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Chapter 6Coupled mass transfer and gelation of TMOS inporous materialsIn the previous chapters the coupled mass transfer and gelation of TMOS in twophasebulk models was considered. Here we turn to the situation in which TMOS,dissolved in oil, is forced into a natural porous material containing (heavy) water.The coupled mass transfer and gel reaction were characterized and monitored byemploying bi-nuclear NMR techniques. The relaxation time T 1 of hydrogen in theoleic phase was measured to determine the mass transfer rate, and the relaxationtime T 2 of deuterium in the aqueous phase was measured to determine the reactionrate. It was found that, like in bulk systems, the mass transfer is predominantlydriven by the hydrolysis rate. Due to the gel treatment, for each core the relativepermeability to water was reduced more than that to oil.6.1 IntroductionWhen an alkoxy-silane solution in oil is forced into a porous material containing water thechemical transfers and reacts with water on a pore scale. Silica gel is then formed, whichalters the pore space or blocks the pores. A series of experiments was done to characterizeand monitor in situ the placement and reaction of TMOS in a natural porous material.Consider a homogeneous porous cylinder (core) with a length L, porosity ϕ and absolutepermeability K abs (see Figure 6.1). The core is fully saturated with an aqueousphase and an oleic phase. The water saturation is given by S w (z, t), where z is the spatialcoordinate along the length of the core. Consequently, the oil saturation S o (z, t) isequal to 1 - S w . The volume fraction of TMOS in the oleic phase is given by φ T (z, t). Amixture of TMOS/oil, with a fraction φ T = φ 0 , is injected with an injection rate Q atthe inlet of the core, which is located at z = −L/2, and subsequently fluids are producedat the outlet which is located at z = L/2. As the TMOS comes in contact with water,the TMOS partitions between both phases. In the aqueous phase the TMOS undergoesa heterogeneous reaction with water and forms a gel.To our best knowledge, the coupled mass transfer and gel reaction of TMOS have sofar not been monitored in situ in natural porous materials. To do so, we developed andemployed dedicated NMR techniques to measure the mass transfer and reaction rates ofthe chemical within small sandstone cores which were prepared at either irreducible wateror residual oil conditions [118]. Discrimination between both phases was done by usingheavy water instead of normal water for the aqueous phase and by employing both 1 H-NMR and 2 H-NMR techniques to measure the relaxation times of the oleic phase and the77
78 6. Coupled mass transfer and gelation of TMOS in porous materialsQS w (z,t)P outz=-L/2zz=L/2Fig. 6.1: Model of porous system (core), having a length L, cross-section A, porosity ϕ andabsolute permeability K abs . The fluids are injected at the inlet (z = −L/2) witha flux Q. In this example the water saturation (S w ) profile represents a drainageprocess. The outlet pressure at z = L/2 is equal to P out .aqueous phase, respectively. Additionally, the effect of the gel treatment on the (relative)permeability of the cores was determined by measuring the differential pressure over thecores while injecting oil, respectively water at a given flow rate. The effect of pH andtemperature on the reactive transport was investigated and compared to the bulk modelresults described in Chapter 4.The measurement of NMR relaxation times of fluids or gels (containing hydrogen ordeuterium) in porous materials is not trivial. Therefore, we briefly discuss the mainproperties that we need to interpret our experiments. Two aspects need to be considered;the intrinsic relaxation of the fluids or gels, and the effect of the porous material on therelaxation behavior.First of all, within a saturated porous material the relaxation times of the fluid(s)are affected by the solid content of the porous structure. Interaction between the fluidmolecules and the pore wall in general leads to enhanced relaxation rates, depending on thepolarity of the fluid and the surface structure and chemistry of the pore wall. The smallerthe pore, the higher is the relative number of molecules that, due to diffusion (Brownianmotion), come in contact with the surface within a fixed time interval. Therefore thereduction of the average T 1 and T 2 increases with decreasing pore size [119]. The enhancedaverage relaxation rate of the molecules near the surface can be explained by severalmechanisms. For example, when the fluid molecule adsorbs temporarily at the surfaceor forms hydrogen bonds with a solid molecule, the mobility is temporarily reduced and,hence, the relaxation rate is enhanced [94]. Another effect is due to the presence ofparamagnetic elements in the solid which effectively increase the relaxation rates of thefluid molecules through dipolar interaction, especially with respect to elements with alarge gyromagnetic ratio such as 1 H [120].In case of a multi-phase system the mechanisms mentioned above are important onlyfor the wetting phase. The third mechanism, which holds for all fluid molecules in thepores, is due to the difference in magnetic susceptibility between the fluid and the solid.Local magnetic gradients are found on the pore scale, the magnitude of which increaseswith increasing main magnetic field of the NMR scanner [49]. The presence of a magneticgradient reduces the apparent relaxation time T 2 that is measured with the CPMGsequence. This is due to enhanced dephasing of the spins brought about by diffusionof the molecules within the magnetic gradient [121]. The reduction of T 2 increases with
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TWO-PHASE REACTIVE TRANSPORTOF AN O
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2 1. Introductionproducerbarrierlow
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4 1. IntroductionTwo inter-European
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Part ITwo-phase bulk systems7
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148 Appendix B
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150 Appendix CwhereΩ A (θ) = −2
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152 Appendix C
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154 Appendix D
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158 Appendix EAs the saturations, d
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160 Bibliography[12] B. Vinot, R. S
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162 Bibliography[40] M. D. Mantle a
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164 Bibliography[70] F. J. M. van d
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166 Bibliography[97] K. Yoshida, A.
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168 Bibliography[127] G. W. Scherer
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170 Bibliography
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172 Summaryand gelation rates. The
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174 Samenvattingwaterfase. De T 1 v
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176 List of publications
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178 Dankwoordimmer opgewekte stemmi
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