ECO2N / ECO2M: handling of wettability in a dry zone
When exposing the initially brine-filled grid cells near injection point to CO2 injection, a dry zone is established. At a certain condition of flow and rock properties water saturation becoming zero in some grid cells despite having assigned a positive values for irreducible- or residual water saturation (when entering relative permeability endpoints). I wonder what is the rationale behind it? Is it because we think evaporation should also include residual wet phase saturation? The expectation is that Sg+Sw+Ss=1 and Sw=Sw_mob+Sw_immob. Further, we may expect that solid phase replaces first mobile water, then immobile, if it supposed to do so. If that is the case, the solid saturation should get as high as Sw_mobile before any Swr is removed? But in that case, the grid cell might already had lost so much of its porosity that the permeability has already reached zero. If the fraction of lost porosity to reach a “zero-point-permeability” given by input as 0.7 (for instance) and Swr=0.1, what happens when 70% of initial pore space is filled with solid salt, would it be 30% trapped high saline water left there as capillary front has moved far away? What about if the fraction of lost porosity is 0.95 instead of 0.7, would it be 0.05 irreducible water saturation left there?
Appreciate your feedback
When dry CO2, or any other dry gas, is injected in a medium containing water, water is evaporated according to local thermodynamic equilibrium eventually promoting the complete evaporation of the aqueous phase and the precipitation of dissolved solids. This process has been observed at the field during natural gas injection in saline aquifers, as well when producing natural gas from gas reservoirs with high temperature and high saline connate brine.
The process has been replicated at the laboratory and reproduced by numerical simulation when injecting dry CO2-rich mixtures or any dry sour- acid-gas mixture.
TOUGH2/3 assumes instantaneous thermodynamic equilibrium among all the phases present. Thus, as soon as the dry gas enters in contact with the brine, water is evaporated in order to have the same water fugacity (or partial pressure, depending on the EOS used) in both phases.
Looking at the mass balance equations, you can see that there is no distinction between mobile and immobile aqueous phase. Thus, evaporation drives just a reduction of aqueous phase saturation and an increment of NaCl concentration in the bulk of aqueous phase. Similarly, if NaCl reaches the solubility limit for local conditions, the solid halites precipitates reducing the porosity and permeability.
The models used to account for porosity-permeability relationships are a big question mark, as the deposition of solid halite on the surface of rock matrix alters the pore size distribution. As you said, permeability can be reduced to nearly zero when there is still a large fraction of porous volume available for fluid flow, this because it is enough that the pore throaths are clogged to avoid the fluid flow (see Verma & Pruess models and the large literature on the argument published in recent years).
To improve the simulation of the evaporation process, the inclusion of vapor pressure lowering (VPL) could be considered (implemented in EOS4, EWASG and TMGAS). The reduction of aqueous phase saturation produces an increase of capillary pressure which affects the VPL, reducing the vapor pressure of the aqueous phase. The evaporation is slower and liquid water can persist at P lower than the local vapor saturation P. But finally, the aqueous phase is evaporated even with VPL on.