Pressures under the earth, temperature changes, ground movement from constructing nearby wells, and shrinkage crack and damage the thin layer of brittle cement that seals the wells. And getting the cement right as drilling goes horizontal is extremely challenging. Meanwhile, once the cement leaks, attempting to repair it thousands of feet underground is expensive and often unsuccessful. Even if successfully repaired, methane migration might have been occurring for months or years.

Some recent modifications to cementing regulations misguidedly include requirements on cement strength. But it is not a question of stronger cement or better technology. Industry's own documents say that:

"strength is not the major issue in oil well cementing under any circumstances … cement clearly cannot resist the shear that is the most common reason for oil well distortion and rupture during active production."

In other words, the high stresses and rock movements deep underground will cause a significant proportion of wells to fail no matter what.

Other industry documents show that well failure is a widespread problem around the world, that abandoned wells are a major migration pathway to aquifers, and that there are multiple scenarios by which gas and other contaminants can escape a well to contaminate water supplies.

Industry also asserts that the gas reservoirs they target are thousands of feet deeper than water supply aquifers. Therefore, industry officials claim there is no way the water supply could have been contaminated by their operations. But the cement barrier around the wells can fail from many causes, or be absent, allowing gas to migrate out of the well and into the drinking supply.