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94 6. Coupled mass transfer and gelation of TMOS in porous materialsIn bulk systems it was found that the initial decay of T 2 in the aqueous phase wasequally fast for the pH = 9.48 system and the pH ≈ 6 system. Here the relatively slowdecay of T 2,AV in the pH = 9.48 case suggests that the gel reaction is slower compared to theunbuffered case and the pH = 6.20 case. However, the relaxation time T 2,AV is an averageof the T 2 spectrum obtained with the inversion routine. It was found that especially forthe pH = 9.48 case (experiments A09 and A10) the T 2 spectra showed a relatively highcontribution of the long T 2 peak (around 100 ms) throughout the measurement interval,which also explains the high plateau value of T 2,AV around 50 ms. This is indicative ofa fast and inhomogeneous gelation of the water in contact with the oleic phase, so thata large amount of water in the aqueous phase has not turned into a gel. The effect wasobserved within each slice.The fitted parameter κ (see Table 6.3) reveals that, for a consistent set of conditions(experiments A03 through A10), the decay of T 2,AV is the fastest for the pH = 6.20 systemand the slowest for the acid-catalyzed system. Increasing the temperature from 25 ◦ C to50 ◦ C for most systems results in an increase of κ by roughly a factor of 2, except for theunbuffered systems, for which the inconsistent results cannot be explained at this point.The acid-catalyzed system appears to be most sensitive to temperature in terms of κ.The low values of κ of about 0.5 h −1 in the experiments B01 and B02, compared to theexperiments A01 through A04, are expected since the cores were prepared at residual oil(B01 and B02). Therefore, the ratio of water/oil is much higher and the concentration ofTMOS – which partitions between both phases – in the aqueous phase is lower than thatin the experiments A01 through A10.6.3.5 Effect of treatment on permeabilityThe absolute permeability K abs of each core is listed in Table 6.2. The permeabilityfor water at full water saturation was found to vary between 1.3 and 2.0 µm 2 . It isnoted that despite the use of vacuum in the initial saturation step a slight amount ofair was reintroduced in the cores when they were put in the core holder and the tubingwas connected. The presence of air bubbles, as a non-wetting phase, in the core lowersthe apparent absolute permeability. Some of the experiments showed a high relativepermeability to oil (k ro ≈ 0.8) in the primary drainage cycle. Since the oil would replacethe air bubbles present, this indicates that the measured K abs should be higher andk ro lower. However, quantitative corrections cannot be made. The effect of the geltreatments is expressed in the residual resistance factor, RRF , which is the ratio ofthe relative permeability for oil or water after treatment and the relative permeabilitybefore treatment. Both of these are based on the same K abs so that the accuracy of theabsolute value becomes less important. The relative permeability k rw after the secondaryimbibition cycle (step 3) was found to vary between 0.21 and 0.36, and the relativepermeability k ro after the secondary drainage cycle (step 4) was found to vary between0.51 and 0.84. The much lower water relative permeability compared to the oil relativepermeability is expected in a water-wet core [118].In the post-treatment injection steps (step 6 and step 7) the differential pressureshowed a highly erratic behavior at a given flow rate during the initial 10 to 20 minutes.This is attributed to displacement of or damage to the gel which is formed in the pore