The Heat Transfer Characteristics of Vacuum Insulated Tubing

Abstract

Abstract Thermal effects in vacuum insulated tubing (VIT) have been studied in previously reported experiments2. VIT is usually characterized by a single thermal conductivity (-value) based on a one-dimensional radial heat transfer analogy. Despite its simplicity, however, this physical model masks the underlying complexity of the two-dimensional axisymmetric heat transfer field. Recent theoretical insights have demonstrated that VIT behaves as a fin around the weld and coupling regions, where up to 90% of the overall heat loss can occur. As a result, VIT does not have a single k-value; instead it has a potentially wide range of k-values, depending on boundary conditions. Recent numerical and experimental investigations of vertically aligned VIT demonstrate the highly sensitive nature of thermal performance with respect to these boundary conditions1. With this insight, previous test data2 is reinterpreted in a more revealing light. It can be demonstrated that the 10-fold spread of k-values from these tests is attributable to explainable boundary phenomena. Another issue which requires clarification is the k-value definition. A standard definition is presented here which 1) is physically related to the effective thermal conductivity of the vacuum and 2) is relatively stable for different VIT configurations. Transient behavior, including startup, cool down during shut in, ground soak, and convection cell development will be explained, as well as the implications for long-term thermal stability. The discussion concludes with calculations of k-value for VIT under significantly varied boundary conditions: different annular fluids, different soils, with and without interior or exterior coupling insulators, and using high-temperature steam injection. Introduction Over the past 25 years, VIT has matured as a technology capable of thermal isolation in increasingly challenging environments1–4. Historically, because VIT performance in some applications, such as cold startup and permafrost subsidence prevention, proved adequate, design tools have remained undeveloped. In other applications, such as paraffin formation and annular pressure buildup (APB), insulated tubing design requires a deep understanding of the performance boundaries in order to make rational cost/benefit judgments. In some notable cases (steam flood and heavy oil flow applications), such understanding is necessary to determine whether the economic exploitation of a field is feasible at all. Unfortunately, the inherent complexity of the subject does not lend itself to simple answers. Nevertheless, with regard to VIT and insulated tubing in general, establishing performance, while far from being simple, is not an altogether intractable problem. VIT performs well for tasks that require a combination of 1) compact space such as those encountered downhole and in riser sections, 2) high load capability, able to pull 200,000 - 500,000 lbs tension load, and 3) high thermal insulation value, with overall thermal conductivity (U-values) less than 1 Btu/hr-ft2-°F (5.7 W/m2-°K). This paper addresses issue (3), concentrating on the thermal performance of VIT under as wide a range of boundary conditions as is presently encountered in the industrial use of the product. Conceptual issues about performance will be addressed. To standardize requirements for all users, a choice of metric is presented, together with a rationale for its use.