What Vacuum-Insulated Tubings and Casings Bring to Thermal Wells
This paper describes both design and economic considerations that lead to the selection of vacuum-insulated tubing (VIT) or vacuum-insulated casing (VIC) for the completion of thermal wells.
This paper describes both design and economic considerations that lead to the selection of vacuum-insulated tubing (VIT) or vacuum-insulated casing (VIC) for the completion of thermal wells. The results shared in this paper are some of the parameters considered during thermal-well design: temperature on the casing and the tubing, and heat loss. Knowing these parameters, well integrity can be studied and the overall efficiency of the process estimated.
The most common thermal enhanced-recovery methods are cyclic steam stimulation, steamflooding, and steam-assisted gravity drainage, which is widely used in Canada. The role of these thermal-recovery methods is to convey heat into the reservoir, mainly by convection. The temperature of the oil increases and its viscosity decreases significantly in the reservoir. These methods can substantially increase the oil production of a field, increase the recovery factor, or unlock some heavy-oil assets.
Typical steam-injection temperature is between 250 and 310°C; in a few cases, it could be greater than 310°C. This high-temperature application requires adapting the design of the injector wells to avoid any mechanical failure of the well or to mitigate heat loss through the well. VIT or VIC are solutions to some of these issues.
VIT technologies have evolved significantly since their first application. Different materials are used as insulation material, and several designs are proposed by the industry to achieve the best thermal-performance-vs.-cost equation.
A model was developed to assess the temperature of the different components of a well—tubing, casing, cementation, and ground—and to provide the heat loss occurring during the steam injection. The results show that the wide range of VIT thermal performance provides a range of casing temperatures. Thus, the casing thermal expansion or thermal loads resulting from this temperature increase range significantly.
A VIT is a double-walled tubing, one inner tubing and one outer tubing with an annular space between. The connection could be on either the inner tubing or the outer tubing. Two main aspects will define the VIT performance and dimensions: the insulation material and how the connection between two VIT joints is insulated. There are two main types of insulation. The first, selected when VIT was first developed, is based on the insulation material used by the space industry. It requires a high vacuum (10–5 Torr) coupled to a multilayer foil. The high vacuum will reduce the thermal conduction and thermal convection in the annulus space. The multilayer foil will reduce the last mode of heat transfer, the radiation between the inner pipe and the outer pipe. The phenomenon of hydrogen permeation through the carbon steel of the tubing increases the vacuum pressure of the annulus, thus reducing the service lifetime of VIT using high vacuum as insulation. The other insulation material used is a microporous material applied between the inner tubing and the outer tubing. The microporous effect is the restriction of air-molecule collision that leads to heat transfer. Applying a soft vacuum, typically 10 Torr, the pore inside the microporous material is smaller than the mean free path of air molecules, thus reducing the heat-transfer modes. This material requires only a soft vacuum (seven-order-of-magnitude difference from the high vacuum required for the multilayer-foil technology). One major advantage of using a soft vacuum is that the hydrogen permeation does not affect the long-term performance.
The insulation of the connection of two consecutive VITs is the other major aspect that is critical to the overall thermal performance. The first VIT technologies had limits because of the poor insulation at the coupling. The thermal-insulation weaknesses are even more substantial at high temperature because of radiation and convection caused by potential fluids in the casing annulus because both of these effects depend strongly on the temperature difference across the annulus. The concept of the insulation at the connection is to use an insulator sleeve above the coupling. This sleeve is typically made of Teflon. An alternate design, now available on the market, uses intermediate pieces to cover the coupling with a layer of vacuum insulation. Thus, the conduction, convection, and radiation are reduced to a minimum.
The analysis model is a steady-state thermal model of a horizontal slice of a well. It includes the different layers of a thermal well: the bare tubing or the VIT including the inner tubing, the outer tubing, and the gap in between; the casing annulus; the casing; the cementation; and the ground. Only the radial heat flux is considered. The thermal resistance of each layer is calculated following formulae that are provided in the complete paper. The casing annulus could be filled by air; therefore, radiation has to be taken into account with conduction. The heat flux through each layer has the same value. This provides several equations having temperatures as the unknowns. An equation solver provides the temperature results in each case.
The inputs to the model are
- The steam temperature
- The ground temperature
- The VIT U-value, or the overall heat-transfer coefficient of the VIT
- Outer diameter (OD) and inner diameter (ID) of each layer
- The thermal conductivity of carbon-steel pipe, air, cement, and the ground
- Convection coefficient between the steam and the inner tubing
Well-Temperature Analysis During Steam Injection
The study was based on a typical thermal injection well. The main characteristics of the well are
- Inner tubing OD is 3½ in.
- Casing OD is 9⅝ in.
- Cement thickness is 2 in.
- Ground thickness is 15 ft.
- Steam temperature is 300°C.
- Ground temperature far from the well is 30°C.
- Steam/oil ratio is equal to 3.
The thermal performance indicator of the insulated tubing is the overall heat-transfer coefficient, or the U‑value; the unit expression is Btu/(hr-ft2-°F) [or W(m2•K)]. The choice of the U-value over the other commonly used thermal conductivity, K-value, is guided by the following:
- The U-value gives the intrinsic thermal performance of an insulated tubing, including the connection.
- U-value measures the overall heat transfer of a tubing; multiplied by the surface reference and the temperature differential, it provides the heat flux in Watts or Btu/hr.
- Only one surface reference needs to be defined, whereas the K-value requires two reference surfaces.
- Two definitions are commonly used for the K-value, leading to possible misinterpretation of thermal performance. The K-value could be defined relative to the outer diameter of the outer pipe and inner diameter of the inner pipe, or relative to the inner diameter of the outer pipe and the outer diameter of the inner pipe.
In this work, all U-values are defined relative to the external surface of the inner pipe. Currently, almost all technologies of insulated tubing use a double-walled pipe with an inner pipe and outer pipe. The fluid is conveyed through the inner pipe.
Results and Conclusion
The results from this study show that behind the generic name of VIT, the overall thermal performance of the selected product will yield significant variation in the well temperature or heat loss. In some pressure conditions, the casing temperature can be higher than the water-vaporization temperature or can be high enough to create annular-pressure buildup. The thermal stress induced by the temperature increase in the casing is far from negligible. With a VIT U-value higher than 1 Btu/(hr‑ft2‑°F), the thermal stress induced represents greater than 30% of the yield strength of L80-grade casing.
The basic economics, based on the results of the model, show that any improvement of the overall thermal performance of a VIT will lead to additional oil production. That means that, apart from well-integrity issues, for which VIT can be the answer, the saved energy itself can make a return on investment sufficient to justify VIT investment.
The wide range of VIT thermal performance is representative of the market availability. Thus, during the selection of a VIT string for a thermal well, the specification of the application should be clearly identified to select the proper VIT.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 182515, “What Vacuum-Insulated Tubings (or Vacuum-Insulated Casings) Bring to Thermal Wells,” by J. Damour and D. Johannson, Majus, prepared for the 2016 SPE Thermal Well Integrity and Design Symposium, Banff, Canada, 29 November–1 December. The paper has not been peer reviewed.