gas riddles, page-6

Gday SG,

I am with you now sorry. I went back over the threads and found your initial q, sorry I missed it :)

ESGs seams are rather deep, so based on the reported nature of cleating/fracturing in the seam, being vertical, this is consistent with what you would expect to find at depth because of the vertical stress the seam is under.

what I meant by field effects was that, as production from your fields progresses, you are doing two main things to the seam - drawing water as well as desorbing gas - which has effects on your most critical commercial factor - PERMEABILITY.


Dr Klink
The first 'effect' relates to some correctable assumptions in modelling. So as with all mathematical models, there are a vast number of assumptions that have to be made in order to establish an outcome. Remember it is only modelling, not the real thing. When measuring darcy flow (to establish perm), the equation assumes that gas closest to the actual desorption point does not move (when in fact it does). This means the effective permeability is going to be greater as you have gas flowing from areas you have assumed it not to be. This effect becomes more relevant as you approach the end of the well life and pressures are reducing. This reevaluated effective perm can be corrected accounting for the amount of flow (slippage factor) at the corresponding pressure. This has a moderate increase on the effective perm, but still it can help you extend the time til abandonment of the well as your pressure that you switch the well off at, can actually go a bit lower because you will flow more.

The other two effects are a bit simpler in nature and relate to the converse effects of gas production and water production from the same reservoir.

Matrix Shrinkage
You might have previoulsy heard of matrix shrinkage. No this isnt what happens when you put Keanu Reeeves in the fridge : )

The methane and other gases live in the coal matrix. As the gas is produced, the matrix volume shrinks because it is already artificially propped open because of the pressures allowing it to hold more gas. This is actually a good thing. If you think about the matrix being in a bounded space, if it is shrinking, then it is opening up the larger face and butt cleats; so technically you get more flow. Technically.

Stressing the Matrix
Can I now keep with the Keanu theme and conjure up some immagery of a stressed geologist Keanu at the well site. Keanu, if you dont keep this well above 8mD your NPV will be shot. What do you punk. What do you do?

Well, firstly, make sure you have a top quality resource like ESGs :)

Remember, you are also producing water. Water is by far the largest and most immediate change to volume in the seam. This reduces the pressure in the coals which has the converse effect of increasing the effective stress. Think of the effective stress as a relationship based on the pressure differential between the coal reservoir and the overburden. As you reduce the pressure in the coal, then the pressure differential actually increases. hence, our resulting effect is an increase in effective stress which is a bad thing because our perm is reduced.

What is the most important "Field Effect"
All in all, water production probably has the biggest 'field effect' we are concerned about as this has the biggest effect on reducing perm in the early stages of the well life - thus having a proportionately greater impact on the well NPV.

The former to effects that enhance our perm are important for extending our well life beyond the assummed abandonment pressures.


Sorry again for snobbing you, not intentional :)

Anyway, time for a visual!

Cheers,

SF


PS: apologies in advance to all those offended. I agree, all images of stars in the 80s should be stricken from the internet. maybe a great use of an internet filter eh?















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= matrix shrinkage