Seeking solutions to mining the Foord seam (May 25, 1992)

A methane explosion ripped through the Westray coal mine at Plymouth, N.S., just one week after it received the Canadian Institute of Mining, Metallurgy and Petroleum’s Ryan Safety Award. In the days that followed the May 9 blast, 15 bodies were brought to surface. Eleven more men remain underground and are presumed dead. The mine is owned by Curragh Resources (TSE).

How could such a disaster happen? Was it failure of technology, negligence, an error in judgment or simply an accident, an unforeseen act of nature? And, beyond Justice Peter Richard’s judicial inquiry into the tragedy, can the Foord coal seam which hosts the Westray mine be safely and economically exploited?

Coal mining has always been a hazardous occupation, and its popular image might color at least part of the inquiry’s proceedings. That image has largely been shaped by novels such as A.J. Cronin’s The Stars Look Down and films such as John Ford’s How Green Was My Valley. These novelists and filmmakers have rarely been at a loss to exploit the drama of coal mine fire and flood, but they do not accurately reflect modern mining practices. The Ryan award is offered annually to the mine achieving the lowest accident frequency. (In Westray’s case, the accidents that did occur up to May 9 were not serious in nature.) Westray was equipped with the latest electrical technology, including sophisticated gas detection devices, and was recognized as one of the country’s most modern coal mines.

The Foord coal seam, which Westray was mining at the time of the explosion, is one of the richer coal deposits in Nova Scotia. Up to 9.5 metres thick, it has a high heating value and is low in sulphur (no acid rain complications), and the reserves are exclusively underland (Cape Breton’s reserves are undersea).

It is also a seam high in methane gas and there has been the grim price of 178 fatalities to pay for working its coal.

Some of those directly linked to the disaster have urged that the mine be closed forever — a natural human response to tragedy. But there may be ways the mine can be redesigned to overcome safety concerns.

All coals contain more or less gas. Methane forms from the same vegetable matter that produces coal. Seams close to surface may lose most of their gas to the atmosphere over geological time but deeper seams tell a different story. Much of the deep-mined coal in the U.S. carries 19 cubic metres or more of methane per tonne of coal; the volume of gas is 15 times that of the coal mined.

Elsewhere in the world, gas volumes greater than 170 times that of the coal are known. It has not been publicized how Westray coal ranks but anecdotal evidence points to a high figure. Technical data on the methane content will become available as the inquiry proceeds.

Several coalfields have investigated methane drainage for years. Some of the work has been experimental in nature but other mines have developed the technology to the point they produce not only coal but also marketable methane. The undiluted methane is tapped at the coal face, piped to surface, cleaned and distributed to consumers as natural gas.

The gas is usually contained in the pores of the coal and frequently in the surrounding rocks. It is under pressure, bleeds off naturally from the freshly cut coal face and passes into the mine atmosphere. The mine’s ventilating requirements are determined (plus other considerations) on the basis of the expected discharge of methane from the coal face. To the calculated volume of fresh air is added an excess to dilute the gas as quickly as it is released and consequently make it harmless. Up to 5% methane in the air can be tolerated (it is “safe”) but between 5% and 15% it forms an explosive and readily ignitable mixture. This then might have been the condition of the underground atmosphere when the explosion struck Westray. An electrical spark or one caused by stone striking steel could have been sufficient to cause detonation.

Automatic gas monitors at Westray were calibrated to sound a general alarm whenever the mine atmosphere registered more than 0.25% methane. Because no alarm was triggered, it must be assumed the buildup of gas was so rapid there was no time for the system to respond. In other words, an active coal face broke into a reservoir of pressurized gas, or, into a geological structure leading into such a reservoir. There was a “blow-out.”

Gas and oil reservoir structures, or traps, are the stock-in-trade of petroleum geologists and it follows that if there is to be realism and responsibility in the reopening of Westray, these professionals must be involved to determine where these reservoirs are prone to occur. Are they above the seam, are they below, are they confined to the seam itself, or are they all three possibilities?

There may already be a substantial literature on the subject from other coalfields and this will give valuable pointers. And this work would be followed by drilling to confirm where these structures form.