ANALYSIS OF THE BEST GEOMETRY TO BE USED IN RHEOMETRIC TESTS FOR A DRILLING FLUID

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1 ANALYSIS OF THE BEST GEOMETRY TO BE USED IN RHEOMETRIC TESTS FOR A DRILLING FLUID 1 Filipe C. Ferreira, 2 Diogo E. V. Andrade, 3 Admilson T. Franco e 3 Cezar O. R. Negrão 1 Scientific Initiation fellow of FUNTEF-PR, student of Mechanical Engineering 2 PhD student at Federal University of Technology - Paraná - UTFPR 3 Professor at Federal University of Technology - Paraná - UTFPR 1,2,3 Research Center for Rheology and Non-Newtonian Fluids CERNN, Federal University of Technology Paraná UTFPR. Av. Sete de Setembro, 3165 Curitiba PR, , Brazil filipeferreira@alunos.utfpr.edu.br, diogoandrade@utfr.edu.br ABSTRACT In this work are shown the results obtained with rheological tests performed with two rheometer, TA Discovery and HAAKE MARS and with a BROOKSFIELD-DV-II Pro viscometer, at constant shear rates, from 0.001s-1 to 1000s-1, using a non-aqueous drilling fluid. These tests were done with different kinds of standard test geometries. The obtained results showed that both rheometers and viscometer are comparable, mainly for practical purposes, while the most reliable measuring geometry, for this specific fluid, are the serrated parallel-plates Key-Words: Rheometry, Test Geometry, Drilling Fluid. INTRODUCTION The down hole drilling operation, for oil prospection, is a task job. In most cases, the drill needs to pass through layers of different kinds of rocks, while undergoing deep depths, being exposed to high temperatures, pressures, and friction. In face of these conditions, drilling fluids were formulated to help in the drilling process, decreasing friction and temperature of the drill. In some cases, it would be impossible to drill a hole without a required drilling fluid. According to LYONS et al. (2011) [1], the drilling fluid is composed by several substances, as carbonates, bentonite, hematite and zinc chloride. Due to the gel like structure of most drilling fluid, as the one here studied, it behaves as non- Newtonian fluid. While it is flowing, when sheared, the gel like structure is broken, permitting it to flow. The drilling fluid is good to accomplish best work conditions, and, in addition, it brings up all the gravel up to surface through the annular space. When the drilling operation stops f or procedures as maintenance or even in case of a problem, and the fluid is not sheared, the fluid builds up a gel-like structure. The gelation of the drilling fluid is desirable to avoid cuttings and weighting agents precipitation at wellbore. Once the drilling fluid plays such an important part in the drilling process, this study was conducted in a way to find the best standard test geometry, for a common drilling fluids used for different work conditions. The fluid was given by Petrobras. Rheometric tests were performed in two rotational rheometers, using several standard test geometries, and with a rotational viscometer, with shear rates between s -1 to 1000 s -1. The results obtained with different equipment s and test geometries were compared, permitting a really precise and trustable conclusion for the best test geometry. The possibility of using a viscometer to characterize the drilling fluid was also evaluated. Never the less, better knowledge of the drilling fluid behavior was enhanced, mainly due to the rheometer and viscometer comparison. EXPERIMENTAL SECTION Materials and methods In the current work, rheometric tests were performed in two rotational rheometers, TA-DHR3 and HAAKE-MARS III, and in a viscometer, BROOKSFIELD- DV-II Pro, using a formulated non-aqueous drilling fluid. The temperature of the experiments was controlled by a peltier Thermostatic Bath System, for the rheometers, and only a Thermostatic bath for the viscometer. The used test geometries are, serrated and sand blasted parallel plates, concentric cylinders, cones with 2 degrees angle, and a double gap concentric

