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Stabilization of the mixture of bentonite and sepiolite as a water based drilling fluid
Journal of Petroleum Science and Engineering 76 (2011) 1–5
Contents lists available at ScienceDirect
Journal of Petroleum Science and Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p e t r o l
Stabilization of the mixture of bentonite and sepiolite as a water based drilling fluid Ertuğrul İşçi a, Sevim İşçi Turutoğlu b,⁎ a b
Istanbul Technical University, Department of Petroleum and Natural Gas Engineering, Maslak 34469 Istanbul, Turkey Istanbul Technical University, Department of Physics, Maslak 34469, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 27 October 2009 Accepted 16 November 2010 Available online 3 December 2010 Keywords: bentonite sepiolite mixture of bentonite and sepiolite PVA rheology drilling fluids
a b s t r a c t In this study, the rheological properties of Na–bentonite (NaB), sepiolite (Sp) and the mixture of 2% NaB and 1% Sp from Enez and Eskişehir clay deposits of Turkey were investigated. According to flow properties of the mixture, it was observed that the mixture is not adequate to be a suitable drilling fluid. The main idea was to make stable dispersions with bentonite and sepiolite by using a water soluble polymer as a stabilizer. The changes in the rheological properties of bentonite and sepiolite were studied with various concentrations of polyvinyl alcohol (PVA) to find out the stability of the dispersions. The concentrations, which showed stability for NaB and Sp, were used to stabilize and to change the flow properties of the NaB–Sp dispersion. To determine the interactions, the clay content of the mixture was kept less than the drilling fluid standards of API. The rheological properties of the dispersions were determined at different temperatures. Using the results, the drilling fluid was prepared according to API standards. Moreover, the standard API tests were applied to the drilling fluid to determine the properties of dispersions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Clays are natural raw materials known since old eras and it can be used for different industrial purposes. The different physical and chemical structures of clay minerals supply different properties and industrial application fields like drilling fluids. The rheological and filtration properties of these fluids are greatly controlled by the features of the clay particles. Bentonite has adequate rheological and adsorption properties to be made as a drilling fluid except at high temperatures due to the increasing flocculation. Sepiolite also has adequate rheological properties but can be stable at high temperatures. However, sepiolite has poor fluid loss control and sensitive to contamination. Therefore, the mixture should have the adequate properties as a drilling fluid. Mixing of bentonite and sepiolite causes losing of the rheological properties for both dispersions. Hence, it has unacceptable rheological properties for drilling fluids. When bentonite and sepiolite stabilized with a polymer, the mud shows excellent rheological stability and good filtration control even at high temperatures (Zilch et al., 1991). Mostly, low molecular weight copolymers are used to stabilize bentonites, sepiolites and the mixtures. PVA is a linear, water soluble, cheap and easy accessible polymer. Besides, it is odorless, nontoxic and has high tensile strength and flexibility, also fully degradable. Therefore, PVA is compatible with the natural environment.
⁎ Corresponding author. Tel.: +90 212 285 72 27; fax: +90 212 285 63 86. E-mail address: [email protected] (S.İ. Turutoğlu). 0920-4105/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2010.11.021
Polymers can interact with clay particles according to their ionic or non-ionic character. PVA is a non-ionic and hydrophilic structural polymer. Non-ionic polymer does not interact electrostatically with charged clay particles. At some points, the clay is tied to the surfaces of the particle with hydrogen bond and can stick to the surface. As a result, bridging flocculation or steric repulsion can occur depending on the concentration and chain length of the polymer (Isci et al., 2004, pp. 1975–1978). In this study, the rheological properties of bentonite, sepiolite and the mixture of both were determined. The effects of PVA on the bentonite and sepiolite dispersions were investigated to find out the suitable concentration range to make a stable mixture of bentonite and sepiolite. Then, the effect of temperature on the flow properties of the stabilized mixture and the thermal analyses of the clays were investigated to determine the stability of the dispersions at different temperatures. Using these results, a drilling fluid, which is a stabilized mixture of bentonite and sepiolite with PVA, was prepared and the properties were determined according to API standards. 2. Experimental study 2.1. Materials Natural bentonite—Ca–bentonite (CaB)—was supplied by Bensan Co., Turkey. According to the supplier, the Ca–bentonite was obtained from the Enez area of Turkey. The sepiolite (Sp) sample was obtained from the clay deposits in Eskişehir-Margi, Turkey. X-ray diffraction (Philips PW 1040 model), Infrared (IR) (Jasco Model 5300 FT/IR spectrophotometer) and Differential Thermal Analysis (DTA) (Rigaku
