Coal Bed Methane Reservoir Engineering Book

[vinomarky]

Pwf has more to do with your wellbore and producing conditions than it does your reservoir. In your case, the '100 psi' Pwf is probably a minimum producing BHP that a water pump can maintain without burning up. As your reservoir pressure reduces, theoretically (operational limits permitting) you should still be able to maintain that 100 psi FBHP albeit with reduced water rates

As the coal dewaters and the Pr falls below the saturation pressure, gas will start being desorbed - ultimately (assuming minimal water ingress) you will be operating at quite low Pr, FBHP and water rates as you step down the langmuir sorption curve - At this point, if you had a 50 psi WHP, you could quite easily have less than 100 psi BHP assuming fully dewatered as most of these plays are relatively shallow.

[jesmamohan]

HI, I have a problem in CBM production profile. I am doing an analysis to see the effect of various CO2 injection rates on CBM production. The curves do not look like the typical CBM curve. ie, increase and then stabilize and finally decline. Instead am having a curve that looks like a sandstone reservoir curve. Ie, decline from the start of production. Please let me know what parameters affect this. I realise that my reservoir pressure is 1100 psia and my lang pressure for methane is 610 psia. Please provide me links to understand my case better. i really need help

[vinomarky]

Two variables are typically used to characterize the adsorption isotherms, namely the Langmuir Volume (VL) and the Langmuir Pressure (PL). Together using the Langmuir equation you can describe the gas content remaining in your saturated coal for a given pressure.

The Langmuir pressure though has little to do with whether or not your coal is at saturation

Usually people take some core samples and desorb the coal to first figure out how much gas was in it under insitu conditions. They then take that coal and add methane to it (re-adsorbing) to characterize the full adsorption isotherm

If the initial gas concentration/pressure point fell to the right of the Langmuir isotherm, then the sample is under saturated, and you need to drop the pressure until it hits the Langmuir curve before a single molecule of methane will be coming off. If it lies on the curve, then gas will be coming off the cleats from the beginning

Go back to your data;
1. find the VL and PL values
2. Find the initial gas content of the coal and the insitu pressure
3. Draw the Langmuir curve and see where the initial gas content sits on the curve - this will tell you whether or not you are indeed saturated or not
4. Ensure you initialize your simulation run at the appropriate pressure and gas content to describe its degree of saturation (Check your units as well, Eclipse in particular uses mscf/cuft vs pressure for the Langmuir description)
5. Ensure your Langmuir curve data for saturated gas content is properly described in the simulator deck

Good luck

[jesmamohan]

Thank you so much for the information. I am going to following your recommendations Thanks again

[jovyan]

hi,
the link is broken please help

[zeliustitan]

hi people..

i m kinda stuck with my CBM gas storage capacity calculation in adsorption isotherm

i was able to find out the gas content for every pressure steps but i have a problem

The langmuir equation related to gas storage capacity is

V= VL [ P / (PL +P)]

then i convert it into y = mx + c and became

P/V = (1/VL)*P + PL/VL

basically slope = m = 1/VL and interception is c = PL/VL

my problem is i got a negative slope.

My question is does this negative slope can represents Langmuir Volume? or it shouldnt be negative slope?

I m stuck here.. Hope anyone can help me.. thanks

[vinomarky]

I think your math is wrong somewhere, your equations are correct I believe.

I've uploaded a worked very simple example for you to look at;

[link Point to another website Only the registered members can access]
Where I've simply started with values of PL & VL, created a Langmuir curve, and then fitted that curve back to original PL & VL constants

[zeliustitan]

Thanks.. if the slope shouldnt be negative.. i have to re-check the pressure...

vinomarky, i have another question..

i m doing a preliminary study for potential CBM field.. the field has some sub-bituminous and some lignite..

would be possible for me to use KIM A.G. method to calculate the gas content? (by assuming it to be bituminous)

2nd question, i m doing adsorption isotherm test.. but i did only 1 test because i mixed different coal from different location into 1 sample to make it an average. and so i got 1 langmuir volume value and 1 langmuir pressure.. how am i going to explain it in results and discussion? what are normally reader anticipate to know more?

hope you can help me with your ideas.. thank you

[vinomarky]

2nd question first - mixing the coal to get an 'average' gas content was bit of a mistake in my opinion. Gas content (and indeed saturation) of coals is highly variable across a field, and you can often map trends - helpful when trying to narrow expectation bounds between bore holes. Think of coal gas content & saturation as being another geological value like perm that you need to try to understand what possible depositional conditions controls the variation over reasonable distances. After good field appraisal you should have not only maps of (for example) kh, but also gas content and saturation maps

You can always combine/average results of individual tests later, but you can't go the other way....... Now that you are in this position though, you need to try to establish what range P10/P90 of gas content you could possibly expect - and it needs to be broad - possibly through analogs to other fields. If you get challenged as to why you did what you did, I strongly recommend simply telling the truth, which would be (if I'm right) that you made a mistake, and perhaps follow it up by requesting that you go on some CBM appraisal course - Rose & Associates do a nice short one that will give you the basics - if you try to bluster though, refusing to accept that your approach was 'less than perfect' then you'll lose credibility which is far worse than being thought to have made a mistake (just my 2-cents based on 2 decades of industry experience).

Am unfamiliar with Kim's method sorry, but my gut feeling is that if you need to assume it is something that it isn't that your results will be wrong. The question is how far wrong - if you can establish this method will tend to yield higher, or lower values then you can possibly use them as an upper or lower bound.

[Shakespear]

You may find this of use

[link Point to another website Only the registered members can access]

[link Point to another website Only the registered members can access]

[SEUNCAROLINE]

Please someone reupload the cbm eooks....on 4-s-hared. The links are not valid anymore...thanks

[Ledy]

To pablocien,
why i cant access the link that u gave above?
can u give me again?
thank you