2 cylinder. All used equipment are matched with their respective tests geometries at Table 1. Table 1 - The used geometries for the performed tests Geometries Parallel plates Double Gap Cone Concentric Cylinders TA DHR3 Serrated and sand blasted (40mm HAAKE MARS III Serrated and sand blasted (35mm Viscometer 27mm 2º angle (40mm 2º angle (35mm Mooney Ewart (32mm SC4-31 (11.75mm In Table 2 the all geometries are matched with their respective image, according to Schramm (1998) [2]. Table 2 - Figures of the used geometries. 1 * Geometries Parallel plates Figures It is important to say that in the TA rheometer, a sand sheet was glued on the top middle of peltier, since the original surface is too slippery for the analyzed drilling fluid. The shear rates of the tests were 0.001s -1, 0.5 s -1, 1s -1, 5s -1, 10s -1, 50s -1, 100s -1, and 1000s -1. The drilling fluid sample came in a 50 liters gallon, after that, a little part was divided in smaller containers, for convenience, with volumes around 1 liter. To begin the test, the fluid is firstly agitated for 2 minutes in a Hamilton Beach professional agitator, in a way to guarantee homogeneity to the fluid. After that, a small specimen is taken and placed for test in the rheometers and viscometer, the excess should be removed. The volume of the sample varies with the used geometry and with the instrument. For parallel plates it is one milliliter, while for the concentric cylinders of viscometer it is used nine milliliters, for example. The tests are than programed in their specific testing software. Firstly, the samples rest for 5 minutes, in a way to equalize the temperature at 25 Celsius degrees. After the procedure is started, the fluid is maintained in a constant shear rate deformation for 30 minutes. In the end, when all data is collected. The tests passed through an analysis on their steady state regime, which is the region that the shear stress is close to a horizontal line, by collecting the shear stress of the last 10 points of each test, and then calculating the average stress of these 10 points. These averages are plotted (shear rate versus steady stated shear stress) to build up the flow curve of the drilling fluid. Double Gap Cone Mooney Ewart and SC4-31 Cylinders * All images were taken from Schramm (1998) RESULTS AND DISCUSSIONS Initially there was an assumption that the best results would be from cones and the concentric cylinders, as mentioned by FERNANDES (2014) [3] these is the best geometry for non-newtonian fluid, once the speed along the shear surface is equal in all points. This way, the fluid would be equally deformed in all points. Regarding concentric cylinders, it is interesting to say that they have a conic bottom which prevents from boundary effects [4]. With respect to the parallel plates, the shear rate is unequal along its surface, since the speed in the outer radius is bigger than the speed in the middle. This way, problems such as particle migration can occur [5]. However, contradicting the literature, cones showed no good results, alike the concentric cylinders. On Figure 1 are shown several tests performed with all geometries in the HAAKE rheometer, for a shear rate of 5 s -1,

3 Analyzing the results showed on this table, the shear rate is noticeably not linear, without a clear and steady line, as the results obtained by Fernandes (2014) for example, except for the parallel plates test (letter a). Instead, some of them, like the double gap cylinders, had very had high and low values points of shear stress, even when the analyzed points are just beside the other. The best supposition for these irregularities is the clamping phenomenon. The best result was obtained with a Parallel Sand Blasted Plate, which shows a clear tendency line, and a clear steady state region. Satisfactory results were also obtained with a Mooney Ewart Concentric Cylinder, presenting a similar shear stress when compared to the Parallel Plates. Although, it was not so clear, presenting some fluctuations in its shear stress values. These oscillations might be caused due to its difficulties when measuring in low shear rates [6]. Figure 1 - Comparison between geometries, for tests performed at shear rate of 5s -1. In, (a) parallel sand blasted plates, (b) 2º cones, (c) double gap cylinders, (d) Mooney Ewart concentric cylinders. The results of the tests showed on Figure 1, present clamping for all tests, besides the one performed with a parallel plate in its usual gap condition (one millimeter). The highest occurrence of this phenomenon was with double gap cylinders and with a parallel plate with a 0.23mm gap, the lowest occurrence with a Mooney Ewart concentric Cylinder. Although most experiments showed clamping, some still have a tendency line, which