The consequences of permafrost thaw resulting from increased surface temperatures during drilling and production, affect casing design in a number of ways.
Permafrost is a permanently frozen soil. Soil at temperatures below freezing point may or may not contain ice, depending upon pore fluid salinity, pore pressure, and soil type. Permafrost can be continuous from the surface or discontinuous with intermittent unfrozen zones.
The following practices are for permafrost well casing design. They directly influence casing setting depths (part of the preliminary design phase) and casing load determination. Casing capacity is assumed to be unaffected.
The experimental data upon which these practices are based is mostly location-specific and should not be assumed to be generally applicable.
Casing setting depths
Stove pipe
As the well is drilled and produced, heat generated will cause the permafrost to thaw around the wellbore, thereby reducing or eliminating any cement-formation bond within the permafrost interval. This in turn may cause casing subsidence and/or annular gas migration, as well as a disturbance to the permafrost environment. To offset this effect, the stove pipe, usually set at around 60 ft (18 m), consists of:
- One or more joints of insulated casing made by welding a joint of 26 in (0.66 m) casing inside a joint of 30 in (0.762 m) casing and either leaving a dead air space or placing insulation material in the annular area between the casings.
- One joint of refrigerated casing at surface made by a process similar to the insulated casing but instead running a number of refrigeration coils in the annular area between the casings. A refrigeration unit is used to continually circulate a refrigerant through the coils during both the drilling and production phases to ensure no permafrost melting.
Conductor casing (permafrost casing)
A string of 20 in (0.508 m) conductor casing, also known as permafrost casing is required by Canadian government regulations when the permafrost is in unconsolidated formations and/or is more than 500 ft (150 m) deep. This requirement is also recognised as prudent operating practise in order to limit exposure of permafrost to the warm drilling mud and hence minimise hole enlargement and subsequent hole cleaning/cement displacement problems. The string is set at 500 ft (150 m).
Surface casing
Assuming permafrost casing is set, a 16 in (0.406 m) surface casing is set to ensure all remaining permafrost is cased off as soon as possible. Canadian government regulations require this string to be set between 500 ft (150 m) and 1650 ft (500 m). To allow for the greatest flexibility in setting the intermediate casing, the surface casing is usually set at 500 m.
Detailed casing design
Two effects have to be taken into account when designing casing set in permafrost.
External freezeback
When a well is shut-in during or after drilling, or after a short production period, thawed permafrost and waterbase fluids outside the casing will refreeze and generate inward radial loads around the wellbore. This process, referred to as "external freezeback", can produce significant collapse pressures.
The difference between the external freezeback pressure and the internal fluid pressure must not exceed the casing collapse pressure. This problem can be overcome in three ways:
- modify drilling practices to limit permafrost thaw and freezeback;
- cement any annulus within the permafrost zone;
- if not cemented, ensure the annulus is displaced to high salinity waterbased fluid or non-freezing fluid.
Axial strains resulting from permafrost thaw subsidence
Experience indicates that stove pipe, conductor casing and surface casing failures may occur as a result of thaw-subsidence generated compressive and tensile loads. As the pore ice melts and reduces in volume, a decrease in pore pressure results. This in turn leads to an increase in intergrannular stress, and soil compaction takes place. This compaction is not reversible during external freezeback.
Permafrost lithology determines the nature of loads induced by such thaw subsidence and field tests indicate the presence of both compressive and tensile loads within a string - depending on depth and formation.
Strain, not stress, is the governing criterion for axial load design of permafrost casing. As a result, it is the casing strain limit, not the yield stress, which is important for casing design. Since the movement is gradually imposed, and also limited, large post-yield pipe body strains can be sustained without catastrophic failure.