Permafrost Freeze-Back Pressure Behavior
- Malcolm A. Goodman (Exxon Production Research Co.) | Donald B. Wood (Exxon Production Research Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- August 1975
- Document Type
- Journal Paper
- 949 - 950
- 1975. Society of Petroleum Engineers
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- 137 since 2007
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Drilling and production in the Arctic will thaw permafrost. At Prudhoe Bay on Alaska's North Slope the permafrost. At Prudhoe Bay on Alaska's North Slope the permafrost is nearly 2,000 ft thick. If thawed permafrost permafrost is nearly 2,000 ft thick. If thawed permafrost is allowed to freeze back, significant collapse loads near the bottom of the permafrost will be generated that must be considered in casing design. The loading mechanism is associated with the phase-change expansion of pore water in the thawed permafrost. The magnitude of the pressure buildup depends on the mechanical response of the frozen permafrost.
This article describes the formulation of an analytical freeze-back model and its correlation with freeze-back field-test data from Prudhoe Bay. As such, it is a condensed version of a paper in which the analytical model was formally developed. The model and field test yield the following conclusions.
1. The 13 3/8-in., 72-lb, N-80 casing used in the field test and commonly used at Prudhoe Bay can safely withstand the maximum freeze-back pressures.
2. For freeze-back from large thaw radii (50 ft of production thaw), the maximum pressure is not significantly production thaw), the maximum pressure is not significantly greater than that for freeze-back from small radii (3 ft of drilling thaw).
3. The maximum freeze-back pressure depends on the elastic and yield properties of permafrost, but is most sensitive to the Young's modulus of frozen permafrost.
4. Based on laboratory studies and supported by the field-test data, the creep or viscoelastic behavior of permafrost subject to freeze-back is negligible compared permafrost subject to freeze-back is negligible compared with the purely elastic and yield behavior.
5. To limit the freeze-back pressure, the model is useful in the design of methods to limit the amount of initial thaw or to limit the extent of freeze-back.
The Freeze-Back Model
At each depth, the permafrost is initially thawed to a radius rb and, after refreezing for some time, is frozen back to radius ra. These two radii are input information to the model and serve to determine the amount of phasechange expansion at each instant in time. At the phasechange expansion at each instant in time. At the beginnig of freeze-back, the thawed permafrost is nearly saturated because of vertical drainage, water influx from drilling fluids, or compaction of the soil structure.
The freeze-back process occurs in three stages: relief of effective stress, elastic behavior, and elastic-yield behavior (see Fig. 1). In the first stage, as the pore water freezes, the ice either expands into the fluid-filled pores, increasing the porosity, and at the same time compressing the pore water, or expands locally, causing lens formation.
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