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Double protection drilling fluid: Optimization for sandstone-like uranium formation
Applied Clay Science 88–89 (2014) 233–238
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Double protection drilling fluid: Optimization for sandstone-like uranium formation Shu-Qing Hao a,b,c,d,⁎,1, Sungho Kim d,⁎⁎,1, Yong Qin a, Xue-Hai Fu a a Key Laboratory of CBM Resources and Reservoir Formation Process Ministry of Education of China, School of Resource and Geosciences, China University of Mining and Technology, Xuzhou 221116, China b State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221008, China c Department of Civil Engineering, Monash University of Mining and Technology, Victoria 3840, Australia d Geotechnical and Hydrogeological Engineering Research Group, Monash University, Victoria 3842, Australia
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Article history: Received 8 August 2013 Received in revised form 22 October 2013 Accepted 18 November 2013 Available online 17 December 2013 Keywords: Carboxymethyl cellulose Double protection drilling fluid Montmorillonite Potassium humate Sandstone-like uranium formation Vegetable gum Orthogonal testing
a b s t r a c t Nuclear energy, an alternative for fossil fuel, is accompanied by the excavation of uranium ores and in turn the use of drilling fluids. Thus, the objective of this study was to optimize the formula of drilling fluid for sandstone-like uranium formation representing the Xinjiang's Yili Basin uranium formation in China. Additives such as Na2CO3, potassium humate (KHM), sodium carboxymethyl cellulose (Na-CMC), and vegetable gum (VG) were introduced to montmorillonite (Mt) slurry to maximize the performance of drilling fluid in terms of rheological behavior and filtration characteristics. Orthogonal testing, soaking test, and pressure-bearing test were conducted to find the optimum formula of drilling fluid. The maximized performance was achieved with a drilling fluid consisting of 4% Mt, 1.6% Na2CO3, 0.1% KHM, 1.5% Na-CMC, and 0.5% VG. The lowest density and water loss, with a sufficiently high viscosity were achieved with this concentration combination of each additive. Soaking and pressure-bearing tests exhibited no visual changes of a specimen treated with the drilling fluid, which confirmed the result of optimization. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Fossil fuels have been a main energy source making up over 87% of the world's energy consumption in the past 200 years (Demirbas, 2010). As a result, the conventional energy resources decline and the price is ever increasing, which will lead to an energy crisis in the near future (Turner, 1974). Nuclear energy has alternatively been used in several countries and proven to be cost-effective (Dresselhaus and Thomas, 2001; IAEA, 1997; Paker and Holt, 2007) and stabilize global climate (Hoffert et al., 2002). However, the use of nuclear energy is accompanied by the excavation of uranium which includes difficulties due to the complexity of uranium formation (Dahlkamp, 1978; Lyons et al., 1982). Core samples of uranium drilled from the formation are often necessarily analyzed to understand the properties of mineral reservoir (Anderson, 1969; Ebenhack, 1987; Fitch, 1980; Meinrath et al., 1999; Morgan et al., 1997). During drilling core samples, a drilling mud has effectively been used to improve the borehole stability (Cherepanov,
⁎ Correspondence to: School of Resource and Geosciences, China University of Mining and Technology, Xuzhou, China. Tel.: +86 13952208471. ⁎⁎ Corresponding author. Tel.: +61 3 512 26411. E-mail addresses: [email protected] (S.-Q. Hao), [email protected] (S. Kim). 1 These two corresponding authors contributed equally to the development of this manuscript. 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.11.036
1982; Hao, 2011; Zeynali, 2012). For example, a drilling mud has successfully been applied in uranium sandstone formation (Keas, 1974; Petri and Neto, 2010). A drilling mud itself has also been developed in terms of its properties and composition. For example, a thixotropic drilling mud has been applied to form a radioactive filter cake for a purpose of analyzing the radioactivity of a formation (Ebenhack., 1987). The addition of polymers has been applied to improve the instability problem of shale field (Khodjaa et al., 2010). Nevertheless, there is a demand for protecting drilled core samples because those drilling mud technologies only consider the protection of wellbore. Water invasion often destroys core samples, preventing estimation of the potential producing capacities of the formations penetrated (Bailey and Keall, 1993; Patel and Salandanan, 1968). This is especially important for uranium formations or similar reservoirs, where its poor engineering properties, including loose soft/hard inter-bedding, water-containing strata, poor degree of consolidation, and cracks, often result in collapse of drilled samples as well as of wellbore (Borivoje et al., 2007; Bowes and Procter, 1997). The objective of this study was to create a suitable drilling fluid for use in a sandstone uranium formation, which protects both borehole and core samples (double protection). In order to modify properties of drilling mud (bentonite), additives including Na2CO3, potassium humate (KHM), sodium carboxymethyl cellulose (Na-CMC), and vegetable gum (VG) were utilized. The orthogonal experimental method was employed to optimize the formula of drilling fluid. Soaking and
