BTU/Mcf - Heating Value

[wood5896]

If a natural gas well is drilled in the rockies at 6000 feet should the calculation for heat content take into account the reduced atmospheric pressure (11.87 psia instead of 14.73 psia) eventhough the gas won't see the atmospheric pressure until the burner tip. The gas I am dealing with has a specific gravity of 0.608 and depending on the Pressure I use for the calculation changes from 1072 BTU/Mcf at sea level to 864 BTU/Mcf at 6000 feet.

A web site I found on engineering at high altitudes:

http://www.esmagazine.com/Articles/Cover_Story/BNP_GUID_9-5-2006_A_10000000000000020238

says that for aerated burners that the 11.87 psia calculation is correct and there is a reduction of heating value. I guess what I am confused on is should I be using the lower pressure for the high altitude and if so is the reduced heating value due to less moles of natural gas/mcf compared to sea level or a decrease in the amount of oxygen needed to burn the gas.
THANK YOU!

 

[zdas04]

Are you trying to calculate a BTU value for gas sales or to use it?

For gas sales, you have to look at the contract and the contracts almost always say that the gas conditions will be adjusted to a contract pressure (usualy 60F, the pressures I see are 14.696 psia, 14.73 psia, and 15.025 psia), so the 6000 ft is irrelevant to the actual settlement value.

If you are sizing a burner, you need to understand the impact of elevation on combustion and size your burner with the expectation that you'll be getting about 20% less energy than you are paying for.

David

 

[wood5896]

I was trying to calculate BTU value for gas sales. I understand the in our state we use 15.025 for contracts, but is that just a made up value, a legal agreement to standardize everything, (we are at much higher elevation and therefore like you said you will get 20% less energy when your at the burner tip). I guess where I am stumped is the Standard Pressure being set at 15.025psia. When I do volumetrics for a new gas well I have always utililzed 15.025 psia when I calculate a Bg (formation volume factor), but if I was to use the actual Patm my volumetrics would increase be 20% and the drainage for older wells will decrease by 20%, I'm lost! This has huge implications and I can't find any documentation anywhere on this, why do all handbooks tell you just to accept the standard temp and pressure bases, it's crazy to think the pressure up in the mountains of the rockies will be higher than in the midcontinent, yet the pressure bases are that way.

 

[zdas04]

You have to get over it. In New Mexico, most of our gas-sales contracts use 14.73 psia, but we have to report to the state at 15.025 psia--none of it has any physical significance (if sea level is 14.696 psia, then a field at 7,000 ft would be closer to 11 psia than 15 psia, but politics are what they are).

When I booked reserves on a CBM play in the San Juan Basin, I figured how much mass of CBM and how much mass of contaminants (CO2 was the worst) was in place (a physical parameter not affected by contractual nonsense). Then I converted those numbers to a volume at 14.73 psia and 60F and guessed how much of it I was going to recover with existing technology (and some really iffy guesses about the change in CO2 over time), and conservative economic assumptions to book reserves. The recovery factor, price deck, and heterogeneity of the reservoir were pretty big uncertainties--the choice of a pressure base just wasn't within the accuracy of the calculation so I didn't care.

If you have eliminated the BIG uncertainties and are concerned about this pimple on the tail of the flea on the dog then more power to you. It is an entertaining academic exercise, but is rarely material for reservoir work.



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.

Remember, being ignorant isn't your fault; staying ignorant is.

 

[dcasto]

ok, BTU is energy, energy can not be created or destroyed.

Mass cannot be created or destroyed.

The well will make a fixed amount of energy, but depending on the pressure standard used when measureing, the STANDARD volume will change in proportion to the standard preesure. If the standard pressure is increased, the volume will decrease, but the BTU per STANDARD cubic foot will INCREASE exactly at the same rate the volume Decreased.

what you are looking at is the pressure at the delivery pressure, so you have selected to set your standard to 11.87 psia. The volume required to release the same exact energy will increase, and the BTU per volume will decrease.

Since your burned is a fixed volume consumption device (an orifice with a fixed inlet pressure), the volume will drop and so the net BTU's feed into the burner will drop. If the heater has no forced air blower, your heater will not release as much energy at altitude. Your burner will require less air and because there is no blower, the burner will seek equalibrium as if it were at sea level.

To compensate for the altitude, do what car engines do, put a blower on the air and put more gas into the burner by adjusting the burners "fixed volume" system to make it larger. this means either more pressure upstream of the orifice or a bigger orifice.

 

[wood5896]

So you are saying "what you are looking at is the pressure at the delivery pressure, so you have selected to set your standard to 11.87 psia. The volume required to release the same exact energy will increase, and the BTU per volume will decrease." is that the BTU/Mcf will go down so if you sold gas based upon a BTU/Mcf standard at 14.7 you would be cheating the customer out of money because there is less energy per unit of gas at higher elevation? Sorry if I got that wrong, I think that is what you were saying.

 

[dcasto]

No, you would not be cheating your customer, because at higher elevations, the btu per STANDARD condition cubic foot times the VOLUME in cubic feet at the same STANDARD condtion is always equal.

The energy in one cubic foot at a standard pressure of 14 psia and at 11 psia base are different, but you will use less cubic feet at 14 psia than at 11 psia.