Light hydrocarbons, Summer Temperatures causing gas to develop in piping 2

[jharris3]

I am an ME commissioning a small oilfield produced water filtration plant and we have some problems with gas developing in the suction lines to the pumps. I would like to know what sorts of chemical additives could be used to help the gases separate out of the produced water upstream in the associated Oil Cleaning Plant (OCP) if any.

The OCP is a "light oil" plant that has some deep wells where fuel gas injected for gas-lift. Operations claims to remove the gas at the gage setting before the OCP. The OCP is a Wash-Skim-LACT tank configuration, however, the Skim tank is out of service for nearly two years. We have already made plans to clean the skim tank ASAP and put the plant back into a more normal configuration or at least into a more clean tank. We have in the meantime installed a connection to the wash tank that is on the outlet side of the tank.

The holdup time for the PW in the Wash tank is between 5 and 7 hours normally, but due to the solids buildup in the tank the holdup time is down to 4-6 hours (based on interface). When the water reaches the injection plant (WIP) there are two surge tanks that are connected together and operate with approximately 4-6 hours more hold up time combined. After the filters (walnut shell - adsorbers) there is an equally sized tank for the injection water that operates at a higher level so that the hold-up time would normally be 5-8 hours.

With all that hold up time gas is breaking out in the suction piping to all the pumps. The injection pumps aren't even running but a pretty good amount of gas breaks out, and no I haven't been able to obtain a consistent result of how much gas breaks out for a given volume of liquid. All I can say is that the suction lines are not sloped back to the tanks sufficiently and about 1"-2" of space in the top of the pipe fills with gas, but for what length of pipe I don't know. After I bleed the line I can come back in a few hours and bleed nearly the same amount of gas off again. This can continue for over a week.

I have had some testing done to tell us what is in the PW which by the numbers appears to be C10-C36 in various ppm quantities mostly C10-C20. The gas breaking out is 85% methane (mol), 2.42% ethane, 1.195% propane, 1.653 % hexane, and 6.154 % CO2.

Can an additive be used to help the gas break out faster so that it does so in the tanks rather than the pipe? Can ana additive be used to keep the gas from breaking out at all? This is preventing performance in the pumps.

The pumps we realize are high energy pumps (Nss = 12000) and require a 1.7 Margin on NPSHr and that is likely part of our problem. We might persue some other options with the pumps but for now I would like to know what can be done on the process side.

 

[zdas04]

In direct answer to your specific question--you're kidding right? No there is not a chemical that will keep gas from breaking out of your produced water. The mix of gases you are getting are probably (the mole percentage you gave adds up to 111%, I normalized it) 50% by mass entrained gas and 50% flashing liquids (the dissolved gas will be a really small number according to Henry's Law). There are chemicals that can be added to change the boiling point of a liquid, but they tend to change the chemical composition of the chemical and can be really hard to get out of the process. There are no chemicals that can force the disengagement of an entrained gas.

What you need is a more effective water/gas separator. I have an example of one on my web side (under "Samples", right column at the bottom) if you want to see how it is done. That vessel is patented, but it should show you something about the dynamics of the process. That vessel is currently undergoing tests as a desilter (the CFD models showed the velocity is very low in the bottom of the vessel and the test is showing it to be really effective at removing solids).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[LittleInch]

The only "additive" you could look at is heat somewhere upstream of your final tank. That would encourage release of gas from the liquid before you cool it a bit in you final tank. Other option is to reduce pressure to below atmospheric in one of the tanks as well. I have never heard of a chamical additive to release gas.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Compositepro]

How do you bleed the lines now? Place an automatic gas vent valve there.

 

[racookpe1978]

I guessing here about his field arrangement, but could he add a surge tank before the large pumps: It may be the original purpose of the tank now getting clogged. With a tall surge tank, the suction pressure of the process pumps can be restored to original expectations.

Then, with today's atmosphere release restrictions, you can't expect to just vent off the gasses coming out of solution, so you need to add a small (vacuum) pump to the top of the surge tank pulling the entrapped gasses out of the top of surge tank, which are subsequently either burned locally to provide power for the unit's service needs, or compressed and stored for disposal or recycling or processing.

 

[cloa]

I just added up the mol%-85+2.42+1.195+6.154+1.653=96.422%- I assume the rest is C7+ and inerts and water vapour (were other gases tested for?)

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[BigInch]

Ice

Independent events are seldomly independent.

 

[jharris3]

Cloa and zdaz: The gas test I gave data for was for ONLY the gas that was removed from the pipe. Mole, weight and Lv were given in the report but I only put the approx. mole % in my description because the goal was to simply show that for a non-process guy like myself it is very confusing to have gas continue to break out that is overwhelmingly methane even after the bulk liquid has had up to 18 hours to settle at temperatures between 85 and 110 F. Yes the balance of the fluid in the pipe was a couple other lights up to C6 and then C7+ and water vapor. The C7+ primarily remained in the water as a liquid. Liquids C9-C32 were a total of 30 ppm.

zdaz - no...I'm not kidding. I asked the questions that I needed some answers to. Since there are products that break the surface tension of a gas/oil mixture it seemed reasonable from my green experience to ask for the first time ever if there was a product used to make the surface tension of a gas and any other fluid greater. There isn't for this type of application which matches the answer from two other sources that I have. However, there are de-foaming agents that may help improve the release of gas in the Wash Tank. As for solids removal the hold up time in the wash tank + baffles means that we often have to clean the tank very often because of how quickly the solids build up.

racookpe1978: normally there are two tanks at the beginning of the OCP and our water would come from either tank or both depending on the interface level/needs of operations. Taller tanks would be nice in the WIP however, the goal in this case is to avoid a major addition to the plant first. If we need more equipment then we would add some kind of Micro Bubble Floatation or Induced gas floatation etc or maybe even switch to WEMCOS. Also, we do have a vapor recovery system installed already - however the bolted tanks don't take vacuum very well. Operations has plans farther in the future to install welded tanks. The vented gas is compressed and used for gas assisted lift of the column of oil in the well.

BigInch: That is effectively what mother nature gave us last weekend when our ambient dropped and the gas problem went away. That or the better connection downstream of the outlet baffle. I guess we will find out next summer. I agree with your quote by the way.

Compositepro: thank you - we are planning to do something like that and its good to see that is a valid approach for a near atmospheric suction line.

LittleInch: Thank you - we may push for ops to turn on their heaters upstream and hope the water cools enough when it reaches our plant. We may also insulate the lines in our plant to keep the sun off of them during the summer.

 

[BigInch]

Can pumps or vane pumps may offer future solutions.
You need something with low NPSHR or to increase the NPSHA.

Independent events are seldomly independent.