THERMAL ANALYSIS OF VACUUM INSULATED TUBING (VIT) FOR OFFSHORE OIL WELLS

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1 Proceedings of ENCIT 204 Copyright 204 by ABCM 5 th Brazilian Congress of Thermal Sciences and Engineering November 0-3, 204, Belém, PA, Brazil THERMAL ANALYSIS OF VACUUM INSULATED TUBING (VIT) FOR OFFSHORE OIL WELLS Marcus Vinicius D. Ferreira, mvdferreira@petrobras.com.br PETROBRAS/CENPES, Rio de Janeiro, RJ Johann Barcelos, johann@polo.ufsc.br Universidade Federal de Santa Catarina, Florianópolis, SC Camilo Augusto Santos Costa, camilo@polo.ufsc.br Universidade Federal de Santa Catarina, Florianópolis, SC Jader Riso Barbosa Jr, jrb@polo.ufsc.br Universidade Federal de Santa Catarina, Florianópolis, SC Alexandre Kupka da Silva, a.kupka@ufsc.br Universidade Federal de Santa Catarina, Araranguá, SC Abstract. A recurring problem that affects mainly high pressure and high flow rate wells during oil exploration and production in ultra-deepwater scenarios is the phenomenon called APB (Annular Pressure Buildup), which can be understood as the heating process of the borehole caused by the upward flow of heated hydrocarbons towards the wellhead. This heating process affects the annular space of the borehole, which is often sealed and completely filled with drilling fluid, causing its expansion. The expansion of the fluid increases the annulus pressure to levels that can be detrimental to the well integrity. In order to avoid the tube collapse several strategies are commonly used to mitigate the APB, such as the Vacuum Insulated Tubing (VIT). The VIT has been extensively used in steam injector landshore wells in Northeast and Southeast Brazil, even though its application in production offshore wells is still limited. The technique consists in the use of two concentric metallic tubes that are welded at their extremities, forming an annular space, which between then is evacuated, minimizing the radial heat transfer. However, since the junction between every two tubes is not insulated, heat transfer might be significant at this location. Therefore, in this paper, we study numerically the heat transfer behavior of the VIT system using a commercial code aiming to evaluate the relative influence of the junction when compared with the insulated main body of the VIT. In addition, a study was conducted using a -D analytical model to estimate the overall heat transfer coefficient and the effective thermal conductivity of the VIT system as an integral unit based on a one-dimensional model. Computational results show that a 2D heat transfer model can capture important phenomena present in the VIT behavior. Keywords: Annular pressure buildup (APB), vacuum insulated tubing (VIT), heat transfer, modeling, thermal insulation.. INTRODUCTION Exploration and production of the Pre-Salt areas bring about numerous technical challenges, such as well construction in harsh conditions (ultra deepwater, thick salt layers, high hardness reservoir rocks, high pressure wells), complex flow assurance strategies (hydrate and wax formation inhibition, scale mitigation) and the choice of suitable enhanced oil recovery methods. Heat transfer plays an important role in several processes involved in deepwater oil production. For instance, the temperature gradient along the well depth is of crucial importance in well design and construction, since the fluids and equipment used in drilling and completion are selected based on the local pressure and temperature conditions. Therefore, controlling thermal events and their consequences has been a concern for oil companies since the early days of deepwater production in the 960s (Ezell et al., 200). It is well known that extreme pressures in the annulus can be reached in deep, high pressure, high temperature and high flow rate wells (Bradford et al., 2004). Consequently, Vacuum Insulated Tubing (VIT) technologies are a good alternative to minimize the radial heat transfer from the inner tubing to the well annular space. A schematic diagram of a VIT system is shown in Fig.. It consists of an inner production tubing welded to an outer tube at both ends, forming an annular cavity that is evacuated. The VIT connection (junction) is not thermally insulated. Recent studies (Ellis et al., 2002; Azzola et al., 2007) have shown that the heat loss through the connection region represent between 45 to 90 % of the total radial heat flux, depending on the external environment conditions. VIT systems can be manufactured according to different material and dimensional specifications. Typical ranges of overall tubing length, coupling length, tubing diameters and tubing material of commercial VIT systems are shown in Table. Despite the many years of experience by the industry in applying well insulation technologies to oil and gas production, the technical literature on VIT systems is still somewhat limited, so many questions concerning its thermal performance, applicability and most relevant physical mechanisms remain unanswered. For instance, aspects related to the effective thermal conductivity, heat loss through the connection and main body, and the impact of the vacuum level

