/fire hazard.

The beam compressor idea is a good one. I've never run the pumps from Permian Production Equipment, Inc. so I don't know if they are non-lube or not. I've run a couple that required a can of oil to be placed on the Sampson post and someone had to climb up once a week and fill the can. These had a really short run life because the pumpers kept "getting busy and forgetting to fill the reservoir". I've had good luck with the Basil Beam Compressor

http://www.basilinternational.com/

which uses non-lube materials within the compressor and I've had them run for years without service or lubrication

Another option would be to send the pump discharge through an eductor that sucks on the casing. It probably won't pull as low as a beam compressor can, but it should be trouble free (eductors are thermocompressors that use an incompressible power fluid, not terribly efficient with gas on the suction side, but could easily be efficient enough).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[KMBush]

zdas04,

Thanks for taking a good look at it. I suppose you are correct. I just know that we try to eliminate any and all leaks to prevent the possibility. The idea of oxygen in our gas stream is one we do not prefer.

We has one event a few years ago in which a PSV (set at 5 in H2O) hung open and allowed a great deal of air to be drawn into one of our tanks. The combined vapors were removed from the tank during normal operations with several vapor recovery units (VRU). The resulting mixture created no BOOM but instead generated a fire inside the gas discharge line that extended for close to mile from the VRU's. The pipeline had been in service for some time and had a great deal of Iron Sulfide build up. The resulting fire heated the outside surface of the 6" line enough to catch surrounding brush on fire. The failing PSV was determined to be the culprit.

It is very likely that you would see no problems in most vacuum operations if everything is operated correctly. Just as you were saying with compressor explosions there is always the ability for operator error. You never know when someone is going to leave a suction blowdown valve open!

 

[dcasto]

I've worked vacuum casing head systems for 20 years now (yikes) and no explosions. We had producers that would open up a 1" valve on their wellhead as the pumper went home at night and then close it when he went to work in the morning. All night long, lots of air coming in. Up until the late 90's I've never seen anyone happy with the beam pumps (compressors). The best solution is a small casing head gathering system with a central compressor pulling a vacuum.

The newest trend in the gas world is to limit the O2 to 100ppm. There are a few pipelines that have 2000 ppm limits. The reason the pipeline says 10 ppm is the limit is for safety BS!!!! O2 in a gas line is not a safety issue at 2000 ppm. But the pipelines convince un-enlightened regulators and they win. (another one of those remarks that should ruffle a few feathers).

 

[zdas04]

One of the gatherers in NW New Mexico is on a full-time mission to force a 10 ppm standard, contracts-be-damned. They say it is because of "hugely increased corrosion risks". When I demonstrate to them that they could take the oxygen-related corrosion risk to nearly zero by pigging the lines or by maintaining the installed CP they generally say "next question". [Oxygen can set up a micro cathodic cell in standing water, but if you get rid of the water or impress a current on the pipe to maintain the integrity of the passivation layer then the cells just can't use the pipe as the anode] There is an amazing amount of fear and superstion in this stuff.

David

 

[dcasto]

Here's what I know about the 10ppm limit. There is 1 major pipeline that stores gas in the upper midwest in underground aquafiers. They never had an O2 limit. O2 was pumped into the storage. As they came out of storage, they also produced water at rates over 20,000 bbls/day. The water was trucked and piped around to aboveground tanks before reinjection back to the formation.

Jump ahead 20 years later. The gas coming out of storage was contaminated with H2S at over 10 ppm, up to 100 ppm from some wells. Aw, the biologist said that bateria underground went from anerobic (sic) to anarobic (sic). So they blamed the gas gathers and set a new tarriff based on 10 ppm O2. What about all the air from hauling the water? Who put the bacteria down there in the first place?

Since all the pipeline interchange with each other AND with the FERC staying out of the WOBBIE INDEX and gas interchangeability with LNG, the pipelines are using the stance that they have to meet the most stringent specifications.

I always tell the pipelines I want to deliver my gas to the Denver front range market where they inject 10% by volume air into the gas a 700 psig before entering the city gates or TBS's. That way I only have to meet a 2% O2 spec.

 

[DrillerNic]

If you look at the way that most offshore platform remove all teh oxygen from the injection water 9to prevent corrosion of teh injection wells) and then, as a consequence have to make a huge effort removing any bacteria (to stop souring of the reservoir), to reinject raw produced water, after trucking it to above ground tanks and storing it there for a while does sound pretty stupid!

 

[zdas04]

dcasto,
Can you tell me which companies inject 10% air into the sales gas stream? I'm working on a project where that information would be amazingly useful.

David

 

[jjwamp]

I looked the the Basil Compressors for installation in the old shell field in michigan, even talking to Basil himself. Problem up there is the cold weather causing freezing. We were afraid that ice or salt may form and cause problems. Anyone had problems with this?

 

[zdas04]

Basil is certainly an interesting character. The ones we installed were at 6500 ft, so winter was definitely a factor. We never had any freezing or salt issues with the beam pump (salt was a problem other places in the field, but not there).

David

 

[dcasto]

the Public service company of Colorado. They presented a paper in the late 90's titled "To Air is Human".

 

[zdas04]

Thank you. I just did a Google search and the first entry was the last time you told me that here at eng-tips.com. A good mind is a terrible thing to lose.

David

 

[dcasto]

I remember being amazed that the produced water that got in our gathering system from separators hanging up didn't freeze up in Montana in the winter. The water was loaded with salt. Yeah we did get freeze ups, but not at the rate or the temperatures you'd expect.