When a casing or tubing string is subjected to axial compression it may form a buckle that can impair well access due to increased local well curvatures.
The amplitude (amount of curvature) and shape of the buckle depends on:
- the amount of compression and;
- how much lateral constraint is provided by the well cement and surrounding formations.
If there is no lateral constraint, a global buckle (also called an Euler buckle) can occur that has the appearance of a gradual bow in the casing in one plane. This can occur where an interval of casing is uncemented or with a tubing string inside casing.
These buckles can occur before the compressive loads exceed the yield strength of the casing. In such instances, these buckles are elastic and therefore can “spring back” if the compressive loading is removed.
If the global buckle progresses to the point of contacting the surrounding formation or casing, the casing can yield and the buckle may transition to a multi-wave buckle. These buckles still occur more-or-less in the same plane but have a shorter wave length than the initial global buckle. If axial compression continues to increase the buckle may transition to a helical (corkscrew) mode.
If the pipe is fully constrained by the well cement, buckling is not likely to occur. Small amplitude buckling can be observed where there is only a micro-annulus around a cemented casing, but this does not typically affect well access as the buckle amplitude is so small.
Therefore, buckling deformations indicate that there are two conditions occurring:
- Increased axial compressive loading on the casing, and
- Lack of lateral support for the casing or tubing.
These conditions can occur in situations where subsidence of the overburden occurs caused by the following mechanisms:
- Pore pressure reduction
- Shallow water flow
- Solids production
- Permafrost thaw
Buckling caused by pore pressure reduction
Ongoing production of oil or gas reduces the pore pressure in the producing formation causing more of the overburden weight to be carried by the reservoir rock (higher effective stress). The increased effective stress can cause compaction of the reservoir, especially in soft sediments such as chalk and diatomite where several metres of “reservoir shortening” can occur over time.
Buckling caused by shallow water flow
In offshore wells near large river deltas where rapid sedimentation occurs, alternating deposits of loose sands and low permeability clays and silts can trap pore pressure in the sand layers.
As more sediment accumulates, the trapped pore pressure continues to increase, causing the uncompacted sand to become overpressured compared to the normal hydrostatic gradient. When these shallow, overpressured layers are penetrated while drilling, the overpressure can cause the formation to flow sand and water into the well.
If this flow is not controlled, massive quantities of sand and water can be produced. The combined effect of removing the sand and reducing the pore pressure in the remaining sand causes the originally overpressured zone to compact and the overlying materials to subside.
This is not necessarily a problem for the well being drilled as no casing is in place to deform but the compaction may be enough to impact offset conductor pipe and wells that have already been drilled on a subsea template. This subsidence can result in global and helical buckling of the casing as well as shear in the existing wells.
Buckling caused by solids production
In heavy oil operations in weak formations such as in Canada’s oil sands, formation sand can be produced continuously with the oil over the entire life of the well. This process is usually referred to as Cold Heavy Oil Production with Sand (CHOPS).
Some of the most prolific CHOPS wells have produced over 1,000 m3 of sand from a pay zone that is only 10 m thick. Various theories exist on how the sand is released from the formation and where the sand comes from.
One theory suggests that sand production forms multiple, small diameter “wormholes” that are driven primarily by pore pressure gradients in the formation. Alternatively, analysis of well deformations in several mature CHOPS wells suggests that the sand may be produced from an elongated disturbed zone where the growth of the zone is controlled by in-situ stresses and shear failure of the sand.
Regardless of the mechanism of sand production, the effect of removing sand from around the wellbore has the combined effect of allowing overburden subsidence and removing lateral support for the casing, resulting in large amplitude global buckling. These buckles can be large enough to cause bending fatigue failures in downhole progressing cavity pumping systems and impaired well access.
Solids production related deformations have also occurred during aggressive drawdown in gas wells in weak sand reservoirs. Again, the effect of removing the sand causes overburden subsidence and provides a cavity where a large global buckle can occur.
Buckling caused by heating
In deep wells where the producing formation is significantly hotter than the formations near surface, the casing and tubing temperature may increase significantly as the produced fluid travels to surface.
If the pipe is uncemented and is free to move, it will expand as it warms, increasing in radius as well as length.
If the pipe is completely constrained axially and laterally by cement, this increase in temperature will result in an increase in axial compressive stress in the casing. It cannot get longer as it warms.
If the pipe is only partially constrained, or only constrained over some intervals, varying degrees of elongation and stress increases can occur at intervals along the wellbore.
The result is buckling where the cement does not provide lateral support for the casing. In mild cases of buckling, the stresses may not exceed the yield strength of the casing material allowing the buckles to relax if fluid production stops and the well cools down. Buckles that cause trouble during production may not be apparent if the well is logged after being off production for some time.
Buckling caused by permafrost thaw
In northern regions, wells may penetrate several hundred metres of permanently frozen sediments, generally termed permafrost. When the warm fluids are produced from the underlying formations, heat loss through the wellbore can melt the permafrost near the wellbore.
In some soil types, such as fine silts, the frozen soil may contain excess water that causes the soil to lose strength and compact significantly when it thaws. This can cause localized compression of the casing in the intervals that compact and tension in adjoining intervals that do not compact.
Over the full permafrost interval, the net effect of the melting permafrost can be surface subsidence. The result is severe compressive loading of the well casing. The melting permafrost also reduces or removes the lateral support for the casing that, combined with the compressive loading, can result in global buckling of the casing.