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6.3. Results and discussion 83The amount of TMOS/oil mixture injected in step 5a was either 2 or 4 ml (about 1to 2 PV) and was injected with a flow rate Q equal to 1 ml min −1 . During the injectionthe mixture displaces the existing oil phase. Due to the difference in viscosity betweenthe mixture and pure oil, which is about two times more viscous than the mixture, agradual drop in differential pressure was observed while the oil was being displaced bythe mixture. After injection, the core was quickly taken out of the core holder, sealedand put into the PVC sample holder which was preheated at either 25 ◦ C or 50 ◦ C. TheNMR measurements were subsequently started with a delay of about 3 minutes after theinjection of the mixture was ended. Continuous T 1 and T 2 measurements were performedfor at least 20 hours. In case the temperature of the sample holder was 50 ◦ C the sampleneeded to warm up from room temperature, which took about 30 minutes. This impliesthat for these experiments the relaxation times acquired in the initial 30 minutes of theexperiment were affected by the temperature change. At the end of the shut-in periodthe core was removed from the PVC sample holder and put back again in the core holder.The final steps consisted of oil injection (step 6) followed by water injection (step 7) andthe post-treatment relative permeabilities, k ∗ ro and k ∗ rw, were determined.Two experiments were done with an alternative preparation scheme (see Table 6.1).The cores were initially prepared at full oil saturation conditions. Several PVs of TMOS/oilwere injected to displace the oil with the mixture. Subsequently, 4 ml of D 2 O was injected(step 3a) in a primary imbibition cycle yielding the condition of residual oil, where theoil contains 20 vol% of TMOS, followed by a shut-in period (step 3b). The final stepsconsisted of water injection (step 4) followed by oil injection (step 5a). An overview ofall core injection experiments, listing the main experimental parameters, is given in Table6.2.6.3 Results and discussion6.3.1 NMR relaxation times of fluids and gels in bulkThe longitudinal relaxation time T 1 as a function of temperature was calibrated for themineral oil and for mixtures of oil and TMOS with volume fractions of 0.20 and 0.40,respectively. The results are shown in Figure 6.4. For the pure oil T 1 is 154 ms at 15◦ C and increases with temperature to 440 ms at 55 ◦ C. As the TMOS concentration isincreased T 1 of the mixture increases accordingly. In case of 20 vol% TMOS in oil T 1 is359 ms at 15 ◦ C and increases to 700 ms at 55 ◦ C, and for the 40 vol% mixture T 1 is 595ms at 15 ◦ C and increases to 1130 ms at 55 ◦ C.The transverse relaxation time T 2 was calibrated for bulk gels that were prepared bymixing TMOS with heavy water, i.e. pure D 2 O and the buffered solutions. The resultswere discussed in Chapter 4. T 2 is shorter for the gels than for bulk D 2 O. The maincontribution to the reduction in T 2 is due to the interaction (e.g. adsorption) of the D 2 Omolecules with the silica content. In case of a gelling solution T 2 is initially similar tothat of bulk D 2 O but decreases as the TMOS hydrolyzes and combines into silica clusters.The final reduction in T 2 increases with increasing initial TMOS content of the gellingsolution.