2 Proceedings of ENCIT 204 Copyright 204 by ABCM 5 th Brazilian Congress of Thermal Sciences and Engineering November 0-3, 204, Belém, PA, Brazil on thermal performance, need to be properly quantified before VIT systems become consolidated in Brazilian offshore wells. Table. Typical dimensions of commercial VIT systems Overall length Coupling length Outer tubing external diameter Inner tubing internal diameter to 2.92 m to 0.46 m to m (7.000 to in) to m (5.500 to in) Coupling Outer tubing Weld Inner tubing Vacuum Figure. Vacuum insulated tubing concept and detail of the connection region. In this paper, a theoretically based methodology was developed to assess the thermal performance of VIT systems through a parametric evaluation of their thermal properties and parameters. Two independent formulations were tested, namely a 2-D fully discrete numerical differential model and a -D thermal resistance network model. The similarities and differences of each model are explored through a fundamental analysis of the VIT performance. 2. MODELLING The -D thermal resistance network model was implemented in EES (Engineering Equation Solver) (Klein, 204). The VIT system was divided into five regions (labelled to 5): the VIT body region, the body and coupling interface, the weld region, the outer tubing and coupling interface and the coupling region, as shown in Fig 2. Each region was modeled as an equivalent radial thermal resistance network (Incropera et al., 2007). In this way, it was possible to calculate the overall thermal resistance in each region, starting with prescribed temperatures on the inner surface of the VIT system (T i ) and in the bulk fluid surrounding the VIT (T ). Thermal contact resistances have been neglected. The overall (total) thermal resistivity of each region j, R total,j, (in m.k/w) and the corresponding overall heat transfer coefficient based on the internal diameter of the inner tube of the arrangement, U j, (in W/m 2.K) was calculated according to the following equations (Incropera et al., 2007): ( r ii ) ( r io) R total, = = ln r oi ln r oo 2 π r ii U 2 π r oi h r,c 2 π k s 2 π r oo h r,o ( r ii ) ( r io) R total,2 = = ln r oi ln r coup 2 π r ii U 2 2 π r oi h r,c 2 π r coup h r,coup () (2)

3 Proceedings of ENCIT 204 Copyright 204 by ABCM 5 th Brazilian Congress of Thermal Sciences and Engineering November 0-3, 204, Belém, PA, Brazil ( r ii ) R total,3 = = ln r coup 2 π r ii U 3 2 π r coup h r,coup ( r io) R total,4 = = ln r coup 2 π r ii U 4 2 π r coup h r,coup ( r oo ) R total,5 = = ln r coup 2 π r ii U 5 2 π k s 2 π r coup h r,coup (3) (4) (5) In the above equations, h r,c, h r,o and h r,coup are radiation heat transfer coefficients associated with the annular region (annular cavity) and external surfaces (outer tubing and coupling), respectively. These are given by: h r,c = h r,o = h r,coup = ε ε r oi ε r io σ ( T oi T io ) T 2 2 oi T io εσ ( T oo T ) T 2 2 oo T 2 2 ( T coup T ) εσ T coup T (6) (7) (8) Outer tubing Coupling Inner tubing Vacuum insulation Region q 4 q 2 q 3 Weld Region 2 Region 4 Region 3 q 5 Region 5 Figure 2. The -D VIT heat transfer model. The 5 regions are schematically illustrated together with their equivalent thermal networks. where r ii is the inner tubing inner radius, r oi the inner tubing outer radius, r io the outer tubing inner radius, r oo the outer tubing outer radius and r coup is the coupling outer radius. k s is the thermal conductivity of carbon steel (60 W/m.K), which has been assumed uniform for all metallic components, including the weld region. ε is the emissivity of the tubing surface and is the heat transfer coefficient associated with the external surface. A baseline value of 0.8 was considered for the surface emissivity, and a Pa (0.002 Torr) internal pressure was assumed in the vacuum space. It was assumed a VIT design which has the following specifications: length of 0 m, m (3 /2 in) of outer tubing external diameter, m (2 3/8 in) of inner tubing internal diameter and a coupling length of 0.26 m. In the present simulations, was estimated via the Churchill & Chu (975) natural convection correlation for a long horizontal isothermal cylinder. Although this may represent an over simplification of the situation encountered in real applications, it serves the purpose of designing an experimental setup for validation of thermal models. The physical

