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6.4. Conclusion 95space by the flow of oil or water, though the flow rate was always limited to keep thepressure below 1500 mbar. Due to the mass transfer of TMOS during shut-in the volumeof the non-wetting oleic phase decreases by about 20% and the volume of the aqueousphase increases by the same amount. The injection of oil in step 6 is therefore likelyto result in a partial displacement of the gelled water. The water that is injected aftertreatment in step 7 will either displace the gel, be forced to flow between the gel and thenon-wetting phase or between the gel and the pore wall, or will channel through the gel.The intrinsic permeability of the wet gel is typically between 1 and 100 nm 2 [123, 125] sothat the contribution of flow through undamaged gel can be neglected. It was observed inthe experiments that the differential pressure stabilized at a fixed flow rate after the initialperiod of 10 to 20 minutes. Subsequently, the oil and water relative permeability weredetermined in step 6, respectively step 7, by using K abs determined in the first injectionstep. One could argue that the pore structure is modified by the formed silica gel, andthat the absolute permeability changes accordingly. Nevertheless, the original K abs wasused in order to quantify the effect of the treatment on the relative permeabilities.The residual resistance factors for oil (RRF o ) and water (RRF w ) are given in Table6.2. The gel treatment always led to a reduction of the permeability to oil. The measuredparameter RRF o varied between 1.4 and 2.0 for most of the experiments. Slightly highervalues (RRF o ≈ 3) were found when the acid buffer was used. Acid-catalyzed gels areprone to syneresis [123, 124] so that the silica density and the stiffness are increased.This may explain the relatively high resistance to flow even for the non-wetting phase.The values for RRF w are within a broad range between 1.9 and 27. The unusuallyhigh value for experiment A02 might be due to partial blocking of the outlet by thegel. Nevertheless, in each experiment RRF w was found to be 1.5 to 4 times higher thanRRF o , and therefore the gel treatment effectively results in a permeability reduction forwater without a significant reduction of the oil relative permeability. The significantvariation in RRF w suggests that the flow reduction is sensitive to parameters other thanthe conditions during the gel formation (pH and temperature). The relative permeabilitieswere not measured in experiments B01 and B02.6.4 ConclusionIn situ characterization of the coupled mass transfer and gelation of TMOS within anatural porous material under two-phase conditions was achieved with the bi-nuclearNMR measurements on TMOS/oil and heavy water. Despite the uncertainty, for eachslice and time step, in the measured average TMOS concentration φ T of about ± 0.05,the TMOS appears to transfer almost completely from the oleic to the aqueous phase ona time scale of less than an hour to more than 5 hours. Like in the bulk, the mass transferof TMOS is driven by the rate of hydrolysis. Around pH = 6 and for the unbufferedsystems the rate is the lowest, whereas the hydolysis rates, and thus the mass transferrates, are increased in the acid- and base-catalyzed systems. At a temperature of 50 ◦ Cthe rates are increased roughly by a factor 2 with respect to a temperature of 25 ◦ C. Thecharacteristic gelation times, derived from the average T 2 versus time profiles, revealedthat the gelation rates were the highest for the pH = 6.20 systems (gel time on the order