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.
Over the past decade, petroleum operators have shown renewed interest in exploring both onshore and offshore for oil and gas reservoirs in Arctic regions. This growing interest can be attributed to the depletion of the hydrocarbon reservoirs and fewer good prospects in more readily accessible regions, as well as the development of new techniques/equipment such as subsea well capabilities that will better facilitate the recovery of oil and gas under such demanding conditions.
In addition to the many challenges imposed by the harsh isolated environment, another significant factor affecting the complexity of arctic oil and gas developments is the fact that vast areas are overlain by a thick permafrost layer that can extend to depths of 700 m or more [DeGeer and Cathro, 1992]. Permafrost soil layers may be present both onshore and offshore and they can be either continuous from the surface, or discontinuous with intermittent unfrozen zones. Figure 1 shows the extent of continuous and discontinuous permafrost regions throughout the northern hemisphere [Ferrians et al., 1969]. Figure 2
shows a schematic cross-section of the permafrost layer extending from onshore to offshore that is representative of some coastal areas within the MacKenzie Delta - Canadian Beaufort sea region.
Author: Xie, J., & Matthews, C. M.
Publisher: SPE Arctic and Extreme Environments Conference and Exhibition, 18-20 October, Moscow, Russia
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