2
E. İşçi, S.İ. Turutoğlu / Journal of Petroleum Science and Engineering 76 (2011) 1–5
Thermoflex) techniques were used to determine the clay mineral types. The dominant clay mineral for bentonite was found to be dioctahedral montmorillonite with minor amounts of illite and kaolinite. Quartz was always present in the clay fraction. Na– bentonite (NaB) was produced by the company from natural bentonite (35% humidity) by treating the natural bentonite (CaB) with 4 wt.% NaHCO3 solutions. The chemical composition of the NaB was determined by atomic adsorption spectroscopy (Perkin Elmer 3030 model) and the silica content was determined by a gravimetric method. The sample has the chemical composition (wt.%): SiO2 58.8, Al2O3 18.73, Fe2O3 3.71, CaO 3.34, MnO 0.09, Na2O 3.36, MgO 2.62, K2O 2.70 and Ti02 0.49. Clay samples have been identified as sepiolite (Sp) by X-ray diffraction. The chemical composition of the sample was determined by atomic adsorption spectroscopy and the silica content was determined gravimetrically. The sample had the chemical composition (wt.%): SiO2 61.03, Al2O3 0.83, Fe2O3 0.10, CaO 0.55, MgO 25.28, Na2O 0.01 and K2O 0.01, TiO2 0.01. Polyvinyl alcohol (Sigma, Germany) is a mixture of synthetic polymers produced by the polymerization of vinyl acetate and partial hydrolysis of the resulting polymer's acetate groups [–(CH2CHOH)n– (CHOCOCH3)m–]. Chemical and physical properties of commercial polyvinyl alcohol (PVA) varied depending on its degree of polymerization and degree of hydrolysis. The degree of hydrolysis was determined as 85% (Cameron, 1977). The average molecular weight was calculated from a single point viscosity value and was found to be ~ 30,000 (Lindemann, 1971, pp. 14, 168). 2.2. Preparation of clay dispersions
Fig. 1. The flow curves of NaB, Sp and NaB–Sp.
performed in a Perkin Elmer Diamond at a heating rate of 20 °C/min under an argon atmosphere and in the temperature range of 30 and 900 °C. 3. Results 3.1. The flow properties of water based NaB, Sp and NaB–Sp dispersions Bentonite and sepiolite particles showed Bingham plastic behavior and highly thixotropic flow according to their swelling properties in water (Guven, 1992). However, the mixture of them showed
Bentonite was dispersed in distillated water with ultrasonic bath and then shaken overnight. Sepiolite is also dispersed in the same way with bentonite, but after ultrasonic bath, instead of shaking, the dispersion blended with Yellowline DI 25 dispersion tool for 8 min to break the long fibers of the sepiolite. The solid contents of the clays should have fewer particles to determine the interactions of the particles. Therefore, the solid content of bentonite and sepiolite was kept as 2% and 1% respectively. Besides, the medium of the mixture was preferred as bentonite dominant dispersion to keep the properties closer to the bentonite. Hence, the mixture of the bentonite and sepiolite were prepared as 2% of bentonite and 1% of sepiolite and it is labeled as NaB–Sp. 2.3. Preparation of clay-PVA dispersions The clay dispersions (NaB, Sp and NaB–Sp) were mixed with 0.1 to 1 g/l concentrations of PVA. Then, the dispersions were shaken for 24 h and ultrasonicated for 5 min. 2.4. Preparation of the drilling fluid with NaB–Sp–PVA According to API standards, the drilling fluid (350 ml) contained 15 g NaB, 7.5 g Sp and 0.35 g PVA and was mixed with a mechanical mixer. Before mixing, PVA was dissolved, sepiolite was blended, and bentonite was dispersed in appropriate amounts of water. 2.5. Methods Rheological properties such as viscosity, shear rate (γ̇) and shear stress (τ) of dispersions were measured using a Brookfield DVIII+ type low-shear viscometer. The flow behavior of the clay dispersions was obtained by shear rate measurements within 0–330 s−1 shear rates. Rheological measurements were carried out in duplicate. The thermal analyses of the clays were determined using thermo gravimetric (TG) and differential thermal analysis (DTA). TG-DTA was
Fig. 2. a) The changes of plastic viscosities versus concentration of PVA b) The pictures of Sp–0.7g/l PVA (steric interaction) and Sp–1g/l PVA (bridging flocculation).
E. İşçi, S.İ. Turutoğlu / Journal of Petroleum Science and Engineering 76 (2011) 1–5
3
Fig. 5. The pictures of NaB, NaB–1 g/l PVA, NaB–Sp, NaB–Sp–1 g/l PVA and Sp dispersions.
Fig. 3. The changes of yield values of the dispersions versus PVA concentration.
