Thermal analysis on permafrost subsidence on the North Slope of Alaska

Abstract

One of the major problems associated with the oil fields on the North Slope of Alaska is thawing permafrost around producing oil wells. In these wells, the heat from the producing well fluid gradually thaws the permafrost. This thawing in turn destroys the bond between the permafrost and the casing and causes instability that results in permafrost subsidence which further causes subsidence of the soil surrounding the wellbore and, subjects the casing to high mechanical stresses. The above problem has been addressed by several engineers, and several preventive measures, such as controlling the subsidence by refrigeration or by insulation of the wellbore, have been analyzed. Understanding the thermal behavior of the permafrost is imperative to analyzing permafrost subsidence and providing preventative measures. The current project focuses on building a scaled-down axi-symmetric model in FLAC 7.0 that will help us understand the thermal behavior (i.e., the heat input to the permafrost interval due to hydrocarbon production) and temperature distributions that result in permafrost subsidence. The numerical analysis estimated the thaw influence of steam injection used for heavy oil recovery and its effect on the area around the wellbore for 10 years. The developed model was compared with Smith and Clegg (1971) axi-symmetric model and COMSOL model and correlations of thaw radius and wellbore temperatures were obtained for different types of soils. Heat transfer mitigation techniques were also attempted which are discussed in the report further.