10 replies to this topic

[morvenlight]

hi all,

i m doing internship at gas processing facility...there we have four wells connected with a feederlines...i have observed that in day timings feederline pressures are less as ambient temp is high...but after evening when ambient temp falls, feederline pressure increses approximately 10 to 20 psig...also during day timings flow is on higher side while in evening flow reduces..please xplain...

[ankur2061]

morvenlight,

The explanation is quite simple:

1. During day time due to the higher fluid temperature the density is reduced. For the same mass flow and with reduced density the volume flow increases.

2. Gases behave differently then liquid as far as viscosity is concerned. Gas viscosity increases with increase in temperature which is quite the opposite to decrease in liquid viscosity with increasing temperature. Since the major impact on pressure drop (friction losses) in any piping is the fluid viscosity i.e. increase in fluid viscosity will cause the pressure drop (friction losses) to increase and vice versa. The increased pressure drop due to higher temperature (higher viscosity) will cause the pressure to reduce at the destination. Similarly, the decreased pressure drop due to lower temperature (lower viscosity) will cause the pressure to increase at the destination end. The above explanation is extremely theoretical in nature because the change in viscosity for gases over a fairly large temperature range is quite small compared to liquids.

Hope I have been able to provide you some explanation about your observations.

Regards,
Ankur.

[kkala]

To my understanding observed pressure reduction in feeder lines does not depend on ambient temperature variation but on the increased flow rate during day, which causes higher frictional pressure drop from the natural gas (NG) deposit to the feeder line through the well.
In an oversimplifying view, assume an under the earth NG volume of pressure P, connected to the well through numerous capillary tubes (existing in the soil). Pressure at the feeder line is P-ΔPstatic-ΔPf, where the frictional pressure drop ΔPf increases with flow, while ΔPstatic remains (almost) constant. ΔPf concerns the flow in mentioned capillary tubes and the well pipe, most of ΔPf is expected to be realized in the capillary tubes (that is within soil).
For an individual well a maximum gas flow rate out is permitted, otherwise the well has a risk to stop supplying gas (P is not restored). Wells operate below this limit, specified by geological surveys (http://www.cheresour...6364#entry36364). Observed flows (causing pressure variation) are apparently within acceptable limits.
This is one interpretation, there may be also others.

Edited by kkala, 16 October 2011 - 09:06 AM.

[ankur2061]

morvenlight,

To provide more clarity:

The combination of higher volumetric flow rate with a slight increase in viscosity is the principal reason for increased pressure drop or in other words the lower pressure observed at the destination end during day time and vice versa for the night time.

Kostas,

You fail to mention how the flow rate has increased in your explanation. The volume flow rate for any gas is a function of the gas density which in turn is a function of the gas temperature. For a pipe exposed to ambient conditions the gas temperature is also function of the ambient temperature. A reduction in ambient temperature is ultimately going to reduce the gas temperature and vice versa. I have doubts that you have ever been involved with hydraulic calculations related to gas or oil wells / reservoirs. Please clarify your position with regards to your experience related to oil/gas reservoirs hydraulic studies because your explanation is totally beyond my understanding. Or is it that you have to answer each and every query posted on the forum.

Regards,
Ankur.

Edited by ankur2061, 16 October 2011 - 09:32 AM.

[kkala]