Newtonian flow and almost non-thixotropic behavior (Fig. 1). Bentonite showed higher plastic viscosity and yield value because of the higher solid content than sepiolite, but the mixture of them has lower values even though the higher solid content than both bentonite and sepiolite. Hence, the pure mixture of bentonite and sepiolite is not adequate to make a drilling fluid. 3.2. The stabilization of NaB, Sp and NaB–Sp with PVA polymer Different clay particles in a mixture can protect their individual properties if they do not interact with each other in a stable dispersion. Starting from that, stabilization of bentonite and sepiolite particles with PVA was investigated. PVA is a non-ionic, water soluble and hydrophilic structural polymer. Non-ionic polymer does not interact electrostatically with charged clay particles. At some points, the polymer is tied to surfaces of clay with hydrogen bond and can stick to the surface. Moreover, there is a possibility that PVA molecules are introduced into layers of clay minerals (Isci et al., 2006, pp. 449–456). The plastic viscosity of clay dispersions is plotted as a function of increasing PVA concentrations and static stabilization concentrations were found for each dispersion (Fig. 2a). For bentonite dispersion, with the PVA addition to the dispersion, the decrease in rheological parameters was determined. The groups existing before the addition of PVA has been dispersed and the medium can display less resistance against to the flow. When PVA is added with increasing amount into the dispersion, new structures were formed by these groups and the first opening groups as a result of interaction is called “steric thrust”
Fig. 4. The flow curves of NaB, Sp and NaB–Sp–PVA.
which occurred between the polymer molecules that stacked to the particle surfaces (Isci et al., 2004, pp. 1975–1978). The same kind of groups were determined for sepiolite but at the earlier concentrations of PVA. The stabilization concentrations of sepiolite–PVA dispersion were determined around 0.5 and 0.7 g/ l addition of PVA when they were 0.8–1 g/l for bentonite. Nevertheless, the further addition of PVA to sepiolite caused the bridging flocculation of the dispersion and a second layer of the PVA hang on to the surfaces of sepiolite particles that caused to lie down the particles in the water medium (Fig. 2b). PVA molecules caused a bridging effect on the mixture of bentonite and sepiolite dispersion. Besides, in the medium where lots of particles interact with each other by bridging effect caused to the occurrence of net structure that was formed by the addition of PVA amount, the viscosity value will increase. The net structure kept the water between the particles so, no sedimentation of the particles was seen in the dispersion. However, local stabilization points were determined in some concentration ranges like 0.9–1 g/ and 1.5–1.7 g/l of PVA. The yield values shown in Figure 3 were determined as a good agreement with viscosity results for NaB and NaB–Sp dispersions. However, the last additions of PVA to Sp dispersions showed increasing yield values while the plastic viscosities showed decreasing values. It shows that the flocculation of Sp particles increased the yield values of the dispersions. After the modification of the mixture of the bentonite and sepiolite with PVA, the flow curve was determined and it was found that the flow properties of the mixture were changed with the addition of PVA. The modified mixture showed Bingham plastic behavior and highly thixotropic flow (Fig. 4).
Fig. 6. The viscosity–speed curves of the NaB–Sp–1 g/lPVA and NaB–Sp–2 g/lPVA dispersions at different temperatures.
E. İşçi, S.İ. Turutoğlu / Journal of Petroleum Science and Engineering 76 (2011) 1–5
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Fig. 7. The flow curves of NaB–Sp–1g/lPVA and NaB–Sp dispersions at different temperatures.
Fig. 9. The TG–DTA curves of Sp.
3.5. Thermal characterization of NaB, Sp, NaB–Sp and NaB–Sp–PVA 3.3. Stability of the dispersions The stability of the dispersions was investigated with the time dependency. The dispersions were put in test tubes to see whether there is any sedimentation or not. The pictures of the tubes were taken after 2 weeks (Fig. 5). As seen in Figure 5, NaB, NaB with 1 g/ l PVA and NaB–Sp had some sediment, but Sp, Sp with 1 g/l PVA and NaB–Sp–1 g/l PVA had no sediment and completely stable. The flow properties of the concentration of 2 g/l PVA addition which also showed steric stabilization with the mixture of NaB–Sp were investigated at different temperatures and it was found that the viscosity values went higher but thixotropic behavior disappeared at that concentration (Fig. 6). As a result of all these, the mixture of bentonite and sepiolite (NaB–Sp) were stabilized with the addition of 1 g/l PVA and labeled as NaB–Sp–PVA.
TG-DTA analyses were used to determine the thermal behavior of the clay samples. The samples were dried at room temperature before the measurements. Bentonite sample showed 2 weight loss events on heating in TG-DTA (Fig. 8). The first one, which occurred at 71 °C, corresponds to the loss of molecular water from the interlayer and adsorbed water. At that temperature, the weight loss of the bentonite
3.4. Effects of temperature on the flow properties of NaB–Sp and NaB–Sp–PVA dispersions The effect of the temperature on the flow properties of NaB–Sp– PVA and NaB–Sp dispersions were investigated at 25, 50, and 75 °C. As seen in Figure 7, the yield values increased for all the dispersions with increasing temperature. The plastic viscosities of the dispersions did not show significant change with the temperature but the hysteresis area of the NaB–Sp–PVA dispersion increased extremely.
Fig. 8. The TG–DTA curves of NaB.
