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With baseline data, team will look for change

The Aquistore project is laid out in a grid of instrumentation about six square kilometres above the injection site.
Aquitore
Ben Rostron made a presentation at last week's open house to talk more Aquistore.

The Aquistore project is laid out in a grid of instrumentation about six square kilometres above the injection site.

Located just west of Boundary Dam Power Station and the carbon capture unit that will be sending some carbon dioxide to the Aquistore’s demonstration site next year, the project is ripe with monitoring equipment listening to the delicate and natural fluctuations about 3.4 kilometres below the earth as well as on the surface.

During an open house at the Saskatchewan Energy Training Institute last week, members of the project’s science and engineering research committee (SERC) explained what sort of baseline data they’ve gathered over the last two years and how that will help them tell the story of what happens in and around the deep saline aquifer after they begin injecting CO2 into the sandstone formation.

The monitoring systems include soil, gas, groundwater, seismic and surface deformation.

The team has a group of five industry experts, one in each area, that make up the SERC. This team has been meeting regularly for many years. Chris Hawkes, associate professor of civil and geological engineering at the University of Saskatchewan, has been involved since the Petroleum Technology Research Centre started the project in 2009.

Two wells were drilled for the project, an injection well 3.4 km deep and an observation well 150 metres away. Once CO2 is injected, the team expects it to take weeks, and hopefully not months, to migrate that distance over to the observation site.

“There is a bunch of monitoring activity that is specific to watching for a breakthrough, the arrival of CO2, at the observation well. There are samples being collected at the observation well continuously while we’re looking for that CO2 to arrive,” said Hawkes.

He said the data collection effort following that first injection will be an intense one as they wait for the migration to reach its first milestone.

“We’ve had baselines for about two years now,” said project manager Kyle Worth. “A lot of the technology is looking at the difference, and then you try to determine whether that difference is associated with the CO2 or not.”

Regarding seismic monitoring, Hawkes said the equipment does a few different things for the research team. They have been passively monitoring the site for awhile, they have already mapped the disposition and properties of the rocks below the site. That gives them the baseline information, a picture of what normal conditions are.

“The neat thing is that introducing a different fluid like CO2 into a rock, that changes its acoustic properties, so the reflecting acoustic signal will actually change as CO2 propagates throughout the rock we’re injecting into,” said Hawkes. “One of the useful outcomes of the seismic monitoring program is to allow us to track the movement of the CO2 in the subsurface.”

Seismic monitoring will occur from three seismograph stations along with geophones spread over a 2.5 km by 2.5 km grid around the site. The finer sounds will be picked up more sensitive equipment attached to the observation well.

An event of -1 on their scale is equal to that of someone firing a rifle, -2 is equal to dropping a dictionary on the floor, and -3 is equal to snapping a stick.

“That is the kind of thing that, logically, we would expect to see here,” said Hawkes, who noted those negative events have been monitored in the Weyburn and Midale oilfields where enhanced oil recovery has been ongoing for more than a decade.

While geophones are typically attached to stakes for temporary seismic monitoring, these ones have been set about 20 metres deep in the ground for a much longer period of time.

“One of the upsides of that is better quality data because it gets them away from the noise of the ground surface, and it means that when you come back and do a survey one year later, what you’re looking for is how that acoustic signal has changed. If the same geophones are in the same ground at the same spot, that means any change is going to be a reflection of what’s really changing in the subsurface,” noted Hawkes.

The team will run seismic surveys annually, returning to the site once CO2 injection begins to see what’s happening below the ground.

Regarding the seismic response, Hawkes said they don’t expect to detect any change.

For monitoring surface deformation or ground surface uplift, the PTRC uses a Differential Interferometric Synthetic Aperture Radar (DINSAR) satellite orbiting Earth to send a signal to the surface, and records the time it takes the signal to return to the satellite. A longer or shorter time of travel will determine if the ground in the area has raised or lowered, and it will detect changes to the millimetre.

Hawkes said they have found the ground at the Aquistore site has been moving upward very slowly.

“It has nothing to do with CO2, it’s just the fact that this part of the world used to be covered with glaciers,” he said. “Even though those glaciers are long gone, the surface is rebounding.”

That raise in elevation they would expect to be consistent, and after injecting CO2, what they would be looking for is any deviation from the pattern they’ve already seen.

With data from 2012 to 2014, the PTRC has lots of information to identify what is normal for the ground and below the ground at the Aquistore site. Once CO2 is injected for the first time, they will see exactly what that means for the rock formations below the surface, the soil and water above it, and whether or not this is a feasible storage option for countries looking to extend the life of coal power by capturing its pollutants.