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6.2. Experimental section 79increasing echo time, gradient strength, diffusion coefficient and gyromagnetic ratio.In a porous material the presence of paramagnetic impurities or internal magneticgradients has little effect on the relaxation of D 2 O compared to H 2 O. Moreover, the useof D 2 O instead of H 2 O allows for a clear separation between the oleic phase (containing1 H) and the aqueous phase. For these reasons, the use of D 2 O is preferred, despite therelatively weak NMR signal of deuterium [122].As for the intrinsic relaxation of the fluids and gels, the relaxation of hydrogen in bulkmixtures, such as TMOS/oil, was discussed in Chapter 2. The relaxation of the deuteriumatoms in the silica gels, prepared with D 2 O, is due to intra-molecular quadrupoleinteraction [41], and is sensitive to the mobility of the molecules. T 2 is shorter for the gelsthan for bulk D 2 O. In gelling solutions, T 2 is initially similar to that of bulk D 2 O, but itbecomes shorter in the course of time and reaches a plateau at the gel time. The finalreduction in T 2 increases with increasing initial TMOS content of the gelling solution.The main contribution to the reduction in T 2 is due to the interaction (e.g. adsorption)of the D 2 O molecules with the silica network.6.2 Experimental section6.2.1 Materials and methodsTetra-methyl-ortho-silicate, Si(OCH 3 ) 4 , was obtained from Aldrich (>99% pure). It wasdissolved in Isopar V, a mineral oil with a viscosity of 10 mPa s at room temperature,obtained from ExxonMobil. The volume fraction of TMOS in oil was always equal to0.20. For the aqueous phase we used D 2 O (CIL, Andover, MA, 99.9% pure). In addition,three different D 2 O buffers were prepared by adding salts and chemicals containing normalhydrogen in little amounts. The pH was measured with a semiconductor type pH meter.The low pH buffer (pH = 2.84) contained 8.47 g of citric acid, 3.22 g of NaOH and 2.18g of HCl per liter, the intermediate pH buffer (pH = 6.2) contained 12.53 g of citric acidand 6.32 g of NaOH per liter, and the high pH buffer (pH = 9.48) contained 3.81 g ofdi-sodiumtetraborat per liter. The pH of the untreated D 2 O was around 5.5. The slightdegree of acidity is attributed to solution of CO 2 from the atmosphere.Bentheim sandstone cores with a diameter of 16 mm were cast into an impermeableepoxy layer, thereby increasing the diameter to 20 mm. The penetration of the epoxy inthe core varied between 1 and 2 mm. The length of the cores is 60 mm. The porosity of asample was determined through a mercury intrusion porosity (MIP) measurement yieldinga porosity of 0.22 and a mean pore size of 33 ± 10 µm. In addition, a Bentheim corewas scanned with a micro-CT (X-ray computer tomography) apparatus (Scanco Medical,type VivaCT 40) with a resolution of 10.5 × 10.5 × 10.5 µm 3 . An example image is givenin Figure 6.2. The image clearly shows the irregular structure of the pore space, and thatthe rock contains a few pores with a size much greater than 33 µm . The pore volume ofeach core is about 2 ml. Bentheim is a water-wet rock, consisting mainly of quartz andlittle amounts of non-swelling clay (less than 1%, see Chapter 7).