Ankur, why such a passionate response? Let me try to explain my interpretation. It may look too simple, perhaps with some errors to be pointed out, but probably not much differing to your viewpoint: "The combination of higher volumetric flow rate with a slight increase in viscosity is the principal reason for increased pressure drop". I have not reported viscosity, assuming it does not play a role.
Reply to your issues according to my understanding is as follows.
1. "You fail to mention how the flow rate has increased in your explanation".
I supposed that gas flow rate increases during day due to external higher demand, or another reason. Having accepted it for granted, I tried to justify riser pressure variations.
2. "The volume flow rate for any gas is a function of the gas density which in turn is a function of the gas temperature. For a pipe exposed to ambient conditions the gas temperature is also function of the ambient temperature. A reduction in ambient temperature is ultimately going to reduce the gas temperature and vice versa."
Except riser (quite a short length compared to well), rest piping is underground, thus not affected by ambient temperature. Temperature drop along the well due to pressure reduction is possible. There are uncertainties stronger than ambient temperature influence, yet an interpretation had to be made.
3. "I have doubts that you have ever been involved with hydraulic calculations related to gas or oil wells / reservoirs. Please clarify your position with regards to your experience related to oil/gas reservoirs hydraulic studies because your explanation is totally beyond my understanding".
Experience is limited to a feasibility study on potential exploitation of a small natural gas deposit (2000). A participating mining engineer had explained the gas moving mechanism from neighboring area to the well base, actually a flow through granular solid beds with appreciable ΔP. The gas taken from the well base should be equal to the ingoing flow from beds around, in order to have stable pressure there. If more gas is taken, pressure there will fall, increasing flow rate through granular beds. To my interpretation this drop in pressure at the well base is reflected as decrease of riser pressure during day (and increase of riser pressure at night). In my post to the student forum this is expressed in a more simplified form.
Besides, if pressure falls below a certain limit (i.e. excessive gas is taken out), beds near well base can be destroyed due to pressure difference.
Hopefully explanation (with probable misunderstandings) is clear now and can be commented. Hot expressions out of technical argumentation have not been considered.
4. "Or is it that you have to answer each and every query posted on the forum. Regards, Ankur".
I should neglect this hot tempered hit! If you want to continue it, please refer to specific cases.
Anyway, comments would be appreciated to improve knowledge. Approximate variation (quantitative) for gas volumetric flow rate (day / night) would be also useful.
Regards, Kostas

Edited by kkala, 16 October 2011 - 04:09 PM.

[ankur2061]

Kostas,

A few points in the OP's post:

1. Nowhere the term 'riser' is mentioned by the OP.

2. No indication that the feeder lines are buried in the OP's post.

3. OP's main question is the variation in the pressure and flow during day time and night time in 'feeder' lines. Feeder lines generally collect to a gas gathering manifold before entering any separation / treatment equipment and not necessarily be buried as assumed by you.

For a fixed mass flow rate, the variation in volumetric flow rate is simply due to the variation in gas density. Fundamentally

Gas Volume flow rate = Gas Mass Flow Rate / Gas Density

Decrease in density will increase the volume flow rate. Gas density being a function of temperature as follows:

rho = P*M / R*T*Z

where

rho = gas density
P = absolute gas pressure
M = gas molecular weight
R = gas constant
T = absolute gas temperature
Z = compressibility factor

Without such specific inputs from the original poster you have assumed so many things. When analyzing a specific problem there has to be a rationale behind all assumptions. This is so fundamental to resolving any problem.

Regards,
Ankur.

Edited by ankur2061, 16 October 2011 - 11:10 PM.

[katmar]

Wow Ankur, what has made you so mad at the world? kkala has offered a far better reasoned response than you have, but you then insult him and question his ability and experience. I sincerely hope that you will soon calm down and offer him an apology.

Leaving the emotional stuff aside, let me address some of the engineering issues.

Ankur's statement "Since the major impact on pressure drop (friction losses) in any piping is the fluid viscosity" is wrong and misleading in this instance. While viscosity is a major influence in laminar flow, with gases the low viscosity and high velocities generally ensure that flow is highly turbulent and viscosity has little influence. This is a good example of the far better insight that is gained when a calculation is done by hand rather than by computer. If you have to obtain the friction factor by reading it off a Moody Chart you quickly see that in highly turbulent flow the friction factor changes very little as the Reynolds Number increases. The viscosity only has an impact on the pressure drop via the friction factor.