Fig. 10. The TG–DTA curves of NaB–Sp.
Fig. 11. The TG–DTA curves of NaB–SP–PVA.
E. İşçi, S.İ. Turutoğlu / Journal of Petroleum Science and Engineering 76 (2011) 1–5
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Table 1 Time dependency of filtration. Time (min)
Water loss (ml)
0.25 0.45 1 2 3 5 7 10 30
1.3 1.8 3.1 4.8 5.0 8.0 9.7 11.9 21.6
Fig. 13. High temperature and high pressure viscosity curves of the drilling fluid.
The rheological behavior of the drilling fluid, which was prepared according to API standards, was determined at different temperatures and the results were given in Figure 12. The viscosities did not show significant change with increasing temperature. The viscosity–speed curves of the drilling fluid at high temperatures and high pressures curves are given in Figure 13. Increasing pressure causes to slightly increase at viscosities. When the temperature increased at a constant pressure, the same effect can be seen from the viscosity–speed curves. 4. Conclusions Fig. 12. The viscosity–speed curves of the drilling fluid at different temperatures.
was 2%. Besides, at 691 °C, the dehydroxylation of the silicate lattice was determined where the sample lost 12.7% (Earnest, 1988, pp. 272– 287; Kok, 2004, pp. 145–151). Sepiolite sample had 3 weight loss peaks shown in Figure 9. The peaks appeared at 70, 313, and 800 °C corresponds to loss of zeolitic water (6% weight loss), dehydration in which sepiolite loses the coordinated water (12% weight loss) and dehydroxylation of sepiolite anhydrite (19% weight loss), respectively (Alver et al., 2008, pp. 835–840; Tartaglione et al., 2008, pp. 161–168). The mixture of NaB and Sp had all peaks from bentonite and sepiolite but at different temperatures (Fig. 10). The adsorbed water of both bentonite and sepiolite left the mixture at 77 °C which means the mixture kept the water better than pristine clays. However, the dehydroxylation started earlier than the pristine clays. The stabilized mixture of bentonite and sepiolite with PVA showed an extra peak corresponds to the melting of PVA at 219 °C (Fig. 11) (Alkan and Benlikaya, 2009, pp. 3764–3774). The adsorbed water left the sample at 72 °C but the zeolitic water of sepiolite left the sample at 332 °C instead of 313 °C. Besides, the dehydroxylation occurred at 693 and 802 °C. The weight loss amounts were higher than the pristine clays but that loses occurred at higher temperatures. 3.6. Properties of the drilling fluid API standard tests were applied to the sample. The density and the pH value of the drilling fluid was determined as 8.7 lbm/gal and 9, respectively. The filter cake thickness was found 3 mm and the time dependency of filtration was given in Table 1.
The polymer PVA can be used as a stabilizer both bentonite-based and sepiolite-based drilling muds. PVA covers the clay particles and causes the steric interaction between the particles. The mixture of bentonite and sepiolite causes to lose the flow properties of each dispersion so the mixture is not a proper dispersion to make a suitable drilling fluid. However, when the bentonite and sepiolite particles were stabilized with PVA, the flow properties of the mixture were changed in advance of drilling fluid. References Alkan, M., Benlikaya, R., 2009. Polyvinyl alcohol nanocomposites with sepiolite and heat-treated sepiolites. J. Appl. Polym. Sci. 112, 3764–3774. Alver, B.E., Sakici, M., Yorukogullari, E., Yilmaz, Y., Guven, M., 2008. Thermal behavior and water adsorption of natural and modified sepiolite having dolomite from Turkey. J. Therm. Anal. Calorim. 94, 835–840. Cameron, G.G., 1977. In: Urbanski, J., et al (Ed), Handbook of synthetic polymers and plastics. Poland: Wiley, p. 394. Earnest, C.M., 1988. Thermogravimetry of selected clays and clay products. American Society for Testing and Materials, pp. 272–287. Guven, N., 1992. CMS workshop lectures. Clay minerals soc. Boulder. Isci, S., Gunister, E., Ece, I.O., Gungor, N., 2004. The modification of rheologic properties of clays with PVA effect. Mater. Lett. 58, 1975–1978. Isci, S., Unlu, C., Atici, O., Gungor, N., 2006. Rheology and structure of aqueous bentonite–polyvinyl alcohol dispersions. Bull. Mater. Sci. 29, 449–456. Kok, M.V., 2004. Rheological and thermal analysis of bentonite for water base drilling fluids. Energy Sources 26, 145–151. Lindemann, M.K., 1971. In: Mark, H.F., et al. (Ed.), Encyclopedia of polymer science and technology, 14. Wiley, New York, p. 168. Tartaglione, G., Tabuani, D., Camino, G., 2008. Thermal and morphological characterization of organically modified sepiolite. Microporous Mesoporous Mater. 107, 161–168. Zilch, H.E., Otto, M.J., Pye, D.S., 1991. The evolution of geothermal drilling fluid in the imperial valey. SPE 21786.
