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Shell set to plug leak that created pond at oilsands mine

Never-seen-before problem shows importance of additional seismic work in areas earmarked for mining

By Dave Cooper, edmontonjournal.com October 14, 2011

The flooded pit at Shell’s Muskeg River mine now holds about seven million cubic metres of salty water after a deep crack formed in the rock below the mined-out area last year, allowing water from a deep aquifer to flow upwards. It was the first time an oilsands firm has faced such a situation. Shell is building a drilling pad in the pond and will inject hot asphalt and then cement into the crack to permanently seal the leak next year. Photograph by: Shell Canada, edmontonjournal.com

EDMONTON – When water started appearing at the bottom of a Muskeg River mine pit north of Fort McMurray last October, crews assumed it was normal seepage from surrounding rock.

But it quickly became clear that this was something different — the water was not slowly rising from the basal aquifer, but flowing in under pressure, bubbling up from the bottom of the pit. It was salty, and it stank of rotten eggs, thanks to low levels of hydrogen sulphide.

So it was clearly coming from a deeper aquifer, and that meant it needed to be patched.

But how to do it?

Shell Canada tested the site to learn more about its geology and has recently come up with an innovative plan to permanently seal the crack in the floor of the mined-out pit, named cell 2A. It also has a way to deal with the seven million cubic metres (seven billion litres) of salty water now sitting in the former mine pit — a deep pond that is still growing at 200 cubic metres (200,000 litres) per hour.

“This situation in cell 2A was unexpected and something that has never happened to any oilsands mine before. But what we have now learned is going to change the way we operate, and I think the other firms will be doing the same,” said John Rhind, vice-president of heavy-oil operations for Shell Canada Energy, the operator and majority owner of the Albian Sands project.

And that means doing additional seismic work throughout areas that are planned for mining, so geologists can detect weak areas in the underlying limestone — the 150-metre-thick rock that lies above the deep saline aquifer that is the source of the water in cell 2A.

In this area of the Muskeg River mine, Shell had removed 40 metres of overburden and up to 70 metres of oilsands. Crews were cleaning out the bottom of the cell, down to the limestone base, when the water began gushing in.

“We immediately got the heavy equipment out of there. We had already started to build this cell to hold tailings, so we continued to build up the berms” to contain the salty water, Rhind said.

Water initially gushed in at 2,000 cubic metres (two million litres) per hour.

Geologists know the aquifer under cell 2A originates in Saskatchewan, where fresh water enters the ground and becomes salty as it moves through the salt-rich layer of porous rock. The aquifer eventually seeps into the Athabasca River.

Shell estimates a five-metre-long crack that snakes up through the limestone is the source of the problem.

The firm is currently filling in a small portion of the pond above the leak, dumping sand over a layer of rip-rap (rubble to allow drainage from the leak to continue) to create a base for a drill rig.

“We are going to drill holes from this pad that we are creating, which will allow us to take core samples, and also be a way to inject sealant.”

Shell considered using a floating drilling barge, but if the hydrogen sulphide gas returned it would be a safety hazard for the crew.

“The pad is the safest approach,” he said.

Shell brought in its experts from around the world, people with experience in the Gulf of Mexico and the North Sea, and scientists from Houston and Amsterdam to study the problem.

Normal cement injection won’t work because of the incoming water flow, so Shell intends to inject a hot asphalt material to create a temporary seal. Then grouting cement will be pumped down to make the seal permanent.

Drilling should be completed by January, and Rhind figures it will take another 10 months to complete the sealing process.

Dealing with the remaining water is a simpler problem. Shell can’t use the salty water in its processes, but another oil firm may be able to pipe it to its facility.

Perhaps the easiest solution is to slowly add dry sand from the tailings handling process.

“Dry sand would slowly absorb the water. There would be about 30 per cent water in the sand, and other tailings areas receive wetter sand,” Rhind said.

Reclamation could then proceed as normal, “and as our aboriginal neighbours tell us, the real architects of the land, the beaver, will come in and finish off the landscape,” he adds.

The Energy Resources Conservation Board is closely following the Shell project, spokesman Bob Curran said.

“We believe what Shell is doing is appropriate,” he said.

He could not comment on any moves to ensure all firms do full seismic work at future mine sites to detect any weakness in the limestone cap rock which overlies the deep aquifer.

But Rhind says Shell is sharing all its seismic data with its competitors, Suncor, Syncrude, Canadian Natural and Esso.

“They are happy. Everybody in the industry will learn what we have learned,” he said.

“And from this point forward, we are doing the extra seismic at Muskeg and our new Jackpine Mine so we know what we are dealing with under the oilsands.”

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The Muskeg River Mine Leak

– In October 2010, a five-metre crack developed in a weak area of the 150-metre thick limestone which caps a salty aquifer.

– After mining was completed, but before tailings were added to cell 2A, up to 2,000 cubic metres per hour of salty water flowed in through this crack.

– Containment walls were heightened as the cell filled, but the pressure of the new pond slowed the inflow to just 200 cubic metres per hour.

– Shell is now building a pad in the pond to support a rig which will drill three holes around and through the fracture to understand the geology.

– In January, Shell will begin pumping hot asphalt down these holes to create a temporary seal. Then grouting cement will be pumped in to make the seal permanent by the end of 2012.

– Dry sand will then be added to soak up the salty water, and normal land reclamation will proceed.

© Copyright (c) The Edmonton Journal

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