Importance of Deep Permafrost Soil Characterization for Accurate Assessment of Thaw Subsidence Impacts on the Design and Integrity of Arctic Wells

Abstract

Oil and gas producers have shown renewed interest in developing reservoirs located both onshore and offshore within the Arctic regions of Alaska, Canada and Russia. In many cases, the hydrocarbon reservoirs are known to be overlain by a massive permafrost interval that extends over depths of up to 700 m below the surface active layer. These conditions create unique design and operational challenges for production and injection wells from the perspective of ensuring that well integrity will not be compromised by the inevitable thaw subsidence of the permafrost soil layers.The permafrost soil layers surrounding arctic wells will thaw gradually with time due to wellbore heat loss. As the thaw zone advances radially outward from each well, the ice-to-water phase change within the pore space of the frozen/partially frozen sediments will lead to changes in the permafrost soil properties and to the loading conditions within the thaw column region. These changes will result in soil deformations (including both vertical settlements (subsidence) and horizontal displacements) which can, in turn, induce significant well casing strains that need to be considered in selecting the well design and layout. The magnitude of the soil deformations that occur throughout the permafrost interval are highly dependent on the deposition history, insitu temperature and the physical and mechanical properties of the individual soil layers. Therefore, in order to accurately predict the soil deformations and resultant localized casing strain levels, it is essential to obtain reliable data to properly characterize the lithology (soil types) within the permafrost interval, as well as the frozen state and the relevant mechanical and thermal properties (both frozen and thawed) of individual soil layers. This paper describes the various information and geotechnical test data that has been used to establish the thaw and deformation response of different permafrost soils at a number of arctic locations for the purpose of evaluating the effects of thaw subsidence loading on wells.Overall, the paper serves to highlight the importance of collecting the appropriate geotechnical data to allow thaw subsidence-induced ground deformations and associated casing loading conditions to be properly considered at the well/project design stage. 2 OTC 23747irreversible ground movements throughout the thawed soil column in the vicinity of production wells. The thaw and deformation response is highly dependent on the permafrost lithology and soil properties at each particular location and it is important to recognize that these conditions can differ considerably even within relatively small geographic areas. This paper outlines the key factors which influence thaw subsidence potential and describes the various information and geotechnical test data that has been used to establish the thaw and deformation response of different permafrost soils (e.g. coarse-grained and fine-grained) at a number of arctic locations for the purpose of evaluating the effects of thaw subsid...