it's all about economics. a good recovery factor for a many reservoirs is 40%. and that's after waterflooding. waterflooding is a process of injecting water from surface to maintain the reservoir's pressure. not all reservoirs are good enough to economically do this.

so natural pressure depletion is called primary production. waterflooding is one method of secondary recovery.

now, what is tertiary recovery? let's stick to oil and forget gas for now. as you decrease the oil saturation (through production), the relative permeability to water (the wetting phase) increases. at some point you reach an economic limit, because it costs money to pump the fluids to surface, etc. that is when wells get shut in, plugged, etc. but there is still recoverable resource in the reservoir.

i once worked on a project (15 years ago) where we pumped soap into an underground reservoir. same as washing your hands, right? if oil stayed at a high enough price for long enough, this technology would unlock vast amounts of recoverable oil. we got local recovery factors exceeding 90% of OOIP on that pilot project.

resource vs reserves https://en.wikipedia.org/wiki/Mineral_resource_classification

Blasingame fanboi https://www.youtube.com/watch?v=GIORNcSUbD4

EOR https://en.wikipedia.org/wiki/Enhanced_oil_recovery

ASP flooding https://onepetro.org/SPEEURO/proceedings-abstract/17EURO/2-17EURO/D022S014R003/194812