Contents lists available at ScienceDirect
Journal of Petroleum Science and Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p e t r o l
Stabilization of the mixture of bentonite and sepiolite as a water based drilling fluid Ertuğrul İşçi a, Sevim İşçi Turutoğlu b,⁎ a b
Istanbul Technical University, Department of Petroleum and Natural Gas Engineering, Maslak 34469 Istanbul, Turkey Istanbul Technical University, Department of Physics, Maslak 34469, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 27 October 2009 Accepted 16 November 2010 Available online 3 December 2010 Keywords: bentonite sepiolite mixture of bentonite and sepiolite PVA rheology drilling fluids
a b s t r a c t In this study, the rheological properties of Na–bentonite (NaB), sepiolite (Sp) and the mixture of 2% NaB and 1% Sp from Enez and Eskişehir clay deposits of Turkey were investigated. According to flow properties of the mixture, it was observed that the mixture is not adequate to be a suitable drilling fluid. The main idea was to make stable dispersions with bentonite and sepiolite by using a water soluble polymer as a stabilizer. The changes in the rheological properties of bentonite and sepiolite were studied with various concentrations of polyvinyl alcohol (PVA) to find out the stability of the dispersions. The concentrations, which showed stability for NaB and Sp, were used to stabilize and to change the flow properties of the NaB–Sp dispersion. To determine the interactions, the clay content of the mixture was kept less than the drilling fluid standards of API. The rheological properties of the dispersions were determined at different temperatures. Using the results, the drilling fluid was prepared according to API standards. Moreover, the standard API tests were applied to the drilling fluid to determine the properties of dispersions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Clays are natural raw materials known since old eras and it can be used for different industrial purposes. The different physical and chemical structures of clay minerals supply different properties and industrial application fields like drilling fluids. The rheological and filtration properties of these fluids are greatly controlled by the features of the clay particles. Bentonite has adequate rheological and adsorption properties to be made as a drilling fluid except at high temperatures due to the increasing flocculation. Sepiolite also has adequate rheological properties but can be stable at high temperatures. However, sepiolite has poor fluid loss control and sensitive to contamination. Therefore, the mixture should have the adequate properties as a drilling fluid. Mixing of bentonite and sepiolite causes losing of the rheological properties for both dispersions. Hence, it has unacceptable rheological properties for drilling fluids. When bentonite and sepiolite stabilized with a polymer, the mud shows excellent rheological stability and good filtration control even at high temperatures (Zilch et al., 1991). Mostly, low molecular weight copolymers are used to stabilize bentonites, sepiolites and the mixtures. PVA is a linear, water soluble, cheap and easy accessible polymer. Besides, it is odorless, nontoxic and has high tensile strength and flexibility, also fully degradable. Therefore, PVA is compatible with the natural environment.
⁎ Corresponding author. Tel.: +90 212 285 72 27; fax: +90 212 285 63 86. E-mail address: [email protected] (S.İ. Turutoğlu). 0920-4105/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2010.11.021
Polymers can interact with clay particles according to their ionic or non-ionic character. PVA is a non-ionic and hydrophilic structural polymer. Non-ionic polymer does not interact electrostatically with charged clay particles. At some points, the clay is tied to the surfaces of the particle with hydrogen bond and can stick to the surface. As a result, bridging flocculation or steric repulsion can occur depending on the concentration and chain length of the polymer (Isci et al., 2004, pp. 1975–1978). In this study, the rheological properties of bentonite, sepiolite and the mixture of both were determined. The effects of PVA on the bentonite and sepiolite dispersions were investigated to find out the suitable concentration range to make a stable mixture of bentonite and sepiolite. Then, the effect of temperature on the flow properties of the stabilized mixture and the thermal analyses of the clays were investigated to determine the stability of the dispersions at different temperatures. Using these results, a drilling fluid, which is a stabilized mixture of bentonite and sepiolite with PVA, was prepared and the properties were determined according to API standards. 2. Experimental study 2.1. Materials Natural bentonite—Ca–bentonite (CaB)—was supplied by Bensan Co., Turkey. According to the supplier, the Ca–bentonite was obtained from the Enez area of Turkey. The sepiolite (Sp) sample was obtained from the clay deposits in Eskişehir-Margi, Turkey. X-ray diffraction (Philips PW 1040 model), Infrared (IR) (Jasco Model 5300 FT/IR spectrophotometer) and Differential Thermal Analysis (DTA) (Rigaku
