Gas quality change during production

[Eidos]

Hi,

I am looking for someone who may be able to explain what is occurring on my well. During the past couple months, I notice that the Natural Gas Liquid sale is relatively low, and found that it is related to low 'delivered GPM' value. This 'delivered GPM' is determined by the monthly gas sample analysis, and suggesting that NGL components within the producing gas are decreasing. Thus, there are lesser Ethane, Propane, Iso-Butane, etc.. while Methane is relatively higher. So my question is: Is it usual for the gas quality to change during the life of the well?

Thank you for your time

 

[LittleInch]

Yes, this is normal. As a gas well ages, it is fairly common for the liquids and heavier hydrocarbons fractions to decrease faster than the overall gas rate.

For wells the only constant is change.

My motto: Learn something new every day

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

 

[Eidos]

Thank you for your response. If the decline in heavier hydrocarbon is normal, would you tell me how such heavy hydrocarbon tends to decline faster? For your information, the well has been producing some water from Day 1 of perforation and I am wondering if water is the source of such decline. Thank you again!

 

[LittleInch]

I'm no geo physist, but it pays always to remember we're talking about solid rock here that comprises the reservoir. As the well ages, the molecules need to come from further and further away to the well bore. Smaller molecules (CH4) are able to do this easier than larger ones. If you don't have anything pushing the molecules towards your well (Water injection or gas injection for oil wells), then gradually the molecules getting to the low pressure point represented by your well will comprise more and more of the smaller ones.

Water coming from underneath is also dragged up by the diminishing pressure above it, but usually tends to sweep the hydrocarbon ones in front of it.

You also have natural and man made cracks in the rocks which allow the smaller gas molecules to travel faster than the bigger heavier ones.

I'm sure this is highly simplistic and would welcome any input from more learned posters, but it works for me and when I get predicted reservoir rates from the reservoir engineers, they often show a steep decline in liquids before the steeper decline in gas flow.

However each well is different and each field operation more complex than that.

If you need more definitive information I suggest you contact a geo-physist or reservoir engineer.



My motto: Learn something new every day

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