is close to values obtained by the parallel plates tendency line. In this case the clamping occurrence was thought to be correlated to the gaps between the surfaces, in which the fluid must flow. If the particle is too big for these rooms, it get stuck between the surfaces, and holds the rotor, the spinning part of the geometry, with it, showing a higher stress. When the rotor is released, the stress drops, due to rheometers reaction, this creates a clamping pattern, in which the graph presents very dispersed points of high and low stress, as the ones here obtained. To confirm this hypothesis, all geometries had their gaps checked. For geometries like parallel plates, the gap can be controlled, but for the other ones, the gap is fixed, once it would be impossible to change it as the lateral space in a double gap cylinder. Alternatively, it would cause a wrongly shear the fluid, which is the case of cones, since they were calibrated to work with a specific gap. It should be noted that double gap cylinders have one gap in its inner part, other on its outside lateral, and another in the bottom, between the geometry and its vessel cup. Cones have a bigger gap on their outer radius than in the center, once this geometry is not horizontal. Parallel plates have one gap for all its extension. Concentric cylinders have one gap at the bottom and another gap on its outside lateral. On Table 3 all gaps of the used geometry are listed. Analyzing the results presented on Figure 1 and the gaps geometries showed on Table 3, it is suspected that clamping occurs in situations in which geometries had close gaps between some of its interfaces. For example, at bottom for con-

4 centric cylinders or in the inner portion for Double Gap Cylinders. Table 3 - Geometries gaps. Geometries Parallel plates TA (model) HAAKE (model) Viscometer 1mm 1mm Double Gap Cone Center: 0.063mm Outer ray: 0.761mm Concentric Cylinders Inner side: 0.23mm Outside: 0.62mm Bottom: 0,60mm Center: 0.105mm Outer ray: 0.716mm Outside: 1.56mm Bottom: 0.056mm Outside: 23.59mm It is interesting to notice that geometries with small gaps, within a small interface for one specific gap, like the space between the bottom of a concentric cone and its vessel cup (0.056mm), compared to a bigger gap, but within a larger surface, like the inner space of a double gap cylinder (0.23mm), had a lower level of clamping. Even though the gap at bottom of the cone is almost five times bigger than the one in the inner side of the double gap cylinder. As it seems that area is also an important factor for clamping occurrence, and not just relying on a small gap. To further confirm the relation between gaps and clamping, tests with sand blasted parallel plates with a shear rate of 5s -1, with small gaps, were performed. This was thought once this geometry does have a similar surface to the cones, double gap and concentric cylinders. A gap of 0.23mm was chosen, in a way to compare its result to the one obtained with the Double Gap Cylinders. When analyzing both tests, it is noticeable that the outcomes were similar. Figure 2 shows the test for a Parallel Serrated Plates with a gap of 0.23mm. Figure 2 Sand Blasted Parallel Plates, gap of 0.23mm One more test with Sand Blasted Parallel Plates was done. This one used a 0.5mm gap, in a way to compare the results obtained with a regular gap, and with the 0.23mm gap. In this case, clamping was smaller than the one observed with 0.23mm test. In addition, there was a tendency line, similar to the test performed with a parallel plate using a regular gap, which can be seen on Figure 1 (a). It is noticed that the 0.5mm gap Sand Blasted Parallel Plates test behavior, stayed right in the middle of the other two, as it was with the its gap. This test is showed on Figure 3. Figure 3 - Sand Blasted Parallel Plates, gap of 0.5mm. Since the benefit of using parallel plates was proved, it was obtained the flow curve with the tests results performed with serrated Parallel Plates, Sand Blasted Parallel Plates, and the viscometer. In addition, these geometries results were compared between themselves. In the flow curve was noticed the viscometer had qualitative similar results and precision, when compared to the parallel plates. Even though it depends of the applied shear rate. On Figure 4 the flow curve obtained with the five configurations can be observed.