2
E. İşçi, S.İ. Turutoğlu / Journal of Petroleum Science and Engineering 76 (2011) 1–5
Thermoflex) techniques were used to determine the clay mineral types. The dominant clay mineral for bentonite was found to be dioctahedral montmorillonite with minor amounts of illite and kaolinite. Quartz was always present in the clay fraction. Na– bentonite (NaB) was produced by the company from natural bentonite (35% humidity) by treating the natural bentonite (CaB) with 4 wt.% NaHCO3 solutions. The chemical composition of the NaB was determined by atomic adsorption spectroscopy (Perkin Elmer 3030 model) and the silica content was determined by a gravimetric method. The sample has the chemical composition (wt.%): SiO2 58.8, Al2O3 18.73, Fe2O3 3.71, CaO 3.34, MnO 0.09, Na2O 3.36, MgO 2.62, K2O 2.70 and Ti02 0.49. Clay samples have been identified as sepiolite (Sp) by X-ray diffraction. The chemical composition of the sample was determined by atomic adsorption spectroscopy and the silica content was determined gravimetrically. The sample had the chemical composition (wt.%): SiO2 61.03, Al2O3 0.83, Fe2O3 0.10, CaO 0.55, MgO 25.28, Na2O 0.01 and K2O 0.01, TiO2 0.01. Polyvinyl alcohol (Sigma, Germany) is a mixture of synthetic polymers produced by the polymerization of vinyl acetate and partial hydrolysis of the resulting polymer's acetate groups [–(CH2CHOH)n– (CHOCOCH3)m–]. Chemical and physical properties of commercial polyvinyl alcohol (PVA) varied depending on its degree of polymerization and degree of hydrolysis. The degree of hydrolysis was determined as 85% (Cameron, 1977). The average molecular weight was calculated from a single point viscosity value and was found to be ~ 30,000 (Lindemann, 1971, pp. 14, 168). 2.2. Preparation of clay dispersions
Fig. 1. The flow curves of NaB, Sp and NaB–Sp.
performed in a Perkin Elmer Diamond at a heating rate of 20 °C/min under an argon atmosphere and in the temperature range of 30 and 900 °C. 3. Results 3.1. The flow properties of water based NaB, Sp and NaB–Sp dispersions Bentonite and sepiolite particles showed Bingham plastic behavior and highly thixotropic flow according to their swelling properties in water (Guven, 1992). However, the mixture of them showed
Bentonite was dispersed in distillated water with ultrasonic bath and then shaken overnight. Sepiolite is also dispersed in the same way with bentonite, but after ultrasonic bath, instead of shaking, the dispersion blended with Yellowline DI 25 dispersion tool for 8 min to break the long fibers of the sepiolite. The solid contents of the clays should have fewer particles to determine the interactions of the particles. Therefore, the solid content of bentonite and sepiolite was kept as 2% and 1% respectively. Besides, the medium of the mixture was preferred as bentonite dominant dispersion to keep the properties closer to the bentonite. Hence, the mixture of the bentonite and sepiolite were prepared as 2% of bentonite and 1% of sepiolite and it is labeled as NaB–Sp. 2.3. Preparation of clay-PVA dispersions The clay dispersions (NaB, Sp and NaB–Sp) were mixed with 0.1 to 1 g/l concentrations of PVA. Then, the dispersions were shaken for 24 h and ultrasonicated for 5 min. 2.4. Preparation of the drilling fluid with NaB–Sp–PVA According to API standards, the drilling fluid (350 ml) contained 15 g NaB, 7.5 g Sp and 0.35 g PVA and was mixed with a mechanical mixer. Before mixing, PVA was dissolved, sepiolite was blended, and bentonite was dispersed in appropriate amounts of water. 2.5. Methods Rheological properties such as viscosity, shear rate (γ̇) and shear stress (τ) of dispersions were measured using a Brookfield DVIII+ type low-shear viscometer. The flow behavior of the clay dispersions was obtained by shear rate measurements within 0–330 s−1 shear rates. Rheological measurements were carried out in duplicate. The thermal analyses of the clays were determined using thermo gravimetric (TG) and differential thermal analysis (DTA). TG-DTA was
Fig. 2. a) The changes of plastic viscosities versus concentration of PVA b) The pictures of Sp–0.7g/l PVA (steric interaction) and Sp–1g/l PVA (bridging flocculation).
E. İşçi, S.İ. Turutoğlu / Journal of Petroleum Science and Engineering 76 (2011) 1–5
3
Fig. 5. The pictures of NaB, NaB–1 g/l PVA, NaB–Sp, NaB–Sp–1 g/l PVA and Sp dispersions.
Fig. 3. The changes of yield values of the dispersions versus PVA concentration.