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pressure-bearing tests were conducted to confirm the optimum formula of drilling fluid. 2. Experimental study 2.1. Materials Montmorillonite (Mt) is commonly used as a drilling mud due to its unique characteristics, such as negative surface charge, high specific surface, and high plasticity (Darley and Gray, 1988; Menezes et al., 2010). Calcium bentonite obtained from Xuzhou Wuzi Market (Xuzhou City, Jiangsu Province, China) was used as a source of Mt. Chemical components of the clay were SiO2 (53.82%), Al2O3 (26.96%), Fe2O3 (12.81%), TiO2 (2.96%), MgO (2.91%), and trace elements (0.54%). Additives were used to improve the performance of drilling mud in terms of lubricity, and the ability of protecting cored samples from collapse and viscoelastic vibration generated by a rig. Na2CO3 and potassium humate (KHM) were applied as sources of Na+ and K+, respectively, to control the swelling behavior of Mt. The Ca2+ replacement with Na+ increases the swelling of Mt, while K+ hinders from further swelling of Mt, maintaining the stability of mud cake (Boek et al., 1995). Sodium carboxymethyl cellulose (Na-CMC) was used to decrease the hydraulic conductivity of mud cake, in turn reducing fluid loss (Hebeish et al., 2010). Vegetable gum (VG) was introduced to improve the viscosity of drilling fluid. All the additives were obtained from Xuzhou Wuzi Market (Xuzhou City, Jiangsu Province, China). VG is a water soluble organic polymer consisting mainly of mannose, galactose, xylose, and glucose polysaccharide (Caprita et al., 2010). Due to its hydrophilicity, VG tends to interact with water molecules leading to an increase in the viscosity of fluid (Caenna and Chillingarb, 1996; Finch, 1998; Jackson, 1979). The enhanced rheological property as well as the use of Na-CMC induces a decrease in fluid loss from mud cake. This process prevents Mt particles from flocculating or coagulating which may damage the borehole and cored samples. 2.2. Sample preparation The bentonite was modified with the addition of Na2CO3 to replace the interlayer calcium ions with Na+. The mixture was then mixed with distilled water for 24 h. The fully hydrated clay slurry (drilling mud) was mixed using different proportions of additives. The proportion of additives is tabulated in Table 1. The newly formed drilling fluid was used throughout this study. 2.3. Test procedures 2.3.1. Compatibility test KHM, Na-CMC, and VG are all commonly used as a component of drilling fluids, thereby the compatibility of each additive has been well established. However, it is necessary to test the compatibility of all additives together with the inorganic salt (Na2CO3) used in this study, and the additives with the drilling mud (Mt) because they could be mutually influenced by each other leading to undesirable performance (Hao, 2011). Compatibility tests were conducted by adding 100 ml of mixture of Mt (8%) and additives to the inorganic salt (4% Na2CO3). The content of Mt was obtained from dispersivity tests which determine whether a Table 1 Concentrations of each component of drilling fluids, s. Concentration ID
A B C D
drilling mud remains dispersive at a given content of additives. The content of the inorganic salt was decided based on the API regular criterion (API, 1988). The compatibility tests may provide an appropriate range in the content of each additive. 2.3.2. Optimization Flow of drilling fluids exerts hydrodynamic forces on the borehole and drilled cores. In order to estimate such forces, viscosity, hydrodynamic shear stress created by fluid flow, and dynamic filtration were investigated on every combination. Orthogonal testing is a simple and efficient technique which is often used to find the optimum condition among various affecting factors simultaneously. Orthogonal testing with specimens shortlisted according to the results of compatibility tests would provide insight for the optimum formula of drilling fluids. In this study, four different concentrations of each component of drilling fluids were selected from the pre-determined range of concentrations (Table 1). The performance of drilling fluids was evaluated based on the measurements of rheological properties and filtration characteristics including density, plastic viscosity, Marsh funnel viscosity, yield point, and water loss characteristics of the drilling fluids. The best performance would be achieved when the ratios of the four additives are optimized. Testing details can be found in Hao (2011). 2.3.3. Soaking test Drilling fluid should be formulated to minimize damage due to the invasion of solids and mud filtrate (Jiao and Sharma, 1992). A stable, low-permeability mud cake should form rapidly on the reservoir rock in question. In order to investigate the stability of borehole and core samples, soaking tests were performed. The results of soaking tests imply the stability of the borehole as well as the core samples drilled from the borehole. Sand soil specimens, which simulate the sandstone-like uranium formation, were artificially made for soaking tests. The specimens with a diameter of 1.48 cm and a height of 3 cm consist of white quartz sands with grain diameter of 0.1–1 mm and silt with grain diameter less than 0.075 mm (at a ratio, quartz sands: silt = 4:3). The sand and silt were classified from soil samples obtained from Xinjiang's Yili Basin uranium formation in China. Crack formation and changes in specimen diameter were observed after 72 h of submerging a specimen under each drilling fluid proposed in this study (Table 1). 2.3.4. Pressure-bearing test Laboratory experiments of core samples need to be conducted under simulated field conditions to minimize the differences in the measured properties (Towler, 1986). Pressure-bearing tests were carried out to investigate the stability of specimens in terms of in-situ groundwater pressure and the pressure caused by drilling fluid flow. The specimens used for soaking tests were subjected to a load bar which provides the specimens with a pressure of 2.73 kPa. Unless the specimens collapse after 1 min, an additional load bar was applied leading to the total pressure of 6.14 kPa. Distortion, fracture, or crack formation within the specimens was monitored over the next minute. The pressure of 2.73 kPa is similar to the groundwater pressure in sandstone uranium formation and the pressure of 6.14 kPa is approximately the sum of the groundwater pressure and the pressure caused by drilling fluid flow. The pressures were calculated based on pressure measurements of Xinjiang's Yili Basin uranium formation in China. 3. Results and discussion
