Coupling between Porosity and Pore-Pressure in a Gas Reservoir
Feasible to capture the concept that the porosity may not change – for the changes in effective normal stress – at constant sliding velocity – assuming the magnitude of the porosity change to remain as a constant – in a gas reservoir (where, porosity depends only on the slip velocity)?
If so, whether the changes in effective normal stress - resulting from the respective changes in pore pressure – really lead to the fact that pore pressure changes remain strongly linked with the evolution of porosity?
In essence, whether the ‘magnitude of porosity change’ strongly depends on the changes in ‘effective normal stress’?
If so, how exactly to bring in the role of ‘slip velocity’ associated with the porosity evolution?
What exactly the concept of ‘average reservoir porosity’ does take into account?
Suresh,
Porosity is independent of slip velocity as it is a volumetric property of the rock (porosity = pore volume/bulk volume). Slip velocity comes into play when measuring permeability (not porosity) at high pressure and non-Darcy, or slip-flow accentuates gas permeability. We correct for this using a "Klinkenberg Correction" (there are a couple of these depending on the initial perm of the rock).
Porosity is changed mechanically - shallow with burial and chemically due to chemical compaction" or diagenesis over time. Changes to the original pore system are a function of the maturity of the original rock components (some minerals break down more easily than others - both mechanically and chemically), early chemical diagenesis that helps with later mechanical compaction (clay coatings, e.g. are developed in some depositional environments and at the right level can hold off further cementation), how open or closed the initial pore system is, and what the history of pore fluids is over time.
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