Flow assurance has been a major consideration in deep water completions where undesired heat loss from production tubing to outer annuli can lead to the deposition of sludge, paraffin and asphaltene materials, the formation of gas hydrates, and cause severe flow assurance problems and loss of productivity. In recent years, thermal insulation fluids have been successfully applied in wellbore and deepwater risers to prevent undesired heat loss. Case histories have demonstrated that installation of insulation fluids is an effective alternative to conventional insulation options such as an external insulation or nitrogen gas for deepwater risers. These successes have made this insulation technology the preferred method in many GOM Deepwater projects.
Field practices and flow assurance simulations have shown that the ideal thermal insulation fluid design relies on effectively confining thermal convection and minimizing effective thermal conductivity of the fluid. How do fluid compositions affect these factors? What is required for an insulation fluid to be effective in an oilfield installation? These questions will be addressed.
This paper will discuss the proper testing methods that are relevant to oilfield flow assurance reality and are required to qualify thermal insulation fluids fit-for-use. Fluid rheology and intrinsic thermal conductivity have been assumed by some as two controlling factors for an effective thermal insulation fluid, especially in typical flow assurance simulations. Utilizing proven testing methods, these assumptions will be re-examined and discussed in terms of oilfield thermal insulation reality. Relationship between fluid chemical composition, fluid rheology and effective thermal insulation will be explored. Findings on how to optimize the fluid compositions to achieve the desired insulating potential and make the application of such fluid economical and practical in the field will be presented. Selected field cases will be discussed in reference to these recent findings.
Introduction
Water-based insulating packer fluids for deepwater applications have been successfully installed in the Gulf of Mexico (GOM) since 1999, with over 50 installations in water depths ranging from 1,100 to over 5,000 feet. Applications ranged from controlling pressure buildup in outer annuli, to undesired heat loss from production tubing, especially heat loss through deepwater risers. Applications for deepwater risers have evolved during these same years, from applying insulating fluid to a single annulus riser, then to the outer annulus of a dual-annulus riser system, and more recently to both annuli of a dual-annulus riser system. Development and evolution of these applications are described in detail1–4.
Concurrent with the evolution of deepwater riser developments, the evolution of water-based insulation fluids advanced significantly and new developments have been quickly introduced into the industry. As described herein, a third generation insulation fluid with improved thermal insulating qualities has been evaluated and developed.
In addition to the standard testing to determine basic physical properties and long-term stability of an insulation fluid, explicit testing to determine the "effective thermal conductivity" (defined herein) of an insulation fluid for wellbore or deepwater riser applications, is required. These test methods1–4 include the all-important contribution of "convection". Convection is one of the most important sources of heat loss in a wellbore or riser and the forces that create convection must be incorporated into the test methodology5. When these forces are not present or are minimized in the test methodology6, the "thermal conductivity" values that result, relate more closely to the "intrinsic thermal conductivity" (defined herein) of the fluids, and do not adequately define their impact on heat loss within a wellbore or riser. An over-estimation of the fluid's ability to control heat loss in a wellbore or deepwater riser therefore results.
This paper will discuss the use and implications in using the terms "effective thermal conductivity", "thermal conductivity", and "intrinsic thermal conductivity", and the importance in properly using the correct term.
