Keeping a Lid on It
Caprock is key to ensuring fluids remain underground
If you were to look at a cross section of the subsurface geology, it might resemble a multi-layered cake. But instead of icing and chocolate, you’d find mudstone, silt, and sand layers—sand layers that may be infused with water, gas, and bitumen.
So what keeps the mixture underground?
“Caprock is what keeps everything in the subsurface,” says Felix Chiang, a reservoir engineer with the AER. “It acts as a barrier to ensure that fluids are contained within the reservoir.”
It’s usually made up of fine-grained bedrock of clay and silt.
But in the shallow part of the Wabiskaw-McMurray deposit of Alberta’s Athabasca Oil Sands Area, the caprock in the Clearwater Formation may be partially or completely eroded at depths shallower than 150 metres. This increases the risk that steam and heated reservoir fluids would not be contained and steam may flow to surface when steam-assisted gravity drainage (SAGD) in situ methods are used.
“There was a major incident in 2006, which lasted only a few minutes but caused a significant surface disturbance,” says Chiang.
Soon after that incident, the AER implemented changes to its application processes and requirements, based on preliminary findings from the incident investigation. The AER also began an extensive and complex technical review of the science related to reservoir containment. This included creating five technical reports that analyzed reservoir containment from a number of different perspectives.
Engaging the experts
The AER also engaged with a number of stakeholders on the science behind reservoir containment, including academics, environmental nongovernment organizations, indigenous communities, municipalities, and industry.
“Developing a directive isn’t an overnight affair,” says Chiang. “After studying the science in the reports, we created a draft directive, which was open for public feedback for months. We also engaged with stakeholders and incorporated their feedback into the current directive.”
On December 16, 2016, the AER released Directive 086: Reservoir Containment Application Requirements for Steam-Assisted Gravity Drainage Projects in the Shallow Athabasca Oil Sands Area. The new requirements took effect immediately and specify caprock criteria, requirements for determining a maximum operating pressure, requirements for geological information, requirements for a risk assessment and management plan, a requirement for a monitoring plan, and other application information requirements.
While this directive will apply to all new applications, the AER has evaluated all existing steam-assisted gravity drainage projects in the area and has determined that they all operate within these parameters.