4 Proceedings of ENCIT 204 Copyright 204 by ABCM 5 th Brazilian Congress of Thermal Sciences and Engineering November 0-3, 204, Belém, PA, Brazil properties of the air surrounding the VIT were calculated at the film temperature of the outer surface for a pressure of atm. The heat transfer rate per unit length in each region was calculated from: q. j = T i T R total, j (9) where the overall thermal resistance of each region is the sum of the individual thermal resistances. It should be noted that convection and radiation thermal resistances are connected in parallel. The subscript j refers to each one of the five regions displayed in Fig.2. The 2-D heat transfer model was implemented using the ANSYS Workbench platform. The major advantage of the differential approach is the quantification of potentially high axial heat flux through the tubing walls that cannot be determined using the -D thermal network model. The VIT geometry shown in Fig. 3 was designed in SolidWorks and exported to the ANSYS platform. The mesh was generated in ANSYS ICEM CFD. A more refined grid was applied in the coupling region where heat transfer is more complex. A hexahedral block structured grid was generated with approximately 700,000 finite volumes for a 6 m tubing length. Figure 3. Numerical mesh for the VIT system (2-D model). 3. RESULTS A comparison between the two models is presented in Table 2 in terms of the heat transfer rate per unit of length through each modeled region of the VIT. The simulations were performed varying the prescribed temperature of the inner surface of the VIT and the tubing length from 0.4 to 6.0 m for an external prescribed temperature of 293 K. The considerable difference between the two models in Regions and 2 is due to the influence of the axial conduction through the tubing walls, which is captured by the 2D model, but obviously not by the D model. Furthermore, the heat transfer rate per unit length across Region decreases as the length of the VIT body increases, indicating a reduction of the influence of the axial conduction heat flux through the VIT. Figure 4 shows the results of an analysis of the sensitivity of the fraction of the total heat transfer rate that flows radially through the connection region as a function of the tube surface emissivity and inner surface temperature for the D and 2D models. The results are for a 6-m long VIT. The 2D model, which best represents the VIT system, is less influenced by the tubing emissivity, basically due to heat conduction through the inner and outer tubing walls, which carries heat away from the VIT connection. The values of heat loss through the connection of around 90% of the total heat loss are in agreement with those found in the literature. The external temperature profile along the VIT system for a prescribed internal temperature 373K is presented in Fig. 5. Since the VIT aims to minimize the overall radial heat transfer, the analysis of Fig. 5 reveals a hot spot in the central region of the connection, with the temperature falling towards the end of the outer tubing, confirming the more effective thermal insulation provided by the vacuum. Additionally, it is possible to observe in Fig. 5 that the temperature of the VIT tends to stabilize within less than m from the connection s edge, which indicates that the majority of the VIT s body is not affected by the axial heat transfer originated at the connection.

5 Proceedings of ENCIT 204 Copyright 204 by ABCM 5 th Brazilian Congress of Thermal Sciences and Engineering November 0-3, 204, Belém, PA, Brazil Table 2. Heat loss (W/m) comparison in each VIT region Heat transfer model D 2D VIT body length (m) Internal q (W/m) temperature (K) q q 2 q 3 q 4 q % Connection heat loss (%) 93% 92% 9% 90% Tubing emissivity 323 K - D 348 K - D 373 K - D 398 K - D 323 K - 2D 348 K - 2D 373 K - 2D 398 K - 2D Figure 4. Connection heat loss (fraction of the total heat transfer rate) as a function of tubing emissivity and internal temperature. A temperature map of a 0.8 m-long VIT device is depicted in Fig. 6 for a 373 K prescribed temperature assuming a 0.8 surface emissivity. The axial temperature gradient near the connection is clearly observed. The effective thermal conductivity (k-value), k ef, is the thermal design parameter of VIT systems (Azzola et al., 2004). This parameter is based on a one-dimensional analogy of conduction heat transfer between T i and T and represents the heat transfer rate per unit length per unit temperature difference. The k-value defined based on the outer diameter of the outer tube and the inner diameter of the inner tube is given by, (0) where is the total heat transfer rate per unit length of the VIT. The effective thermal conductivity has been used extensively to compare the effectiveness of different VIT designs (Azzola et al., 2004). k ef is related to the overall heat transfer coefficient based on the internal diameter of the inner tube of the VIT, U, via the following equation, 2k D ln ef U = () ii ( D D ) oo ii

6 Proceedings of ENCIT 204 Copyright 204 by ABCM 5 th Brazilian Congress of Thermal Sciences and Engineering November 0-3, 204, Belém, PA, Brazil Outer temperature (K) Z (m) Figure 5. External temperature profile along the VIT for a 373 K prescribed internal temperature. Figure 6. Temperature map for a 0.8 m VIT length simulation. Table 3 presents the k ef values calculated from Eq. (0) for the D and 2D models for a tubing emissivity of 0.8. The ratio between the VIT k ef calculated from the D and 2D models is also shown. Regardless of the prescribed inner temperature and VIT length, the k efd / k ef2d ratio is approximately equal to 0.8, which is consistent with the data of Table 2, that shows that the 2D model heat transfer rate per unit length can be much higher in regions and 2 due to axial conduction. In order to quantify the influence of each VIT region on the equipment overall thermal insulation capability, Figs. 7 and 8 depict the percentage of the VIT radial heat loss to the outer environment. In fact, despite of the effective insulation capability of the VIT body provided by the vacuum, due to the large length difference between the VIT body region and the other device pieces, it is clear from Figs. 7 and 8 that the heat loss (in W) through the VIT body region is much higher than the others. In addition, comparing the D and the 2D heat transfer models it is possible to see that in the 2D model the heat loss from region 2 represents a significant portion of the total heat transfer. This happens since region 2 is very close to the welded area, where a significant portion of the heat flux comes around the weld from the internal to the external tubing. Table 4 can be used to support the investigation on the above mentioned plots.

