Insulated Hot Oil-Producing Wells In Permafrost

Abstract

An analysis is presented for the transient heat flow in and around production wells in which insulated casings may be used to reduce production wells in which insulated casings may be used to reduce permafrost thaw in the Alaskan North Slope. Results show that insulated permafrost thaw in the Alaskan North Slope. Results show that insulated casing systems can greatly restrict potential thawing of permafrost. Introduction A potential problem in the production of hot (180 to 200 degrees F) crude oil in unprotected wells on the Alaskan North Slope is the thawing of permafrost adjacent to the wells. Thawing of permafrost could result in soil consolidation, subsidence, and possible damage to the well. BP Alaska, Inc., and Arthur D. Little, Inc., have collaborated in developing a method to analyze the thermal performance of insulated wells in permafrost and in performing analyses of typical insulated well-casing systems. The ultimate objectives of this work were to understand permafrost thermal behavior under oil production conditions and to assist the design, development, and testing of an insulated well-casing system. The heat flow along and through the well casing and into the surrounding soil is quite complex. Standard, closed-form, or integral solutions available for melting in one or two dimensions are not applicable to the heat flow in the surrounding soil because of the complexity of heat exchange between the flowing hot oil and the ground, and the variation of thermophysical properties with temperature and depth. Previous studies for wellbore heat transmissions have neglected vertical beat flow in the soil and have not taken into account the effects of latent heat of fusion. Couch et al. have developed a mathematical model for thawing the soil surrounding unprotected wells that included the effects of latent heat of fusion of the soil. The effects of the latent heat were shown to be significant. Their analysis treated the well casing as part of the thermal resistance between the hot oil and the soil. Heat flow along the casing was not accounted for. In the present analysis, we have considered the details of the construction of the casing itself, as well as the heat flow in the casing, to evaluate the effects of "thermal shorting" across the insulation where casing joints are coupled. In addition, the model allows an examination of the effects of localized failure of one or several insulation joints. Analysis Problem Definition and Mathematical Models Problem Definition and Mathematical Models Fig. 1, a sketch of a typical configuration oil-producing well in a permafrost region, shows the flowstring for hot crude oil, a surrounding insulation system (extending only over a selected depth of the permafrost region) and casing, and a region of cement surrounding the well casing. Hot crude oil enters the flowstring at a depth significantly below the permafrost level and flows upward, transferring heat to the innermost pipe by convection. Heat is conducted along the pipe by convection. Heat is conducted along the flowstring, through the casing and insulation, through the cement, and into the surrounding permafrost and unfrozen soil. Since the well-casing components provide a thermal short circuit in the vertical direction, heat also may be conducted along the casing from warmer to colder regions. JPT P. 357