How Does Carbon Capture and Sequestration
Differ from Other Subsurface Operations?
Three common subsurface processes are often confused as
one and the same: hydraulic fracturing (also known as fracking);
wastewater injection, and carbon capture and sequestration (CCS).
But each operation is different, done in different ways, and for
different purposes.
•
Hydraulic fracturing is a means of stimulating oil and gas wells
by pumping specific fluids at high pressure and rate to create
new and open existing fractures, which increases production.
•
Wastewater injection is used to safely return excess water
from subsurface activities by pumping it back into the ground.
•
CCS captures and permanently stores carbon dioxide (CO2)
from sources such as fossil fuel power plants, to minimize
greenhouse gas emissions and their impact on the climate.
Each of these operations is governed by different organizations
and regulations, is carried out at varying depths, pressures, and
in different rock formations, and last for different lengths of time.
comes, the injection operation is adjusted for local conditions to ensure
safe injection.
There is a large difference between the detail of the site
characterization for CCS wells and the much lower level of
characterization required by regulators for wastewater disposal
wells. Much of the effort spent on CCS wells lies with the fact that it is
important to know where the CO2 is within the reservoir and where it
may move over the life of the project, possibly tens to hundreds of years.
As a result, having a complete understanding of the reservoir is crucial.
One other major difference with CCS projects is that the selected site is
continually monitored before and during injection, and then for decades
after injection ceases.
These three underground operations, then, are quite different from
each other. CCS is the most regulated, the most carefully planned,
constructed, and monitored technique of them all.
A hydraulic fracturing operation itself only lasts a few days, creating
better fluid paths within the formation for the oil or gas to flow back
to the well and then safely brought to the surface. Often, grains of
sand are mixed with the treatment fluid to keep the fracture open
after the treatment is complete. Its effects on oil and gas production
are intended to last through the life of the well (decades).
Wastewater injection pumps treated water from oil and gas
production into the ground (well below fresh water) for permanent
disposal. The fluid is injected into a disposal zone, usually between
impermeable layers of rock, which contain any seepage out of
the storage reservoir. Water disposal wells are designed and
constructed to meet regulatory standards which assure that
they are properly built, operated, maintained, and eventually
permanently plugged. These wells operate with minimal monitoring
and reporting requirements.
Just as oil and natural gas have been stored safely in the ground
for millions of years, the sequestration of CO2 storage is viewed as
permanent. Proper geology is the key to choosing a site for the geologic
sequestration of CO2. This includes making sure that the reservoir can
effectively hold large volumes of CO2 as well as reviewing the integrity
of the reservoir to ensure that there are no potential leakage pathways.
The construction of each well is tailored to the site, the monitoring
program is designed specifically for the project and, once the time
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