Newtonian flow and almost non-thixotropic behavior (Fig. 1). Bentonite showed higher plastic viscosity and yield value because of the higher solid content than sepiolite, but the mixture of them has lower values even though the higher solid content than both bentonite and sepiolite. Hence, the pure mixture of bentonite and sepiolite is not adequate to make a drilling fluid. 3.2. The stabilization of NaB, Sp and NaB–Sp with PVA polymer Different clay particles in a mixture can protect their individual properties if they do not interact with each other in a stable dispersion. Starting from that, stabilization of bentonite and sepiolite particles with PVA was investigated. PVA is a non-ionic, water soluble and hydrophilic structural polymer. Non-ionic polymer does not interact electrostatically with charged clay particles. At some points, the polymer is tied to surfaces of clay with hydrogen bond and can stick to the surface. Moreover, there is a possibility that PVA molecules are introduced into layers of clay minerals (Isci et al., 2006, pp. 449–456). The plastic viscosity of clay dispersions is plotted as a function of increasing PVA concentrations and static stabilization concentrations were found for each dispersion (Fig. 2a). For bentonite dispersion, with the PVA addition to the dispersion, the decrease in rheological parameters was determined. The groups existing before the addition of PVA has been dispersed and the medium can display less resistance against to the flow. When PVA is added with increasing amount into the dispersion, new structures were formed by these groups and the first opening groups as a result of interaction is called “steric thrust”
Fig. 4. The flow curves of NaB, Sp and NaB–Sp–PVA.
which occurred between the polymer molecules that stacked to the particle surfaces (Isci et al., 2004, pp. 1975–1978). The same kind of groups were determined for sepiolite but at the earlier concentrations of PVA. The stabilization concentrations of sepiolite–PVA dispersion were determined around 0.5 and 0.7 g/ l addition of PVA when they were 0.8–1 g/l for bentonite. Nevertheless, the further addition of PVA to sepiolite caused the bridging flocculation of the dispersion and a second layer of the PVA hang on to the surfaces of sepiolite particles that caused to lie down the particles in the water medium (Fig. 2b). PVA molecules caused a bridging effect on the mixture of bentonite and sepiolite dispersion. Besides, in the medium where lots of particles interact with each other by bridging effect caused to the occurrence of net structure that was formed by the addition of PVA amount, the viscosity value will increase. The net structure kept the water between the particles so, no sedimentation of the particles was seen in the dispersion. However, local stabilization points were determined in some concentration ranges like 0.9–1 g/ and 1.5–1.7 g/l of PVA. The yield values shown in Figure 3 were determined as a good agreement with viscosity results for NaB and NaB–Sp dispersions. However, the last additions of PVA to Sp dispersions showed increasing yield values while the plastic viscosities showed decreasing values. It shows that the flocculation of Sp particles increased the yield values of the dispersions. After the modification of the mixture of the bentonite and sepiolite with PVA, the flow curve was determined and it was found that the flow properties of the mixture were changed with the addition of PVA. The modified mixture showed Bingham plastic behavior and highly thixotropic flow (Fig. 4).
Fig. 6. The viscosity–speed curves of the NaB–Sp–1 g/lPVA and NaB–Sp–2 g/lPVA dispersions at different temperatures.
E. İşçi, S.İ. Turutoğlu / Journal of Petroleum Science and Engineering 76 (2011) 1–5
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Fig. 7. The flow curves of NaB–Sp–1g/lPVA and NaB–Sp dispersions at different temperatures.
Fig. 9. The TG–DTA curves of Sp.
3.5. Thermal characterization of NaB, Sp, NaB–Sp and NaB–Sp–PVA 3.3. Stability of the dispersions The stability of the dispersions was investigated with the time dependency. The dispersions were put in test tubes to see whether there is any sedimentation or not. The pictures of the tubes were taken after 2 weeks (Fig. 5). As seen in Figure 5, NaB, NaB with 1 g/ l PVA and NaB–Sp had some sediment, but Sp, Sp with 1 g/l PVA and NaB–Sp–1 g/l PVA had no sediment and completely stable. The flow properties of the concentration of 2 g/l PVA addition which also showed steric stabilization with the mixture of NaB–Sp were investigated at different temperatures and it was found that the viscosity values went higher but thixotropic behavior disappeared at that concentration (Fig. 6). As a result of all these, the mixture of bentonite and sepiolite (NaB–Sp) were stabilized with the addition of 1 g/l PVA and labeled as NaB–Sp–PVA.
TG-DTA analyses were used to determine the thermal behavior of the clay samples. The samples were dried at room temperature before the measurements. Bentonite sample showed 2 weight loss events on heating in TG-DTA (Fig. 8). The first one, which occurred at 71 °C, corresponds to the loss of molecular water from the interlayer and adsorbed water. At that temperature, the weight loss of the bentonite
3.4. Effects of temperature on the flow properties of NaB–Sp and NaB–Sp–PVA dispersions The effect of the temperature on the flow properties of NaB–Sp– PVA and NaB–Sp dispersions were investigated at 25, 50, and 75 °C. As seen in Figure 7, the yield values increased for all the dispersions with increasing temperature. The plastic viscosities of the dispersions did not show significant change with the temperature but the hysteresis area of the NaB–Sp–PVA dispersion increased extremely.
Fig. 8. The TG–DTA curves of NaB.