Component (%)
3.1. Compatibility test
Mt
Na2CO3
KHM
Na-CMC
VG
2 4 6 8
0.8 1.6 2.4 3.2
0.1 0.5 1 1.5
0.1 0.5 1 1.5
0.1 0.25 0.5 0.75
The compatibility tests were conducted to investigate the mutual interactions of each additive and to narrow down the range in concentration of each additive. From the experiment, it was seen that up to 1.5% by volume KHM can be dissolved into 100 ml of the modified Mt
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(8% Mt + 4% Na2CO3) without salt formation. Similarly, when adding up to 1.5% Na-CMC or 0.7% VG into the modified Mt slurry, no salt formation was observed. This can be linked to the critical coagulation concentration (CCC) of each additive. Both were used to increase the viscosity, thus to reduce water loss. However, the wellbore stability worsens when the viscosity of drilling fluid is too high. Therefore the CCC of Na-CMC and VG were used as the maximum concentration. The compatibility test result led to the selection of concentration of each additive (Table 1). 3.2. Optimization The results of orthogonal testing and analyzed data are tabulated in Table 2. Selected data are also plotted in Fig. 1 for visual comparison. It shows that the addition of each additive at different proportions simultaneously affects the rheological properties and filtration characteristics of drilling fluid. Because Na2CO3 (swelling promoter) and KHM (swelling inhibitor) were both used to control the swelling behavior of Mt, the ratio between them may affect the properties of drilling fluids, i.e., antisynergistic effect. The impact of the ratio of Na2CO3 to KHM on the density and water loss characteristics of drilling fluid is presented in Fig. 1c. Similarly, the effect of the ratio between Na-CMC and Mt, which both reduce fluid loss, is exhibited in Fig. 1d. Density and water loss properties are the most important properties in determining the optimum formula of drilling fluid (Darley and Gray, 1988; Growcock and Harvey, 2005; Keas, 1974). An increase in the
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density of drilling fluid may cause problems of pump capacity and wellbore stability. According to classical filtration equations, drilling fluid with high viscosity leads to better inhibition efficiency for water loss. Hence, drilling fluid should have low density, and sufficiently high viscosity to maximize the performance. Range values calculated in Table 2 represent the difference between the maximum and minimum values of each property influenced by the addition of each additive. Considering density, Na2CO3 shows the greatest effect on the density. Mt and KHM have the second and third greatest effects on the density respectively. Because the density of drilling fluid should be less than 1.07 g/cm3 (Rosenberg and Pittsburgh, 1962), concentrations A, B, and C of Na2CO3 were selected to be suitable. In the same way, concentrations A and B of Mt, and concentrations A, B, and C of KHM were chosen. All possible combinations of the selected concentrations were matched with Samples 1, 2, 3, 5, and 6. In the same way, considering water loss, Samples 5, 6, 9, 10, 15, and 16 were selected. Among those samples, Samples 5 and 6 satisfy the criteria of density, water loss, and viscosity. The increased density with an increase in Na2CO3 concentration may be attributed to ionic strength caused by Na2CO3 addition which leads to flocculation or aggregation of Mt particles. Relative to the addition of 0.1% KHM, the addition of 0.5% KHM considerably increased the density which remained stable. This phenomenon may be linked to the CCC of KHM. At concentrations above the CCC, coagulation does not increase the density of aggregates (Kim and Palomino, 2009; Theng, 1979; Van, 1977). An increase in the concentration of Na-CMC and Mt decreased
Table 2 The result of orthogonal testing and analyzed data. Sample #
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 AA AB AC AD Range AA AB AC AD Range AA AB AC AD Range AA AB AC AD Range AA AB AC AD Range
Concentration ID Mt
Na2CO3
KHM
Na-CMC
VG
A A A A B B B B C C C C D D D D 1.048 1.043 1.071 1.083 0.040 3.875 6.750 12.875 12.375 9.000 17.450 22.850 28.875 77.125 59.675 0.893 4.271 8.160 16.575 15.683 14.875 11.050 10.950 9.750 5.125
A B C D A B C D A B C D A B C D 1.034 1.053 1.059 1.075 0.041 11.500 10.375 7.125 6.875 4.625 28.025 27.100 46.100 45.075 19.000 7.013 5.355 9.244 8.288 3.889 12.500 11.225 11.250 11.650 1.275
A B C D B A D C C D A B D C B A 1.040 1.067 1.064 1.073 0.027 7.250 8.125 9.500 11.000 3.750 49.375 45.400 24.225 27.300 21.150 11.475 8.033 5.610 4.781 6.694 13.200 12.125 10.850 10.450 2.750
A B C D C D A B D C B A B A D C 1.069 1.058 1.071 1.047 0.243 5.875 7.875 9.750 12.375 6.500 21.025 26.025 47.900 51.350 30.325 2.741 4.973 9.435 12.750 10.009 15.000 12.425 10.550 8.650 6.350
A B C D D C B A B A D C C D A B 1.070 1.066 1.052 1.056 0.017 8.875 8.625 9.625 8.750 1.000 46.325 46.825 28.125 25.025 21.800 8.288 10.646 5.738 5.228 5.419 13.050 11.675 10.900 11.000 2.150
ρ
PV
ηm
YP
FL
1.041 1.046 1.049 1.054 1.051 0.990 1.065 1.064 1.062 1.093 1.040 1.090 1.080 1.080 1.080 1.090
1.5 3 4 7 9 10.5 3.5 4 19 17 8 7.5 16.5 11 13 9
15.90 16.50 17.50 19.90 22.90 32.50 17.80 18.20 33.00 31.20 29.10 22.20 40.30 28.20 N120 N120
0.51 0 1.02 2.04 4.08 9.69 1.79 1.53 14.28 6.12 9.18 3.06 9.18 5.61 24.99 26.52
22 15.7 11.8 10 10.6 8.4 12.8 12.4 8 9.6 12.2 14 9.4 11.2 8.2 10.2
Considering ρ
Considering PV
Considering ηm
Considering YP
Considering FL
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Fig. 1. Results of orthogonal testing.
