Wells in a Compacting Field

Part 10, Part 10: The Producing Field

Learning objectives

Explain how reservoir compaction loads the wells that pass through it: axial compression inside, shear at boundaries
Compute the axial strain and casing stress from compaction and see it survive uniform compaction
Identify the failure hot spots: the reservoir top and bottom and shale-sand slip planes, worse in deviated wells
State the mitigations: high-grade casing, deviation and orientation, compliant completions, and monitoring

The Well Feels the Compaction

The subsidence bowl was the surface feeling the compaction; the wells feel it too, more directly, because they pass straight through the compacting interval. Compaction loads a well two ways. Inside the reservoir the shrinking rock drags the casing into axial compression, shortening it with the formation. At the reservoir's top and bottom, and at any shale-sand bedding plane, the deformation is discontinuous, and the casing is sheared where the rock on one side moves relative to the other.

The uniform axial load is usually survivable. The compaction strain, $\alpha\,\Delta P_p / M$ p/M, is small, a few hundredths of a percent even for a heavy depletion, and the axial stress it puts on the casing, steel's modulus times that strain, is around 100 MPa, comfortably below the yield of a standard casing grade. Wells rarely fail from uniform compaction. They fail at the boundaries, where the deformation localizes.

Where Casing Shears

The failure hot spots are geometric. The reservoir top and bottom concentrate shear because the compaction goes from full inside to zero outside over a short interval. Shale-sand interfaces above the reservoir slip along bedding as the section shortens, and a casing cemented rigidly across such a plane is sheared as the plane moves. And deviated wells fare worse than vertical ones: a well crossing the compacting interval at an angle presents more length to the shear and bends as well as shears. The most common compaction casing failures in the North Sea and California fields sit exactly at these boundaries, not in the reservoir body. Increase the depletion and the deviation and watch the shear offsets grow. The mitigations follow from the mechanism: run high-grade, high-collapse casing through the compacting interval; keep wells as vertical as practical through it, or orient deviated wells to spread the shear; use compliant completions that can take some strain without shearing; and monitor with casing-deformation logs and repeated calipers to catch the shear before it parts the casing.

The Cost of Compaction

Casing shear is expensive: a sheared well loses the interval below the shear, can lose the whole hole, and is often unrecoverable, forcing a sidetrack or a replacement. In a heavily compacting field it is a chronic, budgeted cost, and it is a geomechanics cost, set by the same modulus and depletion that drive the subsidence bowl. The producing field's stress path sinks the surface and shears the wells; the fourth face of the same compaction is the time-lapse seismic signal it writes, which the next section reads to watch the whole process from the surface.

References

Dusseault, M. B., Bruno, M. S., & Barrera, J. (2001). Casing shear: causes, cases, cures. SPE Drilling & Completion, 16(2), 98-107.
Bruno, M. S. (1992). Subsidence-induced well failure. SPE Drilling Engineering, 7(2), 148-152.
Zoback, M. D. (2007). Reservoir Geomechanics (ch. 12). Cambridge University Press.