The OP has said "during day timings flow is on higher side while in evening flow reduces". He did not clarify whether this flow change is on a mass or volumetric basis. Ankur has assumed it is volumetric while kkala has assumed it is a mass flow change. Both Ankur and kkala have correctly predicted the changes that would arise from their own assumptions - but it is important to recognise that both have made major assumptions and to attack another poster because he has made a different assumption is unprofessional.

To morvenlight - I hope this conflagration will not put you off from future participation here. I have not seen this sort of behavior here before. As a new poster can I recommend that when you compose your questions you try to read your own writing through our eyes. Try to be as clear as possible and remember that we do not have the background knowledge that you do. It is unclear to me whether your pressure increases FROM 10 psig TO 20 psig, or if it is increasing BY 10 to 20 psig. Also, please note the other assumptions I listed above as examples of the sort of details that need to be provided if you are to get really useful answers.

[ankur2061]

Harvey,

Thanks for bringing some sanity in this discussion. I must be candid enough to admit that I was totally unjustified in making unsavory comments. My apologies to Kostas and to anyone else who was offended.

The most normal assumption for anybody analyzing the OP's post would be that the flow and pressures are varying considering a fixed gas intake from the wells in terms of mass flow rate. Flow and pressure variation in gas pipelines due to variation in ambient conditions is a phenomena which is normally observed. In fact most of the hydraulic analysis we do for oil/gas flow lines / feeder lines has a summer and winter case simply because the ambient conditons change quite a lot during these two extreme seasons in many parts of the world. Changes in flow (volume flow) and destination pressures over a wide fluctuation in ambient conditons is a very common observation and well documented specifically in gas or gas/condensate systems.

I would still consider that my assumption of a fixed intake from the wells is logically sound and that the variation in flow relates to volume flow and not to the mass flow due to variation in ambient conditions.

Regards,
Ankur.

[morvenlight]

thanks to all of you for your discussion.... this really helped me alot in understanding fluid behaviour with respect to temp changes..i would like to mention here that the reduction in flow indicated in my original post is volumetric not mass flow rate...also the feederlines pressure incresed BY 10-20 psig....so according to above discussions, i can say that

1) flow(volumetric) increases due to reduction in fluids density...and

2) pressure drops at feeder lines during day timings is due to increase in gas viscosity(increased friction losses)...but what about mass and volume of a gas during this condition...as we all know about Gay-Lussac's Law (pressure law). i.e

The pressure of a gas of fixed mass and fixed volume is directly proportional to the gas' absolute temperature.

Simply put, if a gas' temperature increases then so does its pressure, if the mass and volume of the gas are held constant..

Please explain this...

[katmar]

I'm glad to see that you came back with the missing information. The pressure law does not apply here in the way you have stated it because you are not working with a fixed mass. You may have a fixed mass flow, but because the velocity increases there is less mass in the pipe when it is warm. Of course the relationships between pressure, temperature and density always apply - but all three can vary.

Run some example calculations for yourself to determine the effect of the changes in viscosity and density that will occur over the actual temperature range you are observing.. That is the best way to learn. This will allow you to evaluate the relative impact of the viscosity and density changes. As I said earlier, the friction factor vs Reynolds number curve is almost flat so the viscosity will have a small effect. On the other hand for a fixed mass flow as the density decreases the velocity increases and pressure drop is proportional to the square of the velocity. Density effects will dominate over viscosity (in the turbulent flow regime).

