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An oil field in North Dakota. (Deborah Baic/The Globe and Mail)
An oil field in North Dakota. (Deborah Baic/The Globe and Mail)

COLLECTED WISDOM

All that drilling, yet there’s no (sink)hole Add to ...

Hi there. They call me Crude Wildcat – C.W. for short – the roughest roughneck of them all, and I’m going to drill for oil in your backyard. But fear not, your house won’t disappear into a huge sinkhole. Well, probably not.

The question

Since enormous quantities of oil are being pumped out of the ground, why is the ground not collapsing or sinking? Paul Niedermayr of Belleville, Ont., wants to know.

The answer

“In fact, in some rare cases, land subsidence does occur when oil is removed,” writes Murray Gingras, a professor in the Department of Earth and Atmospheric Science at the University of Alberta in Edmonton.

“In North America, the best-known examples include the Goose Creek Field in Texas and the Wilmington Field in Long Beach, Calif.” At Long Beach, he says, subsidence was nearly nine metres at its maximum (in the centre of the oil field). The ground started sinking in the early 1940s and continued into the early 1960s. “The subsidence in these locales,” he writes, “occurred because the oil reservoirs were quite shallow and the reservoir strata were composed of a comparably soft rock, almost like loose sediment.”

In these cases, Dr. Gingras explains, when the oil was removed, fluid pressure within the rock was lowered and the rock volume literally shrank. “In California, the solution was to start pumping water into the reservoir so that fluid pressures would remain stable.” The subsidence was thus halted.

In most oil reservoirs, he says, the porous rock is deeply buried, much more rigid and not prone to serious compaction. Moreover, when the oil is removed, groundwater flows into the space that was occupied by the oil. “As such, subsidence is normally so small that it is not measurable.”

He adds that subsidence can also occur when groundwater is removed from aquifers. “In fact, that is probably more common. For examples there are Mexico City and several areas in Texas, Arizona, Nevada and California,” he writes. “Again, this happens only when the aquifer is shallow and the rock is not rigid.”

The question

How did the Romans do mathematical calculations using Roman numerals? William Clark of North Vancouver wants to know.

The answer

“As far as I’m aware,” writes Aaron Thomson of Chesley, Ont., “Roman numerals weren’t actually used in calculations in the modern sense (as in add five carry the one). Calculations were done on abacuses.”

Indeed so. And CW has learned that stones (called calculi) were originally used on Roman abacuses instead of beads. Hence the word “calculus.”

However, it is possible to do some simple math using Roman numerals. For instance: XV plus XII equals XXVII. You just take the X’s the V and the I’s and shove them all together. Finding the square root of, say, CCLVI is another matter. (In case you’re wondering, it’s XVI.) If anyone has more mathematical musings on this subject, we’d love to hear from you. Send us your e-mails by Nov. XXVIII, MMXII.

Help wanted

Ross Towler of Halifax wonders why Robert Baden-Powell, who was English, chose the French symbol of the fleur-de-lys as the emblem of the Boy Scouts.

Why are some boiled eggs very easy to peel and others impossible? asks Raymond Wiens of Langley, B.C.

“In Britain, the Royal Mail delivers what the Brits call the post,” writes Bruce Little of Toronto. “Here, Canada Post delivers what we call the mail. What’s the etymology of this terminological flip-flop?”

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