RE: Caverns or porous rocks?21 Jun 2021 17:33
just to add to ST's post on the subject (for simplicity on my end):
1. generally reservoir fluids are trapped within porous media (pore spaces within the rock). the pore space sizes are denoted as porosity, the unit is a ratio to 1 (so 0.3 porosity is 30% porous). the larger, the better
2. Pore spaces need to connect to allow fluid to move and be produced, the level of pore space connectivity is called permeability. the higher the value, the better. The unit is Darcy (denoted D)
3. For fluids (oil etc), a few hundred milidarcies (mD) is already very good. we have wells in UK and elsewhere doing 20,000 barrels per day with average permeabilities of 200-400 mD. this is correct for sandstones
4. for gas, microdarcies (mD) is considered very good. this is correct for sandstones and there are elements of "non darcy" to consider during production phase
5. shale however is much tighter than sandstones, in the realms of nano or sub nano Darcies. so you need supporting technologies to connect the pore spaces better (for example via hydraulic fracturing) to make production viable.
6. there are also caverns in the subsurface but less for production and more for storage. A good example is the UK's gas storage (Rough) - our excess UK gas is stored in salt caverns and used like a cookie jar to balance supply to market as and when required.
7. there are other type of sands like carbonates where natural fractures are important but thats another story.
8. for avoidance of doubt, for HE1, we are drilling to target sandstone Helium reservoirs.
9. there are further nuances to permeability and porosity - for example relative permeabilities and apparent porosities to give two examples, and how parameters like wettability and interfacial tensions affects productivity but perhaps no description is needed for now.
GLA and please DYOR
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