Four Failures That Built a Discipline

The Platform That Sank

In 1984, a routine survey in the Norwegian North Sea found something unsettling: the seabed under the Ekofisk platforms was meters lower than when the field was built. The reservoir is a chalk, a rock that is mostly empty space held up by pore pressure, and a decade of production had transferred that load onto a framework too weak to carry it. The chalk compacted, the overburden followed it down, and by 1987 the operators were jacking entire platforms six meters higher to keep the decks above the waves. Water injection was expected to stop the compaction; instead the seawater weakened the chalk further, a humbling lesson in coupled chemistry and mechanics, and total seafloor subsidence went on to approach nine meters. Nobody had priced any of this. The bill ran to billions.

The City That Sank Earlier

Ekofisk had a precedent nobody re-read in time. The Wilmington field under Long Beach, California produced from shallow, unconsolidated sands beginning in the 1930s, and by the 1950s the harbor district above it had subsided up to nine meters, flooding docks and shearing hundreds of well casings until the city was repressurizing the reservoir with injected water just to hold the ground still. Same physics, thirty years earlier: drop the pore pressure, raise the effective stress, and a weak rock compacts, as Part 10 will let you compute. The two bookends of the case list are faster stories. A well whose pore pressure and fracture resistance converge with depth runs out of usable mud weights, takes a kick or loses returns, and is cemented and abandoned: the closing window of Part 6. And beginning around 2009, wastewater injection in Oklahoma raised pore pressures on basement faults that had been quiet through recorded history; the state went from a couple of felt earthquakes a year to hundreds, capped by a magnitude 5.8 near Pawnee in 2016. Part 9 computes exactly how little pressure that takes.

One Diagnosis, Four Charts

Lay the four failures side by side and they are one disease with four presentations: a stress state nobody had measured crossed a strength nobody had checked. Compaction is effective stress exceeding the yield of a weak frame. Casing shear is the strain field of a compacting reservoir finding the stiffest thing passing through it. A lost well is a mud pressure wandering outside a window nobody had drawn. Induced earthquakes are the friction line of Part 3, reached by a pore-pressure pathway of Part 4, on a fault of Part 9. The rest of this course is the checklist those projects did not have, and the next section draws its map.

References

• Sulak, R. M. (1991). Ekofisk field: The first 20 years. Journal of Petroleum Technology, 43(10), 1265-1271.
• Nagel, N. B. (2001). Compaction and subsidence issues within the petroleum industry: From Wilmington to Ekofisk and beyond. Physics and Chemistry of the Earth, Part A, 26(1-2), 3-14.
• Ellsworth, W. L. (2013). Injection-induced earthquakes. Science, 341(6142), 1225942.