9
fracking would introduce large volumes of toxic chemicals underground, which
could then rise up from below and contaminate vast tracts of aquifers that supply
drinking water to thousands of people. This seemed logical to many, because fracking takes place underground, and well water comes from underground, so surely
there was a risk that one would contaminate the other.
The facts are that “underground” is a big place. Aquifers that supply drinking
water to domestic wells are usually quite shallow, in most cases less than a thousand
feet (300 m) deep. There is a practical reason for this – groundwater gets recharged
from rainfall and snowmelt on the surface infiltrating downward into the rocks. This
input allows the shallow groundwater to remain fresh and drinkable. Deeper aquifers receive little to none of this freshwater input, and groundwater becomes increasingly salty with depth. Water with salt contents above 10 parts per thousand is
considered brackish and undrinkable. Seawater contains about 35 parts per thousand dissolved salt, and the brines recovered at the depths of gas shales are typically
six to ten times saltier than seawater (Hayes 2009; Cozzarelli et al. 2017).
Fracking of shales is done far below the base of the fresh groundwater aquifers.
Because of the weight of overburden on these deep rocks, fractures form vertically
and tend to grow horizontally outward in the direction of maximum principle stress.
They will also grow upward for some distance until encountering a “frack barrier,”
which is an overlying layer of rock with physical properties that are very different
from the target shale, such as a limestone. These natural barriers are included in the
design of most frack jobs to constrain the growth of the fracture. Controlling the
upward growth of the fracture is desirable to keep most of it contained within the
productive zone of the shale.
Data on the upward growth of hydraulic fractures has been collected using
microseismic monitoring techniques designed to record the motion of rocks breaking from the frack, and triangulate their location very accurately (Warpinski 2013).
The data in Fig. 1.3 from the Marcellus Shale clearly show that the tops of the
hydraulic fractures are thousands of feet (hundreds of meters) below the drinking
water aquifers, shown in the solid shade at the top of the chart (Fisher and
Warpinski 2012).
The volume of fluid pumped downhole to create a hydraulic fracture is literally
not enough to grow that fracture to the surface. Field studies using tracer chemicals
in the frack fluid along with geochemical and microseismic monitoring support the
notion that fluids simply do not migrate vertically upward any great distance against
gravity and pressure gradients except in very special or deliberate circumstances.
(Hammack et al. 2014).
Once a fracked shale well begins production, the pressure gradient and direction
of flow are from the formation into the well, and not upward toward shallow aquifers. There are potential leakage points for stray gas to enter the groundwater if
there are wellbore integrity problems, which do seem to be more frequently associated with fracked wells. Several studies on stray gas have been published with contradictory results, however, and the jury is still out. There is also a rare (but not zero)
potential for high-pressure gas to migrate to the surface through a pre-existing conduit like an abandoned well. Neither of these events are very common, and fears of
pervasive gas migration from below are largely a perceived risk.
1.2 Perceptions of Risk
fracking would introduce large volumes of toxic chemicals underground, which
could then rise up from below and contaminate vast tracts of aquifers that supply
drinking water to thousands of people. This seemed logical to many, because fracking takes place underground, and well water comes from underground, so surely
there was a risk that one would contaminate the other.
The facts are that “underground” is a big place. Aquifers that supply drinking
water to domestic wells are usually quite shallow, in most cases less than a thousand
feet (300 m) deep. There is a practical reason for this – groundwater gets recharged
from rainfall and snowmelt on the surface infiltrating downward into the rocks. This
input allows the shallow groundwater to remain fresh and drinkable. Deeper aquifers receive little to none of this freshwater input, and groundwater becomes increasingly salty with depth. Water with salt contents above 10 parts per thousand is
considered brackish and undrinkable. Seawater contains about 35 parts per thousand dissolved salt, and the brines recovered at the depths of gas shales are typically
six to ten times saltier than seawater (Hayes 2009; Cozzarelli et al. 2017).
Fracking of shales is done far below the base of the fresh groundwater aquifers.
Because of the weight of overburden on these deep rocks, fractures form vertically
and tend to grow horizontally outward in the direction of maximum principle stress.
They will also grow upward for some distance until encountering a “frack barrier,”
which is an overlying layer of rock with physical properties that are very different
from the target shale, such as a limestone. These natural barriers are included in the
design of most frack jobs to constrain the growth of the fracture. Controlling the
upward growth of the fracture is desirable to keep most of it contained within the
productive zone of the shale.
Data on the upward growth of hydraulic fractures has been collected using
microseismic monitoring techniques designed to record the motion of rocks breaking from the frack, and triangulate their location very accurately (Warpinski 2013).
The data in Fig. 1.3 from the Marcellus Shale clearly show that the tops of the
hydraulic fractures are thousands of feet (hundreds of meters) below the drinking
water aquifers, shown in the solid shade at the top of the chart (Fisher and
Warpinski 2012).
The volume of fluid pumped downhole to create a hydraulic fracture is literally
not enough to grow that fracture to the surface. Field studies using tracer chemicals
in the frack fluid along with geochemical and microseismic monitoring support the
notion that fluids simply do not migrate vertically upward any great distance against
gravity and pressure gradients except in very special or deliberate circumstances.
(Hammack et al. 2014).
Once a fracked shale well begins production, the pressure gradient and direction
of flow are from the formation into the well, and not upward toward shallow aquifers. There are potential leakage points for stray gas to enter the groundwater if
there are wellbore integrity problems, which do seem to be more frequently associated with fracked wells. Several studies on stray gas have been published with contradictory results, however, and the jury is still out. There is also a rare (but not zero)
potential for high-pressure gas to migrate to the surface through a pre-existing conduit like an abandoned well. Neither of these events are very common, and fears of
pervasive gas migration from below are largely a perceived risk.
1.2 Perceptions of Risk