[kkala]

I suppose that conflagration, used by Katmar in a rather excessive & humorous sense, is same as deflagration, occurring after a flammable gas escape (explosion is the other possibility). It lasts very short, with practically no damage to equipment. A human there is certainly dead, bad luck. Should this not happen, everything is soon back to normal.
A. Below are comments on Ankurs' post of 16 Oct 2011 already prepared, still judged usefull despite the development of the topic. Reference to kkala's post of 16 Oct 11 is made in parentheses.
"Kostas, A few points in the OP's post:
1. Nowhere the term 'riser' is mentioned by the OP.
2. No indication that the feeder lines are buried in the OP's post".
"Feeder line" should have been used instead of "riser". So both terms mean same (in kkala's post).
The feeder lines have been assumed above ground, not buried. But exposure to ambient temperature has not been considered as quite significant (para 2).
3. OP's main question is the variation in the pressure and flow during day time and night time in 'feeder' lines. Feeder lines generally collect to a gas gathering manifold before entering any separation / treatment equipment and not necessarily be buried as assumed by you.
Flow increase in day is an input to kkala's interpretation, not a matter for explanation. It is probably due to capacity increase of Gas Processing Facility during day (para 1), but this has not been clarified. It is necessary to assume for kkala's interpretation that (volumetric) flow increases during day, at an extent higher than the result of ambient temperature on feeder lines [FL].
FL have been assumed above ground, but all rest piping through the well is underground (para 3). Only FL is externally exposed to ambient temperature. It seems to me unlikely that day ambient temperature alone could increase frictional pressure drop along FL by 10 - 20 psi.
Note: Perry, Section Fluid and Particle Dynamics, Chapter Compressible flow (formula 6.114 / p. 6-22 in 7th edition) indicates that P1^2-P2^2 is roughly proportional to gas absolute temperature. Suppose that FL transports gas of t=15 oC (night) under P1=15 Bara and P2=12 Bara. And that gas of same mass flow rate (day) gets a "mean" temperature of 40 oC, which means that gas temperatures are approximately 15 & 65 oC at the two ends of FL. Under new temperatures, Perry's formula gives P2=11.70 Bara, while P1 remains 15 Bara. Increase in pressure drop is 0.30 Bar = 4.3 psi.
For same temperatures and P1=30 Bara, P2=25 Bara, increase in pressure drop is 0.48 Bar=7.0 psi.
Lower P1, P2 would bring lower increases of pressure drop.
"For a fixed mass flow rate, the variation in volumetric flow rate is simply due to the variation in gas density".
Ankur may mean stable conditions along the well, supplying a fixed mass flow rate into FL. So pressure P1 at the end of the well is also fixed. All observed pressure variation is due to change of frictional pressure drop along FL.
Moreover kkala's interpretation assumes increased flow through the well, for friction to act on a much longer path. Apart from that, there is also some pressure reduction at the well base (para 3).
"Fundamentally Gas Volume flow rate = Gas Mass Flow Rate / Gas Density
Decrease in density will increase the volume flow rate. Gas density being a function of temperature as follows: rho = P*M / R*T*Z where rho = gas density P = absolute gas pressure M = gas molecular weight
R = gas constant T = absolute gas temperature Z = compressibility factor"
OK, but also see above.
B. General comments.
Any interpretation has week points, natural to some extent because of unconfirmed assumptions. It will approach reality as long as these assumptions are replaced by real data.
There is difference between "not likely" and "excluded", e.g. increase of FL ΔPf by 10-20 psi may be possible under specific operating conditions, long pipelines and hot days. Others may have a better picture on that. It could be checked, if all necessary data were fixed and available.
Frictional pressure drop from the well upwards, combined with any resulting pressure reduction at well base, as explained, could give a possible explanation of pressure reduction at Feeder Lines when gas flow increases. This according to kkala's interpretation.
A specialist could give precise info on relevant fluid dynamics to the well. It is additionally noted that a small gas flow increase in the well (not known to me, say 10%) may not result in pressure reduction at its base. Even so this increased flow will pass from all well pipe and FL. The total path can be long enough to explain the increase of frictional pressure drop by10-20 psi.
Noon is expected to be time of max gas demand, but no relation to Gas Treatment Plant production rate is known. Quantitative variation of gas flow rate into the Feeder Line has been asked.
Any comment on the above is welcomed

Edited by kkala, 18 October 2011 - 04:23 PM.