The Oil Drum

186 comments on Plateau continues, aided by outages...

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R W Reactionairy on April 15, 2006 - 10:18am

Could you please explain the reservoir dimensions / dynamics of Cantarell that translate into a 300 foot yearly rise in the oil water contact [very disturbing] in an existing 825 foot oil column [otherwise an impressive / thick pay streak even after many years of production.] Are the issues simply limited arial extent and high permiability, production mostly coming from factured porosity with limited porosity / permiability elsewhere within the reservoir or somthing else entirely? Thanks.

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westexas on April 15, 2006 - 3:45pm

Dave posted a link to a good article on Cantarell down the way, but in regard to your specific question, let's imagine a large oil field in the shape of a pyramid that is 2,000' high from base to tip. Let's assume that we have two phases, oil and water, with no gas cap, and let's assume a 1,000' oil column. Measured vertically, we have 1,000' of water and 1,000' of oil.

However, the volume of the pyramid that is full of oil is much less than the volume of the pyramid that is full of water. Let's assume that we drill horizontal wells into our oil leg and let's assume that we can produce the field at a relatively high rate until the water hits the horizontal wells. Let's assume a moderately strong water drive that is augmented by water injection into the water leg, in order to maintain a fairly constant pressure as the oil is withdrawn.

Initially, the oil column would thin at a fairly low rate because the lower portions of the oil column are being depleted first. However, if we look at the oil column in terms of 100' intervals, the volume of each successively higher 100' interval is less than the 100' interval below it. (Key point.)

So, as we continue to produce the field at a fairly constant rate, the rate of thinning in the oil column accelerates dramatically.

If you throw in a gas cap, this is grossly simplified explanation of what is happening in the two largest oil fields in the world, Ghawar and Gantarell, accounting for almost 10% of world crude + condensate production. (Note that Cantarell has had a pretty sophisticated nitrogen injection program.)

In any case, you can see how the fields would have the appearance of robust production right up to the point at which production starts crashing.

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R W Reactionairy on April 15, 2006 - 5:46pm

Thanks for clearing up my foggy thinking. I think I've got it now, my mindset just needed reorientation.

Like a pinacle reef on a grand scale. The offshore environment means high development cost per well. Wells drilled well off the structural high would produce a lot of oil initially but are unnecessary because the reservoir is extremely porous and permiable so that a limited number of wells can adequately drain the structure. There aren't going to be any other entities producing from the structure so there is no rush to produce before someone else gets the oil. Thanks again.