Over the past decade, petroleum operators have shown increased interest in exploring and developing oil and gas reservoirs in both onshore and offshore Arctic areas. In many cases, the reservoirs are known to be overlain by massive permafrost layers on the order of 50 to 700 m thick. These conditions create unique design and operation 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.
This paper presents a methodology for modeling and analyzing the severe casing loading and deformation conditions that can occur under thaw subsidence loading. The well design and evaluation methodology includes several sequential steps as follows: wellbore hydraulic and heat transfer analysis, to determine the heat input to the permafrost interval along the well(s) due to either the production of hydrocarbons or water injection; geothermal and geomechanical analyses, to calculate the extent of the permafrost thaw and the resultant thaw-induced soil stresses and movements; and casing-formation interaction analyses, to establish the structural response and evaluate the mechanical and hydraulic integrity of the well casing under the thaw subsidence loads. Sequential thermal and displacement analysis models are used to establish the extent of the thaw boundary that develops with time around the well(s) and the associated thaw subsidence response of the individual soil layers. These results serve as inputs to the non-linear analyses used to assess casing integrity. Examples are used to demonstrate the potential for thaw subsidence movements to cause casing failures, as a result of excessive compressive strains, buckling or large lateral deformations, in both single and multiple well layout scenarios. The methodology presented is recommended for optimizing well completion designs to minimize the potential for such failures to occur.