water loss to the greatest degree. The high water retention capacity of Mt and Na-CMC-molecule-induced particle bridges may play an important role in reducing the water loss (Growcock and Harvey, 2005). As the Na2CO3-to-KHM ratio increases, both density and water loss start to show a downward trend until they hit the lowest values near the Na2CO3-to-KHM ratio of 16 (Fig. 1c). The density and water loss finish with an upward trend after passing the Na2CO3-to-KHM ratio of 16. In the plot displaying the effects of Na-CMC-to-Mt ratio, the smallest density and water loss were observed near the Na-CMC-to-Mt ratio of 0.4 (Fig. 1d). Therefore, Sample 6 with 4% Mt, 1.6% Na2CO3, 0.1% KHM, 1.5% Na-CMC, and 0.5% VG was chosen to maximize the performance of drilling fluid. The addition of Na2CO3 enhances the miscibility of Mt with water, by the exchange of Na+ forCa2+ to obtain a thicker hydration film on the surface of the clay particles, i.e., swelling. On the other hand, the introduction of KHM supplies K+ ions inhibiting the clay from swelling (Boek et al., 1995). This process leads to an increase in the viscosity of the fluid and thus a decrease in the water loss. Note that the result of this study should be applied with caution in the field because potassium ion is abundant in soft soil and hard rocks. For example, the concentration of pre-existing K+ must be measured and considered. In addition to the anti-synergistic effect of Na+ and K+, the system also includes the synergistic effect. KHM, Na-CMC, and VG all increased the viscosity of drilling fluid by bridging particles together (Caenna and Chillingarb, 1996). Furthermore, they can interact with one another. However, because of the simplicity of the orthogonal testing, this study did not capture the impact of each organic additive and their interactions. Other techniques such as response surface methodology would be required to better understand the effect of each additive used in this study. 3.3. Soaking test and pressure-bearing test Specimens after the soaking test are shown in Fig. 2. Observations from the soaking and pressure-bearing tests are tabulated in Table 3.
Samples 1, 2, 3, 5, 8, 9, and 13 led to slight crack formation on the specimen, while partial collapse or heavy crack formation was observed for Samples 7, 10, 11, 12, 14, 15, and 16. Expansion in specimen diameter was observed for Samples 3, 4, 7, 8, 10, 11, 12, 14, and 16. Pressurebearing tests on the specimen used for soaking tests resulted in complete collapse for Samples 7, 10, 11, 12, 14, 15, and 16, while the rest of the specimens displayed no changes. Sample 6 induced no changes after both soaking and pressure-bearing tests, which confirms the result of optimization. The drilling fluid with 4% Mt, 1.6% Na2CO3, 0.1% KHM, 1.5% Na-CMC, and 0.5% VG (Sample 6) may have produced mud cakes along the surface of sandstone-like specimen. The mud cake significantly reduced hydraulic conductivity of the specimen, thereby inhibiting fluid penetration into the specimen. No changes after the soaking test and the pressure-bearing test represent the stability of wellbore and core samples drilled for the wellbore. Thus, the use of Sample 6 as a drilling fluid has the maximized benefit of protecting both wellbore and core samples. As aforementioned, the high pressure bearing capacity of the specimen may be attributed to the controlled swelling behavior of Mt and particle bridges caused by the organic additives. The introduction of Na+ and K+ controls the swelling behavior of Mt, maintaining the stability of the mud cake (Boek et al., 1995). The particle bridges due to KHM, Na-CMC, and VG molecules inhibit Mt from interacting with the borehole and the cored sample, which may cause damages. 4. Conclusions The objective of this study was to find the optimized formula of drilling fluid for a sandstone-like uranium formation. Na 2 CO 3 , potassium humate (KHM), sodium carboxymethyl cellulose (Na-CMC), and vegetable gum (VG) were added to montmorillonite (Mt) slurries to maximize the performance of drilling fluid with respect to rheological
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Fig. 2. Observations after the soaking tests.
behavior and filtration characteristics. The optimum formula of drilling fluid was found through orthogonal testing. Soaking and pressurebearing tests representing the stability of borehole and core samples were performed to confirm the optimum formula found in this study. A drilling fluid with 4% Mt, 1.6% Na2CO3, 0.1% KHM, 1.5% Na-CMC, and 0.5% VG resulted in the best performance in terms of density, water loss, and viscosity. The lowest density and water loss were achieved with the drilling fluid. Also, it provided sufficiently high viscosity. Soaking and pressure-bearing tests confirmed the result of optimization. The drilling fluid led to no crack formations or deformations on a sandstone-like specimen after 72 h of the soaking test. In addition, the specimen treated with the drilling fluid displayed no visual changes under an external load of 6.14 kPa. The results of this study can be utilized as an effective double protection drilling fluid for a sandstonelike uranium formation.
Acknowledgments This study is supported by the National Natural Science Foundation of China (51104145, 40730422), China Postdoctoral Science Foundation (2011M501282, 2012T50500), the Fundamental Research Funds for the Central Universities (2012QNA64, 2013XK06), Jiangsu Province Postdoctoral Science Foundation (1102087C), 2011 Ministry of Housing and Urban–Rural Construction Scientific Technology Scheme Project (2011-K5-18), State Key Laboratory for GeoMechanics and Deep Underground Engineering Open Foundation (SKLGDUEK0911), National Program on Key Basic Research Project (973 Program: 2009CB219605), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the China Scientific Council Overseas Studying Supporting Foundation (2011832443). References
Table 3 Results of soaking tests and pressure-bearing tests. Sample #
Soaking test result
Pressure-bearing test result
Sample #
Soaking test result
Pressure-bearing test result
1 2 3 4 5 6 7 8
SC SC SC, ED ED SC NO PC, ED SC, ED
NO at 6.14 kPa NO at 6.14 kPa NO at 6.14 kPa NO at 6.14 kPa NO at 6.14 kPa NO at 6.14 kPa CC at 2.73 kPa NO at 6.14 kPa
9 10 11 12 13 14 15 16
SC PC, ED PC, ED PC, ED SC PC, ED HC PC, ED
NO at 6.14 kPa CC at 2.73 kPa CC at 2.73 kPa CC at 2.73 kPa NO at 6.14 kPa CC at 2.73 kPa CC at 2.73 kPa CC at 2.73 kPa
SC = slightly cracked; HC = heavily cracked; PC = partly collapsed; CC = completely collapsed; ED = enlarged diameter; NO = no changes.