5 depending rheometers and viscometer reached qualitative close behavior. Also, both parallel plates had quite close measured values. CONCLUSION Figure 4 - Flow curve for parallel plates and viscometer. Also in the analysis of the flow curve, the average shear stress for Sand Blasted Parallel Plates, in a shear rate of 10-3 s -1, are low, close to zero. This was caused by the a common phenomenon in rotational rheometers, called slipping, which is an apparent speed discontinuity in the speed profile in the regions close to the geometry walls [3]. To complete the flow curve analysis, and have a better comparison between these last geometries, Table 4 presents the average shear stress for each shear rate of Serrated and Sand Blasted Parallel Plates, for Haake rheometer and the viscometer, while in steady state regime. Table 4 - Average shear stress for parallel plates of Haake Mars III, and viscometer shear stress Shear Rate [s -1 ] Serrated [Pa] Sand Blasted [Pa] Viscometer [Pa] Percentage Difference (Serrated/ Viscometer) , % % % % % As it can be seen on Table 4, the viscometer presented higher results than the rheometers. This might be due its precision, which is not as good as the rheometers. As mentioned before, For this specific tested drilling fluid, the following geometries, Double Gap Cylinders, Concentric Cylinders and Cones, working in ideal conditions presented clamping. This phenomenon can be noticed by a sharp discrepancy between the shear stress values. The results showed the clamping phenomenon for some geometries, but for parallel plates. The initial hypothesis that the fluid particle is too big for some gaps, relating it to the gaps between the test geometry and the surrounded surfaces, was confirmed by the three tests performed with Sand Blasted Parallel Plates. Once the only difference between these tests was only the gaps, this could be the only reason for clamping appearance. Never the less, the bigger was the gap, the smaller was the clamping phenomenon, the clearest becomes the shear stress tendency line. The area of these interfaces should also be taken in consideration, not just the gap With respect to parallel sand blasted plates, at a low shear rate of 10-3 s -1, the shear stress was too low, indicating slipping. Once the Parallel Serrated Plates have grooves on its surface, it was capable grab the fluid, and then precisely shear it from the lowest to the highest shear used ratios, unlike the Sand Blasted Plates. Never the less, due to its gap, it did not presented clamping, alike the other remaining geometries. Since the Serrated Parallel Plates performed all tests in all shear rates, without occurrence of any sort of problem, undoubtedly it is the best geometry when using this drilling fluid. The viscometer also presented good qualitative results, even more considering the rheometers precision in perspective. The best results were achieved with the highest used shear rates in the viscometer. This might be especially interesting for field applications, where a rheometer is not practical, or even impossible to be used. In addition, viscometers are stronger and cheaper instruments. Depending of the applied shear rates, viscometers can be good enough rheological properties with certain reliability. Since Serrated and Sand Blasted Parallel Plates achieved similar results, they can be swapped without big problems, what can be interesting for several kinds of purposes. In low shear rates applications that more care taken, due to the slipping phenomenon for Sand Blasted Plates.

6 REFERENCES [1] LYONS, W; CARTER, T; LAPEYROUSE, N. J; Formulas and Calculations, for Drilling, Production, and Workover: All the Formulas You Need to Solve Drilling and Production Problems. Gulf Professional Publishing, [2] SCRAMM, G. A. A practichal approach to Rheology and Rheometry. Hakke, [3] FERNANDES, R. R.; Metodologia para preparação de amostras em testes reológicos e avaliação da tensão limite de escoamento de fluidos de perfuração, Trabalho de Conclusão de Curso Curso de Engenharia Mecânica, Universidade Tecnológica Federal do Paraná. [4] MACOSKO, C. W. Rheology Principles Masurements and Aplications. 1ed.Nova York: John Wiley & Son, 1993 [5] MAHAUT, F. et al. Yield stress and elastic modulus of suspensions of noncolloidal particles in yeld stress fluids. Journal of Rheology (1978) [6] NGUYEN, Q.D.; BORGER, D. V. Measuring the Flow Properties of Yield Stress Fluids. Annual Review of Fluid Mechanics, ACKNOLEDGMENTS The authors acknowledge the financial support of PETROBRAS S/A, ANP, CNPq and FINEP.