7 Proceedings of ENCIT 204 Copyright 204 by ABCM 5 th Brazilian Congress of Thermal Sciences and Engineering November 0-3, 204, Belém, PA, Brazil Table 3. Effective thermal conductivity calculations. Temperature (K) VIT length (m) k ef (W/m*K) D 2D k efd / k ef2d q i /q q/q q2/q q3/q q4/q q5/q VIT length (m) Figure 7. Heat loss to the bulk fluid surrounding the VIT device for the D model and a 398K prescribed temperature. Table 4. Fraction of heat loss, in W, through each modeled VIT region. L (m) q /q q 2 /q q 3 /q q 4 /q q 5 /q Model D D

8 Proceedings of ENCIT 204 Copyright 204 by ABCM 5 th Brazilian Congress of Thermal Sciences and Engineering November 0-3, 204, Belém, PA, Brazil q i /q q/q q2/q q3/q q4/q q5/q VIT length (m) Figure 8. Heat loss to the bulk fluid surrounding the VIT device for the 2D model and a 398K prescribed temperature. 4. CONCLUSIONS Two heat transfer models (D and 2D) for VIT systems have been implemented and their results have been compared. The 2D model results suggested a significant influence of axial heat conduction through the tubing walls at regions distant up to m from the connection, which cannot be captured by the D approach. The heat transfer rate per unit length through the connection region was also significant due to the absence of vacuum insulation in that region. The VIT effective thermal conductivity (k-value), k ef, was calculated for both the D and 2D models. For the conditions of the present simulations, the ratio of the D and 2D k-values was approximately equal to 0.8, which is consistent with the fact that, for a prescribed temperature difference, the heat loss is higher in the 2D model because of the axial heat conduction near the connection. Future developments will include a parametric study of the thermal influence of the connection distance as well as the VIT thermal behavior when the vacuum level, the annular space and the welding distance from the coupling are changed. Furthermore, a nodal.5d model-like will be developed considering the axial conduction thermal resistance and will be compared with the 2D model. 5. REFERENCES Azzola, J.H., Pattillo, P.D., Richey, J.F. and Segretto, S.J., The Heat Transfer Characteristics of Vacuum Insulated Tubing. SPE Annual Technical Conference and Exhibition. Houston. TX. USA September Azzola, J.H., Tselepikadis, D.P., Pattillo, P.D., Richey, J.F., Tinker, S.J., Miller, R.A. and Segretto, S.J., 2007, Application of Vacuum Insulated Tubing to Mitigate Annular Pressure Buildup. SPE Drilling and Completion Journal, Vol. 22, pp Bradford, D.W., Fritchie Jr., D.G., Gibson, D.H., Gosch, S.W., Patillo, P.D., Sharp, J.W. and Taylor, C.E., 2004, Marlin Failure Analysis and Redesign: Part I Description of Failure. SPE Drilling & Completion, June, 04-. Churchill, S.W. and Chu, H.H.S., 975. Correlating Equations for Laminar and Turbulent Free Convection from a Horizontal Cylinder. International Journal of Heat and Mass Transfer. vol Ellis, R.C., Fritchie, D.G., Gibson. D.H., Gosch, S.W. and Pattillo. P.D., Marlin Failure Analysis and Redesign: Part 2 - Redesign. IADC/SPE Drilling Conference. Dallas.TX. February. Ezell, L.g., Fontenot, S., Robinson, E., Cunningham, L. and Patrickis. A., 200. High Performance Aquous Insulating Packer Fluid Improved Flow Assurance and Reduced Annular Pressure Buildup in Ultra Deepwater Wells. SPE Deepwater Drilling and Completions Conference. Galvestone. TX. USA. October. Incropera, F.P., Dewitt, D. P., Bergman, T. L. and Lavine, A. S., Fundamentals of Heat and Mass Transfer. John Wiley & Sons. Inc.. New York Klein, S.A. and Alvarado, F.L., 204, Engineering Equation Solver (EES), F-Chart Software, Professional Version RESPONSIBILITY NOTICE The authors are the only responsible for the printed material included in this paper.