Gas Hydrate Concern -- Need real world experience
Gas Hydrate Concern -- Need real world experience
(OP)
Hi all,
I was wondering if anyone could weigh in on the subject of "when do hydrates REALLY become a problem". I need real world experience with gas production to balance what ProMax is predicting for hydrate formation.
I am currently studying a gas pipeline which will be transporting about 5 MMCSCFD (12,750 lb/hr) at lower velocities (5-20 ft/s) underground for about 10 miles. The ground temperature is expected to reach 60 F, and ProMax is giving warnings about hydrate formation being reached around 67 F.
There is about 20 lb/hr of water entering the pipeline with the gas, so even if this does "freeze", will it really make a difference in actual operation? Will the hydrates necessary plug the pipeline or can the be entrained and moved with the flowing liquid? Do some operators "roll the dice" and see if a problem exists before deciding to inject methanol?
Thanks
I was wondering if anyone could weigh in on the subject of "when do hydrates REALLY become a problem". I need real world experience with gas production to balance what ProMax is predicting for hydrate formation.
I am currently studying a gas pipeline which will be transporting about 5 MMCSCFD (12,750 lb/hr) at lower velocities (5-20 ft/s) underground for about 10 miles. The ground temperature is expected to reach 60 F, and ProMax is giving warnings about hydrate formation being reached around 67 F.
There is about 20 lb/hr of water entering the pipeline with the gas, so even if this does "freeze", will it really make a difference in actual operation? Will the hydrates necessary plug the pipeline or can the be entrained and moved with the flowing liquid? Do some operators "roll the dice" and see if a problem exists before deciding to inject methanol?
Thanks





RE: Gas Hydrate Concern -- Need real world experience
I see hydrate problems in surface manifolds and in roadbore casings with double vents. Rarely see hydrates form in pipe buried in dirt.
Is the water entering the line in liquid form (doesn't participate in hydrate formation) or as water vapor (which would equate to about 96 lbm/MMSCF which is consistent with fully saturated methane at 67F and 150 psia).
I've seen office engineers specify prophylactic methanol injection, but I've never seen anyone who has ever seen a field install it. Methanol is horrible stuff (the MSDS sheet gives me nightmares) and injecting it because of some simulation is reasonably irresponsible. Beyond that, I've never seen it work. We inject it into places where injection is convenient and it accumulates in the first low point.
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Gas Hydrate Concern -- Need real world experience
I take it that you dont have experience from the North Sea? Here injection of MeOH is quite common. We operate several satellite platforms where the LG sometimes may not be dry and the we inject MeOH - and if we dont then we get hydrate and they DO plug up the LG pipline or annulus (or both or wthatever). 5 MMSCFD is not a lot - i think it might be along our LG consumtion rates but im not sure. Our LG lines are around 4" and the pressure in the 140-150 barg ande and the line is exposed to sea water. We dont inject it when its water dry only when its not (that would depend on which compressors are avaiable). And i dont think our operation is that uncommon.
Continious inject in multiphase tiebacks tends to go toward MEG because its easier to recover.
What do you mean by "Is the water entering the line in liquid form (doesn't participate in hydrate formation)"? While hydrate formation can take place in any phase - as long as there is water and light alkens or co2 - free water tends to promote hydrate formation and is generally considered nescessary for hydrate formation as far as i know? I believe that kinetics is important as to why hydares often is less of a problems that what models predicts and the fact the properties of the hydrates e.g. their tendency to aglomerate depends very much on the operating conditions.
Best regards, Morten
RE: Gas Hydrate Concern -- Need real world experience
The thermodynamics of hydrate formation requires that the matrix (water) be in gaseous form. Liquid water will tend to kill the reaction (but is rarely 100% effective at killing it). The reason that people see hydrates in the presence of liquid water in places like the North Sea and Deep Gulf is that the hydrates form from the in situ water vapor and partially block lines which causes a big dP that accelerates evaporation and the newly evaporated water forms hydrates (and leaves behind phase change scale) that feeds the problem. Without the trip event the liquid would stay liquid and would freeze based on the steam tables as long as there is an adequate heat sink to remove the latent heat.
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Gas Hydrate Concern -- Need real world experience
But i guess that the process that you describe is hard to distguish from _not_ having free water since it seem to me what you are saying is that the neuleation of the hydrate takes place in the gas phase but that the gas must be close to water saturation (normally this will also mean that there is a liquid water phase prensent)?
Dr. D. Sloan (from the Colorado school of mines) is usually considered quite knowledgeable about these things and have among other things received the Katz award (GPA) for his work in this field.
Best regards, Morten
RE: Gas Hydrate Concern -- Need real world experience
I went back and my sources (primarily a 1980's vintage course from the company that became JM Campbell). The formation of clathrate hydrates requires a super-saturation of the target gas relative to the matrix species. In a gas/water-vapor system this is the normal case, and introducing saturated or sub-saturated liquid water would tend to kill the reaction.
To get hydrate formation in liquid water, the water had to be saturated with dissolved target gas at a much higher pressure/temperature than is experienced at the hydrate-formation point. This makes perfect sense for hydrate formation to occur in liquid water in a deep, subsea, high pressure reservoir.
What I missed was that the requirement is for super-saturation, not a requirement for water vapor.
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Gas Hydrate Concern -- Need real world experience
Learn from the mistakes of others. You don't have time to make them all yourself.
RE: Gas Hydrate Concern -- Need real world experience
http://hydrates.mines.edu/CHR/Home.html
400psig and under 40f? Now about hydrates at 900 and 75 F. Yes sir. Composition is the other important part of the equation.
Zdas, I've never seen teg injection, teg in mixers and staged contractors and then removed immediately because it gets to viscous to move below 50 degrees. Not picking, just clarifying.
BTW, I was one of Dr Sloans first students at Mines. :)
RE: Gas Hydrate Concern -- Need real world experience
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Gas Hydrate Concern -- Need real world experience
RE: Gas Hydrate Concern -- Need real world experience
I did a HAZWOPER course once where the mock incident we did was a large Methanol spill. First thing was pull out the MSDS sheet on methanol. What nightmare stuff that is. Lethal in small doses, causes organ cancer from skin contact, causes blindness on contact, several different ways to kill a person. And we carry buckets of it in the back of our trucks. I have nightmares about that mock incident 15 years later. Companies that have requirements to lock-out/tag-out locations and wear a full face shield to gauge a water tank allow people to carry open buckets of methanol and just pour it on a freeze. I'm not understanding the whole "it is more important to have a policy than to think about what we do" culture.
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Gas Hydrate Concern -- Need real world experience
With higher pressures, higher formation temperatures are possible.
Learn from the mistakes of others. You don't have time to make them all yourself.
RE: Gas Hydrate Concern -- Need real world experience
RE: Gas Hydrate Concern -- Need real world experience
I'm sure some companies if you are in the border zone just see what happens as many predictions of temperature, pressure and hydrates err on the side of caution. Some hydrate formation in the initial stages can appear like "slush" - I recall some surface laid lines in the desert where the operators were reporting "slugging", but only between 1 am and 5am - we worked out that the winter temperatures were enough to get in the hydrate zone but clearly not quite enough to cause a blockage. Then the sun came up and all was well again....
For your case I would recommend that you advise the client that the operation is very close to or within the hydrate formation zone and a risk of hydrate formation exists at the proposed operating temperature and pressure. Then it is up to them as to whether this well is important enough to spend the money to include an injection skid, but an injection point would seem a good idea at the manifold...
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way