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1.2. Oil-Soluble Chemicals 31.2 Oil-Soluble ChemicalsA novel concept in water shut-off is the application of an Oil-Soluble Chemical (OSC)which is soluble and chemically stable in oil but reacts with water to form a gel. By usingan OSC the gelant will selectively form a gel in the reservoir without blocking the oilproducing layers.The chemical tetra-methyl-ortho-silicate (TMOS) is a potential OSC for water shut-off.Like other alkoxy-silanes TMOS undergoes a heterogeneous gel reaction when mixed withwater according to the sol-gel principle [24]. When an oleic solution of TMOS is injectedin a porous medium (e.g. a reservoir) which contains water, the TMOS transfers to theaqueous phase where it undergoes the gel reaction. Consider a reservoir that consists ofa low-permeable oil-producing layer and a high-permeable watered-out layer (see Figure1.2). A solution of TMOS in oil is forced, for instance through an existing productionwell, into both layers. As a result the mixture penetrates into the reservoir and displacesthe original fluids. In the oil-producing layer the TMOS remains stable and will not react.However, in the water-producing layer the TMOS will react on a pore scale with the waternear the displacement front and with the residual water behind the front. Subsequently,a shut-in of the near-wellbore area is performed to allow the gel to cure. Ideally, uponresumption of the production the gel effectively blocks the water-producing layer whilethe oil-producing layer is not affected by the gel treatment (see Figure 1.1).F H @ K ? A H E 6 5 M E D E C D M = J A H Fig. 1.2: A bi-layered reservoir after water breakthrough (left). A mixture of TMOS and oilis injected in both layers. In the water-producing layer the TMOS is transferredfrom the oleic to the aqueous phase on the pore scale (right), where it undergoes aheterogeneous gel reaction.The application of alkoxy-silanes, in the context of water control, was first proposedby Plazanet and Thomere for the consolidation of sand producing formations during oilrecovery [25]. Later, Thompson and Fogler considered the use of OSCs as diverting agentsin both acid preflush and water control procedures. They performed laboratory experimentsby injecting oleic solutions of either polyurethane or TMOS into micromodels andceramic or limestone cores [20, 26]. Their study was focussed on pressure profiles, tracersand effluent analysis. Polyurethane was found not to be effective for the alteration of the(water phase) permeability [26]. Their micromodel visualization experiments showed themass transfer of TMOS and the fluid/gel distribution on a pore scale, but the mechanismsremained unclear [27].
4 1. IntroductionTwo inter-European cooperation projects devoted considerable effort to evaluate thepotential of TMOS as a bullhead water control agent [3]. The micromodel and coreinjection studies, conducted in these projects, were reported by Grattoni et al. [28, 29]and Bartosek et al. [30]. The effect of aging of TMOS-based gels was analyzed as wellas the mechanisms of permeability modification. Recently, Elewaut et al. and Elewautand Zitha reported experimental studies of TMOS gel treatments in sandstone cores,using pressure data [31] and X-Ray computed tomography (CT) imaging [32]. The masstransfer of the flowing gelant in the initial stages of the gel treatment was interpretedin a macroscopic sense. Other (potential) types of OSCs for water shut-off have notbeen reported sofar, except for tetra-ethyl-ortho-silicate (TEOS) [30], which is related toTMOS. However, the reaction of TEOS with water is much slower than the reaction ofTMOS with water [24].1.3 Scope of thesisThe aim of the research was to investigate, on a fundamental level, the reactive transportof an oil-soluble gelant in two-phase systems, in bulk and within porous materials.A clear understanding of the physical-chemical mechanisms is lacking in literature.TMOS was chosen as a model chemical. The choice is inspired by the potential useof TMOS for water shut-off in hydrocarbon reservoirs. The effect of physical-chemicalparameters, such as temperature, pH and concentration on the reactive transport wasinvestigated, since the gel reaction is known to be sensitive to these parameters [24, 33–35]. The results described in the thesis form a framework for the reactive transport ofoil-soluble gelants in general, as well as a framework for the experimental techniquesneeded in the analysis.# Fig. 1.3: The role of relaxation times in NMR imaging. (left) High-resolution T 1 -weightedNMR image of a glass bead pack saturated with oil in the upper part and water inthe lower part. (right) T 2 map of a slice of a chocolate-peanut candybar.The main experimental tool used in this study is Nuclear Magnetic Resonance (NMR)[36]. Like other techniques (e.g. X-Ray CT imaging [37] or Scanning Neutron Radiography
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160 Bibliography[12] B. Vinot, R. S
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172 Summaryand gelation rates. The
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