Permafrost Thaw-Subsidence Casing Design
Authors: Mitchell R.F. , Goodman M.A. 
Published: December 2007

Casing design for arctic wells requires estimates of maximum casing strains resulting from permafrost thaw subsidence. These maximum strains have been determined for the Prudhoe Bay field by sensitivity studies with an analytical thaw-subsidence model that correlates with field test data.

Introduction
Deep permafrost at Prudhoe Bay consists of permanently frozen sediments about 1,850 ft thick. Thaw subsidence is the soil compaction resulting from the thawing of permafrost by a producing oil well. Thaw subsidence permafrost by a producing oil well. Thaw subsidence should be considered in well design because of the strains induced on well casing by this compaction.
Atlantic Richfield Co. and Exxon Co., U.S.A., conducted a field test to assess thaw-subsidence effects. Subsidence was measured by surveying the motion of the wellheads, using subsidence rods and wires to monitor vertical motion in the soils, monitoring the motion of radioactive bullets shot into the formation, and measuring casing axial strains using a multiple-casing collar locator tool. The casing axial strains are most important because they indicate the casing's resistance to soil motion. An analytic thaw-subsidence model developed at Exxon Production Research Co. gives good qualitative explanations for the field test results and good quantitative agreement with the measured casing strains. The analytic subsidence model provides the tool needed for extrapolating the field test results to other areas in the field. This extrapolation was accomplished through a series of sensitivity studies.
Two types of parameters influence thaw subsidence and are required in the analytical model. The first type is the amount of thaw, which can be controlled through the completion scheme and the production schedule. The second type involves the permafrost description, which cannot be controlled. The permafrost description is defined by the lithology, mechanical properties, and pore pressure. The lithology consists of the arrangement, pressure. The lithology consists of the arrangement, thickness, and location of the layers of the various soil types. The mechanical properties are the soil compressibility (compactibility) and the resistance to shear. The pore-pressure behavior during thawing is required because the pore-pressure reduction is the main driving force in the thaw-subsidence model.
The confidence in determining the values for the input parameters in the analytical model is closely related to the parameters in the analytical model is closely related to the sensitivity of the model to these parameters. If the model is not very sensitive to the lithology, for example, then one does not have to have confidence in determining lithology across the field. Conversely, if the model is sensitive to lithology, then one should be confident in determining the lithology within some reasonable bound. The objective of the sensitivity studies is to determine and, it is hoped, bound the dependence on the various input parameters. The sensitivity studies include the strain dependence onthaw radius,lithology as defined by separation, thickness, and location of the layers, andmechanical properties including the compressibility and shear modulus.
In the course of the sensitivity studies, certain ratios quantities were found to be important parameters when investigating layered systems. The layer separation is characterized by the sand/silt thickness ratio, which is the ratio of the thickness of a given sand layer to the thickness of an adjacent silt layer.
JPT
P. 455