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Double protection drilling fluid: Optimization for sandstone-like uranium formation Shu-Qing Hao a,b,c,d,⁎,1, Sungho Kim d,⁎⁎,1, Yong Qin a, Xue-Hai Fu a a Key Laboratory of CBM Resources and Reservoir Formation Process Ministry of Education of China, School of Resource and Geosciences, China University of Mining and Technology, Xuzhou 221116, China b State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221008, China c Department of Civil Engineering, Monash University of Mining and Technology, Victoria 3840, Australia d Geotechnical and Hydrogeological Engineering Research Group, Monash University, Victoria 3842, Australia
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Article history: Received 8 August 2013 Received in revised form 22 October 2013 Accepted 18 November 2013 Available online 17 December 2013 Keywords: Carboxymethyl cellulose Double protection drilling fluid Montmorillonite Potassium humate Sandstone-like uranium formation Vegetable gum Orthogonal testing
a b s t r a c t Nuclear energy, an alternative for fossil fuel, is accompanied by the excavation of uranium ores and in turn the use of drilling fluids. Thus, the objective of this study was to optimize the formula of drilling fluid for sandstone-like uranium formation representing the Xinjiang's Yili Basin uranium formation in China. Additives such as Na2CO3, potassium humate (KHM), sodium carboxymethyl cellulose (Na-CMC), and vegetable gum (VG) were introduced to montmorillonite (Mt) slurry to maximize the performance of drilling fluid in terms of rheological behavior and filtration characteristics. Orthogonal testing, soaking test, and pressure-bearing test were conducted to find the optimum formula of drilling fluid. The maximized performance was achieved with a drilling fluid consisting of 4% Mt, 1.6% Na2CO3, 0.1% KHM, 1.5% Na-CMC, and 0.5% VG. The lowest density and water loss, with a sufficiently high viscosity were achieved with this concentration combination of each additive. Soaking and pressure-bearing tests exhibited no visual changes of a specimen treated with the drilling fluid, which confirmed the result of optimization. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Fossil fuels have been a main energy source making up over 87% of the world's energy consumption in the past 200 years (Demirbas, 2010). As a result, the conventional energy resources decline and the price is ever increasing, which will lead to an energy crisis in the near future (Turner, 1974). Nuclear energy has alternatively been used in several countries and proven to be cost-effective (Dresselhaus and Thomas, 2001; IAEA, 1997; Paker and Holt, 2007) and stabilize global climate (Hoffert et al., 2002). However, the use of nuclear energy is accompanied by the excavation of uranium which includes difficulties due to the complexity of uranium formation (Dahlkamp, 1978; Lyons et al., 1982). Core samples of uranium drilled from the formation are often necessarily analyzed to understand the properties of mineral reservoir (Anderson, 1969; Ebenhack, 1987; Fitch, 1980; Meinrath et al., 1999; Morgan et al., 1997). During drilling core samples, a drilling mud has effectively been used to improve the borehole stability (Cherepanov,
⁎ Correspondence to: School of Resource and Geosciences, China University of Mining and Technology, Xuzhou, China. Tel.: +86 13952208471. ⁎⁎ Corresponding author. Tel.: +61 3 512 26411. E-mail addresses: [email protected] (S.-Q. Hao), [email protected] (S. Kim). 1 These two corresponding authors contributed equally to the development of this manuscript. 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.11.036
1982; Hao, 2011; Zeynali, 2012). For example, a drilling mud has successfully been applied in uranium sandstone formation (Keas, 1974; Petri and Neto, 2010). A drilling mud itself has also been developed in terms of its properties and composition. For example, a thixotropic drilling mud has been applied to form a radioactive filter cake for a purpose of analyzing the radioactivity of a formation (Ebenhack., 1987). The addition of polymers has been applied to improve the instability problem of shale field (Khodjaa et al., 2010). Nevertheless, there is a demand for protecting drilled core samples because those drilling mud technologies only consider the protection of wellbore. Water invasion often destroys core samples, preventing estimation of the potential producing capacities of the formations penetrated (Bailey and Keall, 1993; Patel and Salandanan, 1968). This is especially important for uranium formations or similar reservoirs, where its poor engineering properties, including loose soft/hard inter-bedding, water-containing strata, poor degree of consolidation, and cracks, often result in collapse of drilled samples as well as of wellbore (Borivoje et al., 2007; Bowes and Procter, 1997). The objective of this study was to create a suitable drilling fluid for use in a sandstone uranium formation, which protects both borehole and core samples (double protection). In order to modify properties of drilling mud (bentonite), additives including Na2CO3, potassium humate (KHM), sodium carboxymethyl cellulose (Na-CMC), and vegetable gum (VG) were utilized. The orthogonal experimental method was employed to optimize the formula of drilling fluid. Soaking and
