I have certainly worked my entire life understanding decline rates and reserves, as well as the type of rock and reservoir fluids which cause them over the life of a well and fields.
In shale wells, the big initial declines are caused by the rapid production of the fractures, whereas the later production declines are caused by the low permeability of the formation.
Past the initial few years of the life of a shale well, there is very low decline, precipitated by the very low amount of movement into the wellbore from low permeability of the rock. This causes the majority of production during the life of a shale well to be at very low declines.
If the world had, to take the extreme case, 100% of its production coming from shale oil production, then the decline rates are correspondingly very low, hence the need for more production to replace is more modest. Adding to production is more challenging for shale.
I know it is tough to understand, but the long lives of shale wells can be a solid contribution of the base production that has a lot of value. The risk of continued exploration of risky ventures to find new production in new traps is diminished when one can fallback on this low decline established by shale wells.
Maybe another way to think of it is the relatively constant 100 year life of a coal mine corresponding to the low decline rates of the bulk of the life of a shale well.
That low decline rate (what I consider the second phase of production, after the steep initial decline) provides the base production, more dependent on the number of producing wells (and their IP) than much else.
But initial production (the first phase, unloading fractures, with steep decline) is what spikes production. While the aged wells will be fairly steady, the new wells will decline fairly rapidly to 10-20% of IP, and that wedge in the production graph will need new completions to replace it until the number of aged wells has risen and produce to that level.
There will be a peak, only replaced by new production. How severely that decline affects the overall picture depends not just on the formation, but the number of new wells being brought on line. As the number of new wells drops, the overall production figures will decline, then stabilize on a flatter curve. If there are a lot of new wells (relative to existing 'aged' producers), the field decline (overall production) will appear more severe.
If you go through the years (graph at the link) selecting every third or fourth year, and unselecting the intervening years of first production, the wedge effect on the graph will be apparent, with each new run of wells causing a spike with subsequent decline in production and then leveling off. That happens for every new well, but is easier to see the effect visually if the years of initial production selected are roughly three years apart, because it takes roughly that long for production from any given well to stabilize. That ongoing (mature) production forms the base production for the asset in question. In successful wells, the 'wedge' from IP to stabilization pays off the well and even provides profit. That initial decline wedge is what must be replaced to maintain peak production levels, but to keep up with the peak, you are chasing your tail.
