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9.3. Water shut-off treatments 1339.3 Water shut-off treatmentsThe advantage of using an OSC, such as TMOS, is that it reacts selectively when placedin a reservoir containing both oil and water. In oil-saturated zones the TMOS will onlyform gel in the presence of connate water. As discussed before this will lead to a decreaseof permeability for both oil and water. However, the permeability reduction for oil willbe smaller than the permeability reduction for water.It is advised to limit, in the application of the OSC, the concentration of TMOS insolution below roughly 20%. This should be done in order to limit the amount of gelformed in the oil-producing zones. After shut-in the production is resumed, the flow of oilwill be reduced but not hindered. Ideally, the water in the watered-out zones is blockedtowards the production well. In case the water channels through the treated oil-saturatedzones it will face a reduced relative permeability due to the formed gel. Reservoir modelscould be used to predict the flow profiles of both oil and water in the near-wellbore afterthe gel treatment, in order to optimize the treatment procedure.The reaction of TMOS with water leads to a relatively quick formation of gel, especiallyat elevated temperatures and in the presence of base catalysts or some other salts. Theconditions found in the field vary, but temperatures above 50 ◦ C and the presence of salts[143, 144] are common in downhole reservoirs. When planning a water shut-off treatmentthe composition of the brine and the temperature downhole should be taken into account.Simple bulk phase experiments could be performed on site to evaluate the gel time. Acontrolled placement of TMOS in a well-defined near-wellbore region without prematuregelation (also in the assembly) is therefore challenging. A preflush of oil could prove to beuseful, first of all in order to cool the wellbore area, and secondly to create a non-wettingpathway in the water-saturated zones.9.4 Outlook9.4.1 Extended physical-chemical surveyWith respect to the chemical mechanisms of the reactive transport the problem wasanalyzed using a limited set of parameters, in terms of temperature, pH, concentrationof TMOS etc. Since the sol-gel reaction is known to be sensitive not only to pH andtemperature but also to concentration and type of catalysts (salts) [24] it is recommendedto extend the physical-chemical analysis. Furthermore, it would be interesting to studythe effect of oil-composition (and viscosity), wettability and rock type on the reactivetransport mechanisms.As opposed to the indirect determination of the gel time (based on T 2 ) one could employrheological measurements. A few test experiments were performed using a Couettetyperheometer (Contraves Low Shear 40) to determine the dynamic shear modulus of agelling TMOS solution (see Figure 9.2). A low-frequency oscillating strain with a smallamplitude was imposed on the solution. The absolute shear modulus, which is initiallyaround 0.015 Pa, increases to about 8 Pa at the gel time. The relatively low and fluctuatingmodulus after the steep increase is probably a result of a poor contact between the gel
134 9. Concluding remarks and outlook108oilTMOS|G| [Pa]64waterspindle200 1 2 3 4 5 6 7time [h]cupFig. 9.2: Absolute shear modulus of agelling TMOS solution measuredin a low-frequency oscillatorymode. The gel time is indicatedby the steep increase in the modulusat t ≈ 5 h.Fig. 9.3: Schematic view of a Couette-typerheometer. The spindle is loweredin the aqueous phase, so that themeasured torque is mainly due tothe rheological properties of theaqueous phase.and the rheometer. Nevertheless, the gel time is clearly indicated by the steep increase ofthe shear modulus. An interesting approach would be to perform a similar measurementof the shear modulus of the aqueous phase in a two-phase system as illustrated in Figure9.3. A custom, non-metallic rheometer could be developed that could be used in theNMR set-up. This would allow to directly link the measured (complex) viscosity or shearmodulus to the transverse relaxation time T 2 in the bulk systems.9.4.2 Fine-tuning of core injection experimentsThe core injection experiments could be improved by employing larger cores (e.g. 15 cm inlength and 5 cm in diameter). This way the differential pressure measurements, and hencethe permeability measurements, become more accurate and robust. Secondly, the drainageor imbibition processes would be less affected by the non-uniform flow development nearthe inlet and outlet, compared to the case of using smaller cores. A new NMR insertneeds to be built when using cores larger than the cores used in this research.The current insert (in the 0.95 Tesla set-up) has a satisfactory sensitivity and signalto-noiseratio, considering the limited time of development. However, the NMR insert(or future inserts) could be optimized to obtain the highest possible signal-to-noise ratioand the shortest acquisition times. Dynamic NMR measurements, e.g. during injection,will be possible in large cores provided the acquisition times can be reduced considerablyto less than one minute per slice. Improved signal-to-noise will also improve the quality(resolution) of the T 2 spectra (of deuterium), so that one could extract more informationon the pore scale gel reaction and distribution. Possible solutions to increase signal-tonoiseand sensitivity are the use of higher field strengths and/or the use of improved coildesign.
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TWO-PHASE REACTIVE TRANSPORTOF AN O
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2 1. Introductionproducerbarrierlow
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Part ITwo-phase bulk systems7
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Part IIGel placement in porous mate
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Page 169 and 170: 160 Bibliography[12] B. Vinot, R. S
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Page 181 and 182: 172 Summaryand gelation rates. The
Page 183 and 184: 174 Samenvattingwaterfase. De T 1 v
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