egm notes, page-18

Thanks Saracen!

18 Tcf is actually 18,000 PJ, not 18. The 2 Tcf figure here is the 'high side' case which is usually defined as the P10, which is the value of which it has only a 10% chance of being higher. It basically relies on the structure being full to spill, the reservoir being optimal (porosity, net pay etc) and everything being perfect. That rarely happens, and the most likely estimate case will be a lot lower than 2 Tcf, but by all accounts it's a massive structure.

2,000 Petajoules is 1.84 tcf to be precise. That's a lot of gas for a conventional field, however, we're talking about Gas Originally In Place. The recoverable should be about 30%.

Yes the standard conversion factor to get from bcf to PJ for pure methane is approx 1.038, but you rarely get pure methane when drilling for gas (except, usually, CSG). When you're talking about a number like 2 Tcf it's a 'geologist number' i.e. nobody really has any idea whether it's 200 bcf or 2 Tcf.

In this case we have are expecting a reasonable level of inerts, not just He but N and CO2, which would reduce the energy content of the gas. But there are also suggestions there may be condensate down there in which gas there may be high ethane, propane and butane, which would increase the energy content per cubic foot.

So these two effects will counteract each other. Depending on how much of each are present, the conversion between bcf and PJ may be more than 1 or less than 1.

Best to just use 1 to 1 equivalence like haasko says. Especially in situations like this where the geological error bars dwarf the effect of gas composition.

Re recovery factor, it's normally higher than 30% for a gas field, sometimes as high as 80%+, but it really depends on how tight the reservoir is. For a structure this big and supposedly tight, it's probably not developable everywhere so 30% may be appropriate. If they get a good flow rate on testing though, it would probably be higher. It depends on water saturation.