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pressure-bearing tests were conducted to confirm the optimum formula of drilling fluid. 2. Experimental study 2.1. Materials Montmorillonite (Mt) is commonly used as a drilling mud due to its unique characteristics, such as negative surface charge, high specific surface, and high plasticity (Darley and Gray, 1988; Menezes et al., 2010). Calcium bentonite obtained from Xuzhou Wuzi Market (Xuzhou City, Jiangsu Province, China) was used as a source of Mt. Chemical components of the clay were SiO2 (53.82%), Al2O3 (26.96%), Fe2O3 (12.81%), TiO2 (2.96%), MgO (2.91%), and trace elements (0.54%). Additives were used to improve the performance of drilling mud in terms of lubricity, and the ability of protecting cored samples from collapse and viscoelastic vibration generated by a rig. Na2CO3 and potassium humate (KHM) were applied as sources of Na+ and K+, respectively, to control the swelling behavior of Mt. The Ca2+ replacement with Na+ increases the swelling of Mt, while K+ hinders from further swelling of Mt, maintaining the stability of mud cake (Boek et al., 1995). Sodium carboxymethyl cellulose (Na-CMC) was used to decrease the hydraulic conductivity of mud cake, in turn reducing fluid loss (Hebeish et al., 2010). Vegetable gum (VG) was introduced to improve the viscosity of drilling fluid. All the additives were obtained from Xuzhou Wuzi Market (Xuzhou City, Jiangsu Province, China). VG is a water soluble organic polymer consisting mainly of mannose, galactose, xylose, and glucose polysaccharide (Caprita et al., 2010). Due to its hydrophilicity, VG tends to interact with water molecules leading to an increase in the viscosity of fluid (Caenna and Chillingarb, 1996; Finch, 1998; Jackson, 1979). The enhanced rheological property as well as the use of Na-CMC induces a decrease in fluid loss from mud cake. This process prevents Mt particles from flocculating or coagulating which may damage the borehole and cored samples. 2.2. Sample preparation The bentonite was modified with the addition of Na2CO3 to replace the interlayer calcium ions with Na+. The mixture was then mixed with distilled water for 24 h. The fully hydrated clay slurry (drilling mud) was mixed using different proportions of additives. The proportion of additives is tabulated in Table 1. The newly formed drilling fluid was used throughout this study. 2.3. Test procedures 2.3.1. Compatibility test KHM, Na-CMC, and VG are all commonly used as a component of drilling fluids, thereby the compatibility of each additive has been well established. However, it is necessary to test the compatibility of all additives together with the inorganic salt (Na2CO3) used in this study, and the additives with the drilling mud (Mt) because they could be mutually influenced by each other leading to undesirable performance (Hao, 2011). Compatibility tests were conducted by adding 100 ml of mixture of Mt (8%) and additives to the inorganic salt (4% Na2CO3). The content of Mt was obtained from dispersivity tests which determine whether a Table 1 Concentrations of each component of drilling fluids, s. Concentration ID
A B C D
drilling mud remains dispersive at a given content of additives. The content of the inorganic salt was decided based on the API regular criterion (API, 1988). The compatibility tests may provide an appropriate range in the content of each additive. 2.3.2. Optimization Flow of drilling fluids exerts hydrodynamic forces on the borehole and drilled cores. In order to estimate such forces, viscosity, hydrodynamic shear stress created by fluid flow, and dynamic filtration were investigated on every combination. Orthogonal testing is a simple and efficient technique which is often used to find the optimum condition among various affecting factors simultaneously. Orthogonal testing with specimens shortlisted according to the results of compatibility tests would provide insight for the optimum formula of drilling fluids. In this study, four different concentrations of each component of drilling fluids were selected from the pre-determined range of concentrations (Table 1). The performance of drilling fluids was evaluated based on the measurements of rheological properties and filtration characteristics including density, plastic viscosity, Marsh funnel viscosity, yield point, and water loss characteristics of the drilling fluids. The best performance would be achieved when the ratios of the four additives are optimized. Testing details can be found in Hao (2011). 2.3.3. Soaking test Drilling fluid should be formulated to minimize damage due to the invasion of solids and mud filtrate (Jiao and Sharma, 1992). A stable, low-permeability mud cake should form rapidly on the reservoir rock in question. In order to investigate the stability of borehole and core samples, soaking tests were performed. The results of soaking tests imply the stability of the borehole as well as the core samples drilled from the borehole. Sand soil specimens, which simulate the sandstone-like uranium formation, were artificially made for soaking tests. The specimens with a diameter of 1.48 cm and a height of 3 cm consist of white quartz sands with grain diameter of 0.1–1 mm and silt with grain diameter less than 0.075 mm (at a ratio, quartz sands: silt = 4:3). The sand and silt were classified from soil samples obtained from Xinjiang's Yili Basin uranium formation in China. Crack formation and changes in specimen diameter were observed after 72 h of submerging a specimen under each drilling fluid proposed in this study (Table 1). 2.3.4. Pressure-bearing test Laboratory experiments of core samples need to be conducted under simulated field conditions to minimize the differences in the measured properties (Towler, 1986). Pressure-bearing tests were carried out to investigate the stability of specimens in terms of in-situ groundwater pressure and the pressure caused by drilling fluid flow. The specimens used for soaking tests were subjected to a load bar which provides the specimens with a pressure of 2.73 kPa. Unless the specimens collapse after 1 min, an additional load bar was applied leading to the total pressure of 6.14 kPa. Distortion, fracture, or crack formation within the specimens was monitored over the next minute. The pressure of 2.73 kPa is similar to the groundwater pressure in sandstone uranium formation and the pressure of 6.14 kPa is approximately the sum of the groundwater pressure and the pressure caused by drilling fluid flow. The pressures were calculated based on pressure measurements of Xinjiang's Yili Basin uranium formation in China. 3. Results and discussion
