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132 9. Concluding remarks and outlookoilφ Tmass transfertime scales (hours)driven by hydrolysiswaterTMOScross-linking(condensation)T 2φ Tplateau/gel pointdecay of T 2pH/temperaturetimehydr.Toil/TMOSratewaterowcond.acidneutralbasepHFig. 9.1: Overview of the main mechanisms and parameters in the coupled mass transfer andgel reaction of TMOS in oil-water systems.9.2 Permeability modificationThe placement of TMOS in a water-wet sandstone at the irreducible water saturation,as described above, leads to an increase of the water saturation and a gel reaction in thewater phase. After treatment the overall permeability is reduced due to the diminishedpore space available for the flow of both water and oil. The water relative permeabilityis reduced more than the oil relative permeability. The modification of the relative permeabilitiesis insensitive to pH and temperature in the system, though the mass transferrate and reaction rates are dependent on temperature and pH (see previous section).Gel formed inside sandstone under single phase conditions (see Chapter 7) leads to adrastic reduction of the permeability of the stone with a factor of about 10 4 . The intrinsicpermeability of TMOS-based gel is between 1 nm 2 (for acid-catalyzed solutions) and100 nm 2 (for base-catalyzed solutions) and depends on the concentration of the TMOSprecursor. Again, the permeability reduction of the gel-treated stones is insensitive topH and is also insensitive to the concentration used. The pore-scale mechanism of thepermeability modification by the gel treatment is poorly understood and remains an issuewhich could be addressed in future studies (e.g. using advanced pore-network models).