I'm a little surprised to see how quickly this beer fermented. Eight days ago I made an Imperial stout with a SG of 1.096 measured by way of refractometer (I don't really use software, I just x4 to get my gravity so brix was originally 24). I took a reading today (8 days fermenting) and had 15 on the refractometer. I remember reading that you have to adjust your readings by refractometer due to the non-sucrose elements in the wort (e.g. alcohol). So by entering this in my brewing software calc, it tells me that 15 Brix of fermented wort is really 1.039.
The type of yeast I used was whitelabs WLP002 English Ale Yeast which is supposed to have an attenuation of 63-70%. I also created a 1L starter and added yeast nutrient along with a fish pump O2 diffuser stone for about 3 minutes. Currently, by my fuzzy math, I have an attenuation of 57% ((1.096-1.039)/1.096)=.57 right? I'd venture to say that I'm about 85-90% through the fermentation.
I'm going to let it sit for another week and check the gravity again. Come next Wednesday or Thursday I will raise the temp and give it a diacetyl rest then keg.
My question after a long winded background is if this is really possible to ferment an imperial stout or any high gravity style for that matter this quickly? Most of the recipes I have dealt with have been 4-6 weeks total fermentation time. I'm happy, don't get me wrong, just a little befuddled that it is fermenting so fast!