Component (%)
3.1. Compatibility test
Mt
Na2CO3
KHM
Na-CMC
VG
2 4 6 8
0.8 1.6 2.4 3.2
0.1 0.5 1 1.5
0.1 0.5 1 1.5
0.1 0.25 0.5 0.75
The compatibility tests were conducted to investigate the mutual interactions of each additive and to narrow down the range in concentration of each additive. From the experiment, it was seen that up to 1.5% by volume KHM can be dissolved into 100 ml of the modified Mt
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(8% Mt + 4% Na2CO3) without salt formation. Similarly, when adding up to 1.5% Na-CMC or 0.7% VG into the modified Mt slurry, no salt formation was observed. This can be linked to the critical coagulation concentration (CCC) of each additive. Both were used to increase the viscosity, thus to reduce water loss. However, the wellbore stability worsens when the viscosity of drilling fluid is too high. Therefore the CCC of Na-CMC and VG were used as the maximum concentration. The compatibility test result led to the selection of concentration of each additive (Table 1). 3.2. Optimization The results of orthogonal testing and analyzed data are tabulated in Table 2. Selected data are also plotted in Fig. 1 for visual comparison. It shows that the addition of each additive at different proportions simultaneously affects the rheological properties and filtration characteristics of drilling fluid. Because Na2CO3 (swelling promoter) and KHM (swelling inhibitor) were both used to control the swelling behavior of Mt, the ratio between them may affect the properties of drilling fluids, i.e., antisynergistic effect. The impact of the ratio of Na2CO3 to KHM on the density and water loss characteristics of drilling fluid is presented in Fig. 1c. Similarly, the effect of the ratio between Na-CMC and Mt, which both reduce fluid loss, is exhibited in Fig. 1d. Density and water loss properties are the most important properties in determining the optimum formula of drilling fluid (Darley and Gray, 1988; Growcock and Harvey, 2005; Keas, 1974). An increase in the
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density of drilling fluid may cause problems of pump capacity and wellbore stability. According to classical filtration equations, drilling fluid with high viscosity leads to better inhibition efficiency for water loss. Hence, drilling fluid should have low density, and sufficiently high viscosity to maximize the performance. Range values calculated in Table 2 represent the difference between the maximum and minimum values of each property influenced by the addition of each additive. Considering density, Na2CO3 shows the greatest effect on the density. Mt and KHM have the second and third greatest effects on the density respectively. Because the density of drilling fluid should be less than 1.07 g/cm3 (Rosenberg and Pittsburgh, 1962), concentrations A, B, and C of Na2CO3 were selected to be suitable. In the same way, concentrations A and B of Mt, and concentrations A, B, and C of KHM were chosen. All possible combinations of the selected concentrations were matched with Samples 1, 2, 3, 5, and 6. In the same way, considering water loss, Samples 5, 6, 9, 10, 15, and 16 were selected. Among those samples, Samples 5 and 6 satisfy the criteria of density, water loss, and viscosity. The increased density with an increase in Na2CO3 concentration may be attributed to ionic strength caused by Na2CO3 addition which leads to flocculation or aggregation of Mt particles. Relative to the addition of 0.1% KHM, the addition of 0.5% KHM considerably increased the density which remained stable. This phenomenon may be linked to the CCC of KHM. At concentrations above the CCC, coagulation does not increase the density of aggregates (Kim and Palomino, 2009; Theng, 1979; Van, 1977). An increase in the concentration of Na-CMC and Mt decreased
Table 2 The result of orthogonal testing and analyzed data. Sample #
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 AA AB AC AD Range AA AB AC AD Range AA AB AC AD Range AA AB AC AD Range AA AB AC AD Range
Concentration ID Mt
Na2CO3
KHM
Na-CMC
VG
A A A A B B B B C C C C D D D D 1.048 1.043 1.071 1.083 0.040 3.875 6.750 12.875 12.375 9.000 17.450 22.850 28.875 77.125 59.675 0.893 4.271 8.160 16.575 15.683 14.875 11.050 10.950 9.750 5.125
A B C D A B C D A B C D A B C D 1.034 1.053 1.059 1.075 0.041 11.500 10.375 7.125 6.875 4.625 28.025 27.100 46.100 45.075 19.000 7.013 5.355 9.244 8.288 3.889 12.500 11.225 11.250 11.650 1.275
A B C D B A D C C D A B D C B A 1.040 1.067 1.064 1.073 0.027 7.250 8.125 9.500 11.000 3.750 49.375 45.400 24.225 27.300 21.150 11.475 8.033 5.610 4.781 6.694 13.200 12.125 10.850 10.450 2.750
A B C D C D A B D C B A B A D C 1.069 1.058 1.071 1.047 0.243 5.875 7.875 9.750 12.375 6.500 21.025 26.025 47.900 51.350 30.325 2.741 4.973 9.435 12.750 10.009 15.000 12.425 10.550 8.650 6.350
A B C D D C B A B A D C C D A B 1.070 1.066 1.052 1.056 0.017 8.875 8.625 9.625 8.750 1.000 46.325 46.825 28.125 25.025 21.800 8.288 10.646 5.738 5.228 5.419 13.050 11.675 10.900 11.000 2.150
ρ
PV
ηm
YP
FL
1.041 1.046 1.049 1.054 1.051 0.990 1.065 1.064 1.062 1.093 1.040 1.090 1.080 1.080 1.080 1.090
1.5 3 4 7 9 10.5 3.5 4 19 17 8 7.5 16.5 11 13 9
15.90 16.50 17.50 19.90 22.90 32.50 17.80 18.20 33.00 31.20 29.10 22.20 40.30 28.20 N120 N120
0.51 0 1.02 2.04 4.08 9.69 1.79 1.53 14.28 6.12 9.18 3.06 9.18 5.61 24.99 26.52
22 15.7 11.8 10 10.6 8.4 12.8 12.4 8 9.6 12.2 14 9.4 11.2 8.2 10.2
Considering ρ
Considering PV
Considering ηm
Considering YP
Considering FL
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Fig. 1. Results of orthogonal testing.