Fig. 10. The TG–DTA curves of NaB–Sp.
Fig. 11. The TG–DTA curves of NaB–SP–PVA.
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Table 1 Time dependency of filtration. Time (min)
Water loss (ml)
0.25 0.45 1 2 3 5 7 10 30
1.3 1.8 3.1 4.8 5.0 8.0 9.7 11.9 21.6
Fig. 13. High temperature and high pressure viscosity curves of the drilling fluid.
The rheological behavior of the drilling fluid, which was prepared according to API standards, was determined at different temperatures and the results were given in Figure 12. The viscosities did not show significant change with increasing temperature. The viscosity–speed curves of the drilling fluid at high temperatures and high pressures curves are given in Figure 13. Increasing pressure causes to slightly increase at viscosities. When the temperature increased at a constant pressure, the same effect can be seen from the viscosity–speed curves. 4. Conclusions Fig. 12. The viscosity–speed curves of the drilling fluid at different temperatures.
was 2%. Besides, at 691 °C, the dehydroxylation of the silicate lattice was determined where the sample lost 12.7% (Earnest, 1988, pp. 272– 287; Kok, 2004, pp. 145–151). Sepiolite sample had 3 weight loss peaks shown in Figure 9. The peaks appeared at 70, 313, and 800 °C corresponds to loss of zeolitic water (6% weight loss), dehydration in which sepiolite loses the coordinated water (12% weight loss) and dehydroxylation of sepiolite anhydrite (19% weight loss), respectively (Alver et al., 2008, pp. 835–840; Tartaglione et al., 2008, pp. 161–168). The mixture of NaB and Sp had all peaks from bentonite and sepiolite but at different temperatures (Fig. 10). The adsorbed water of both bentonite and sepiolite left the mixture at 77 °C which means the mixture kept the water better than pristine clays. However, the dehydroxylation started earlier than the pristine clays. The stabilized mixture of bentonite and sepiolite with PVA showed an extra peak corresponds to the melting of PVA at 219 °C (Fig. 11) (Alkan and Benlikaya, 2009, pp. 3764–3774). The adsorbed water left the sample at 72 °C but the zeolitic water of sepiolite left the sample at 332 °C instead of 313 °C. Besides, the dehydroxylation occurred at 693 and 802 °C. The weight loss amounts were higher than the pristine clays but that loses occurred at higher temperatures. 3.6. Properties of the drilling fluid API standard tests were applied to the sample. The density and the pH value of the drilling fluid was determined as 8.7 lbm/gal and 9, respectively. The filter cake thickness was found 3 mm and the time dependency of filtration was given in Table 1.
The polymer PVA can be used as a stabilizer both bentonite-based and sepiolite-based drilling muds. PVA covers the clay particles and causes the steric interaction between the particles. The mixture of bentonite and sepiolite causes to lose the flow properties of each dispersion so the mixture is not a proper dispersion to make a suitable drilling fluid. However, when the bentonite and sepiolite particles were stabilized with PVA, the flow properties of the mixture were changed in advance of drilling fluid. References Alkan, M., Benlikaya, R., 2009. Polyvinyl alcohol nanocomposites with sepiolite and heat-treated sepiolites. J. Appl. Polym. Sci. 112, 3764–3774. Alver, B.E., Sakici, M., Yorukogullari, E., Yilmaz, Y., Guven, M., 2008. Thermal behavior and water adsorption of natural and modified sepiolite having dolomite from Turkey. J. Therm. Anal. Calorim. 94, 835–840. Cameron, G.G., 1977. In: Urbanski, J., et al (Ed), Handbook of synthetic polymers and plastics. Poland: Wiley, p. 394. Earnest, C.M., 1988. Thermogravimetry of selected clays and clay products. American Society for Testing and Materials, pp. 272–287. Guven, N., 1992. CMS workshop lectures. Clay minerals soc. Boulder. Isci, S., Gunister, E., Ece, I.O., Gungor, N., 2004. The modification of rheologic properties of clays with PVA effect. Mater. Lett. 58, 1975–1978. Isci, S., Unlu, C., Atici, O., Gungor, N., 2006. Rheology and structure of aqueous bentonite–polyvinyl alcohol dispersions. Bull. Mater. Sci. 29, 449–456. Kok, M.V., 2004. Rheological and thermal analysis of bentonite for water base drilling fluids. Energy Sources 26, 145–151. Lindemann, M.K., 1971. In: Mark, H.F., et al. (Ed.), Encyclopedia of polymer science and technology, 14. Wiley, New York, p. 168. Tartaglione, G., Tabuani, D., Camino, G., 2008. Thermal and morphological characterization of organically modified sepiolite. Microporous Mesoporous Mater. 107, 161–168. Zilch, H.E., Otto, M.J., Pye, D.S., 1991. The evolution of geothermal drilling fluid in the imperial valey. SPE 21786.