Summary

Vacuum-insulated tubing (VIT) has been used successfully to mitigate the potentially harmful effects of annular pressure buildup (APB). In a recent deepwater installation, the subject well had lost alternate APB mitigation capability through a series of events. VIT was then chosen as the only viable technology.

A common design companion of VIT is gelled brine, chosen to decrease annular natural convection driven by heat loss around the VIT connections. There are, however, several drawbacks to the indiscriminate use of gelled brine:

  • Tight clearances around the tubing hanger running tool, creating the potential for debris plugging and/or tool-recovery issues from the subsea wellhead.

  • Limitations imposed by VIT collapse and hydrostatic packer setting pressures.

  • Unknown temperature/viscosity response and associated quality-control requirements for displacement through a subsea wellhead.

  • A need to reduce operational time and cost.

These issues led to the consideration of alternative means of controlling natural convection, and an effort was made to understand, improve, and deploy external coupling insulators.

An in-ground vertical experiment with two connected joints of VIT was conducted with and without coupling insulators. Temperatures were monitored at multiple locations. Test results confirm the effectiveness of external coupling insulators. Several theoretical models using different numerical techniques (finite difference, lumped mass/resistance, finite element, and computational fluid dynamics) were found to be consistent with experimental results. These were compared with critical APB temperatures calculated with a commercial wellbore simulator. The net result of these studies was to adopt the external insulators.

This paper reviews the experimental data and presents several models of a vertically aligned VIT in a deepwater completion. A comparison of thermally effective APB solutions is presented, together with a critical assessment of modeling and experimental accuracy.

Introduction

Thermal insulation technology is an evolving science covering a wide range of industries and applications and suitable for energy conservation, transport and storage of fluids, isolation of critical components, and other uses.

The first oilfield insulated tubing applications, based on silica powder and polyurethane media, were designed to prevent flowline subsidence in the Alaskan tundra. From these simple beginnings, vacuum tubing developed into a highly efficient insulation system capable of carrying large axial loads while fitting within a compact form factor.

The reduction of catastrophic risk associated with annular pressure buildup in a subsea well completion requires specialized technology (Moe and Erpelding 2000). In the late 90s, following one notable and several suspected well failures in the Gulf of Mexico (Bradford et al. 2002; Ellis et al. 2002; Gosch et al. 2002; Vargo et al. 2002), interest in APB solutions increased. The BP Marlin failure was attributed to several potential root causes, one of which was APB. To alleviate APB, VIT was recognized as an effective remedy in minimizing heat transfer to outer annuli.

In 2002, BP's King West completion was challenged by several unanticipated technical problems. Standard mitigation methods were considered, but because of wellbore conditions, VIT was chosen as the only viable technology.

VIT consists of an inner and outer tube welded together at both ends. Piping sizes range from 2-in. line pipe to 7-in. pipe in Range 2 or 3. Materials are generally L-80 or 13Cr. The annular space, typically 0.15 to 0.5 in. wide, is evacuated and plug welded. This space contains aluminum foil wraps for radiation reduction and a getter to scavenge hydrogen and other deleterious gases for vacuum preservation over a typical design life of 10 years.

Historically, VIT has been applied successfully in a number of areas: enhanced secondary oil recovery through the use of steam delivery systems [steamflood, steam-assisted gravity drainage (SAGD)], paraffin buildup prevention, alleviation of hydrate formation, and an increase in heavy oil flow through temperature preservation. While VIT has been effective in these applications, the understanding necessary to design systems that are problem-specific, and particularly the analytical tools required to optimize those systems, have not yet been developed.

To understand VIT performance conceptually, one must begin with the vacuum insulation jacket. Thermal insulation in the vacuum body is extremely high. An alternate viewpoint is that the heat flux (heat flow per unit area) through the vacuum space is relatively low (Fig. 1). The complicating feature of VIT is the relatively large heat fluxes around the coupler region (Azzola et al. 2004). There are two distinct heat paths, each preferentially following low thermal resistivity through the relatively high-conductivity steel. The first path runs radially through the coupler (Fig. 1). The second path runs axially along the inner pipe toward the weld, then through the weld and down the outer pipe. From there, heat is lost by convection around a hot zone which extends generally one to two feet axially beyond the weld (Fig. 1). The magnitude of this secondary heat path depends on the convective heat transfer coefficient in the annulus, as well as on other variables, but in general, it exceeds 30% of total heat loss. For this reason, VIT acts like a fin, and as a consequence, it exhibits a heat loss, which can be expressed as a thermal conductivity (k) or overall heat transfer coefficient (U), which in turn depend strongly on boundary conditions such as annular fluid properties, coupling insulation, completion geometry, and ground conductivity (Azzola et al. 2004).

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