water loss to the greatest degree. The high water retention capacity of Mt and Na-CMC-molecule-induced particle bridges may play an important role in reducing the water loss (Growcock and Harvey, 2005). As the Na2CO3-to-KHM ratio increases, both density and water loss start to show a downward trend until they hit the lowest values near the Na2CO3-to-KHM ratio of 16 (Fig. 1c). The density and water loss finish with an upward trend after passing the Na2CO3-to-KHM ratio of 16. In the plot displaying the effects of Na-CMC-to-Mt ratio, the smallest density and water loss were observed near the Na-CMC-to-Mt ratio of 0.4 (Fig. 1d). Therefore, Sample 6 with 4% Mt, 1.6% Na2CO3, 0.1% KHM, 1.5% Na-CMC, and 0.5% VG was chosen to maximize the performance of drilling fluid. The addition of Na2CO3 enhances the miscibility of Mt with water, by the exchange of Na+ forCa2+ to obtain a thicker hydration film on the surface of the clay particles, i.e., swelling. On the other hand, the introduction of KHM supplies K+ ions inhibiting the clay from swelling (Boek et al., 1995). This process leads to an increase in the viscosity of the fluid and thus a decrease in the water loss. Note that the result of this study should be applied with caution in the field because potassium ion is abundant in soft soil and hard rocks. For example, the concentration of pre-existing K+ must be measured and considered. In addition to the anti-synergistic effect of Na+ and K+, the system also includes the synergistic effect. KHM, Na-CMC, and VG all increased the viscosity of drilling fluid by bridging particles together (Caenna and Chillingarb, 1996). Furthermore, they can interact with one another. However, because of the simplicity of the orthogonal testing, this study did not capture the impact of each organic additive and their interactions. Other techniques such as response surface methodology would be required to better understand the effect of each additive used in this study. 3.3. Soaking test and pressure-bearing test Specimens after the soaking test are shown in Fig. 2. Observations from the soaking and pressure-bearing tests are tabulated in Table 3.
Samples 1, 2, 3, 5, 8, 9, and 13 led to slight crack formation on the specimen, while partial collapse or heavy crack formation was observed for Samples 7, 10, 11, 12, 14, 15, and 16. Expansion in specimen diameter was observed for Samples 3, 4, 7, 8, 10, 11, 12, 14, and 16. Pressurebearing tests on the specimen used for soaking tests resulted in complete collapse for Samples 7, 10, 11, 12, 14, 15, and 16, while the rest of the specimens displayed no changes. Sample 6 induced no changes after both soaking and pressure-bearing tests, which confirms the result of optimization. The drilling fluid with 4% Mt, 1.6% Na2CO3, 0.1% KHM, 1.5% Na-CMC, and 0.5% VG (Sample 6) may have produced mud cakes along the surface of sandstone-like specimen. The mud cake significantly reduced hydraulic conductivity of the specimen, thereby inhibiting fluid penetration into the specimen. No changes after the soaking test and the pressure-bearing test represent the stability of wellbore and core samples drilled for the wellbore. Thus, the use of Sample 6 as a drilling fluid has the maximized benefit of protecting both wellbore and core samples. As aforementioned, the high pressure bearing capacity of the specimen may be attributed to the controlled swelling behavior of Mt and particle bridges caused by the organic additives. The introduction of Na+ and K+ controls the swelling behavior of Mt, maintaining the stability of the mud cake (Boek et al., 1995). The particle bridges due to KHM, Na-CMC, and VG molecules inhibit Mt from interacting with the borehole and the cored sample, which may cause damages. 4. Conclusions The objective of this study was to find the optimized formula of drilling fluid for a sandstone-like uranium formation. Na 2 CO 3 , potassium humate (KHM), sodium carboxymethyl cellulose (Na-CMC), and vegetable gum (VG) were added to montmorillonite (Mt) slurries to maximize the performance of drilling fluid with respect to rheological
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Fig. 2. Observations after the soaking tests.
behavior and filtration characteristics. The optimum formula of drilling fluid was found through orthogonal testing. Soaking and pressurebearing tests representing the stability of borehole and core samples were performed to confirm the optimum formula found in this study. A drilling fluid with 4% Mt, 1.6% Na2CO3, 0.1% KHM, 1.5% Na-CMC, and 0.5% VG resulted in the best performance in terms of density, water loss, and viscosity. The lowest density and water loss were achieved with the drilling fluid. Also, it provided sufficiently high viscosity. Soaking and pressure-bearing tests confirmed the result of optimization. The drilling fluid led to no crack formations or deformations on a sandstone-like specimen after 72 h of the soaking test. In addition, the specimen treated with the drilling fluid displayed no visual changes under an external load of 6.14 kPa. The results of this study can be utilized as an effective double protection drilling fluid for a sandstonelike uranium formation.
Acknowledgments This study is supported by the National Natural Science Foundation of China (51104145, 40730422), China Postdoctoral Science Foundation (2011M501282, 2012T50500), the Fundamental Research Funds for the Central Universities (2012QNA64, 2013XK06), Jiangsu Province Postdoctoral Science Foundation (1102087C), 2011 Ministry of Housing and Urban–Rural Construction Scientific Technology Scheme Project (2011-K5-18), State Key Laboratory for GeoMechanics and Deep Underground Engineering Open Foundation (SKLGDUEK0911), National Program on Key Basic Research Project (973 Program: 2009CB219605), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the China Scientific Council Overseas Studying Supporting Foundation (2011832443). References
Table 3 Results of soaking tests and pressure-bearing tests. Sample #
Soaking test result
Pressure-bearing test result
Sample #
Soaking test result
Pressure-bearing test result
1 2 3 4 5 6 7 8
SC SC SC, ED ED SC NO PC, ED SC, ED
NO at 6.14 kPa NO at 6.14 kPa NO at 6.14 kPa NO at 6.14 kPa NO at 6.14 kPa NO at 6.14 kPa CC at 2.73 kPa NO at 6.14 kPa
9 10 11 12 13 14 15 16
SC PC, ED PC, ED PC, ED SC PC, ED HC PC, ED
NO at 6.14 kPa CC at 2.73 kPa CC at 2.73 kPa CC at 2.73 kPa NO at 6.14 kPa CC at 2.73 kPa CC at 2.73 kPa CC at 2.73 kPa
SC = slightly cracked; HC = heavily cracked; PC = partly collapsed; CC = completely collapsed; ED = enlarged diameter; NO = no changes.
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