So, as a mid-case would it be reasonable to expect total world net exports to be reduced by ~50% by 2020 (in 12 years) - that's ~5% a year decline rate (from 46 mbpd in 2005 to 23 mbpd in 2020.)

By 2020 I am expecting world population to be around 7.6 billion, up from 6.6 billion today, a 15% increase overall - not sure yet what percentage it will be for net importer countries.

Sounds reasonable, but a key point to keep in mind is that the year to year net export decline rate starts out slowly, and accelerates with time, but once the decline kicks in, the volumetric decline rate tends to be approximately linear, i.e., close to a fixed rate per year.

So, if the overall world net export decline really kicked in, in 2007, one might expect the decline to be on the order of 1.5 mbpd per year. The 2006 and 2007 data suggest that the top five are dropping at about one mbpd per year in 2006 and 2007.