Methodology to Assess Thaw Subsidence Impacts on the Design and Integrity of Oil and Gas Wells in Arctic Regions

Abstract

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.

Experimental Set-Up for Testing Cement Sheath Integrity in Arctic Wells

“…Due to the harsh Arctic topside conditions, all downhole operations are required to be performed within short time windows. Encountering frozen formations during drilling is challenging, as they can thaw and cause hole enlargening or subsidence in the near-well region (Xie and Matthews 2011).  Drilling challenges.…”

“… Drilling challenges. The well is also likely to be exposed to high loads during its productive lifetime as a result of strong Arctic ocean currents, ice scouring, repeated detaching/relocating of topside structures or subsidence due to freezing/thawing of sediments (Kipker 2011, Xie andMatthews 2011). The latter type of "geohazard" comprises subsea permafrost and gas hydrates, which can be buried as deep as 600-900 m below the seafloor and have thicknesses of several hundred meters (Paepe and Melnikov 2001).…”

“…Arctic drilling operations are commonly complicated by large buried boulders or frozen formations. 1000 m below seafloor when a well was drilled through frozen formations (Xie and Matthews 2011). Encountering frozen formations during drilling is challenging, as they can thaw and cause hole enlargening or subsidence in the near-well region (Xie and Matthews 2011).…”

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

“…These assessments should clearly define the permafrost soil conditions and examine the relative utility of alternative well completion designs and layouts that may be considered suitable for the prescribed location and field development scenario [Xie and Matthews 2011, Degeer and Cathro 1992, Ruedrich et al 1978, Goodman 1977. These assessments should clearly define the permafrost soil conditions and examine the relative utility of alternative well completion designs and layouts that may be considered suitable for the prescribed location and field development scenario [Xie and Matthews 2011, Degeer and Cathro 1992, Ruedrich et al 1978, Goodman 1977.…”