95
have been carried out in all of the major tight oil and shale gas producing regions of
the United States, but the majority of studies were focused on the Marcellus Shale
region, presumably because of the higher population density at risk of exposure compared to other plays.
6.1 Water Quality and Stray Gas
The perceived risk that gas and chemical additives injected into the ground during
fracking operations would migrate upward and contaminate drinking water aquifers
from below has been largely debunked (Fisher and Warpinski 2012; Hammack et al.
2014). It defies the laws of physics, because fractures simply don’t break that way,
and fluids don’t move upward against gravity unless pushed. A hydraulicallyinduced fracture requires the overburden pressure to exceed rock strength if the
fracture is to break vertically upward. In most cases, this occurs at a minimum depth
of 2500 feet (800 m). Any hydraulic fracturing attempted at shallower depths will
create fractures that propagate horizontally instead of vertically in a process known
as “pancaking” that is very inefficient for O&G recovery.
Fracking is a very specialized and competitive business. Referring to the microseismic monitoring data presented back in Fig. 1.3, it is obvious that the vertical
extent of the hydraulic fractures is limited, and they do not approach anywhere near
the freshwater aquifers. Service companies carefully monitor the placement of
hydraulic fractures within the target zone to limit costs and maximize hydrocarbon
recovery.
The main thing working against frack fluids moving upward is gravity. A frack
would literally have to be pumped way beyond design specifications to reach depths
that even begin to bring it near shallow, freshwater aquifers. Once the pumps are
stopped, fluids cease moving upward and gravity brings these down toward the horizontal well bore.
Although it has never been directly observed, a group of hydrologists decided to
model every possible way hydraulic fracturing fluid might be able to move upward
toward shallow aquifers (Birdsell et al. 2015). The migration mechanisms identified
include topographically-driven flow, overpressured shale gas reservoirs, permeable
pathways such as faults or leaky wellbores, increased formation pressures due to
frack fluid injection, and the density contrast of the freshwater frack fluid with
denser surrounding brine. The studies found that without a fast permeable pathway
to the surface like a fault or an unsealed wellbore, the frack fluid would never reach
shallow aquifers. These fluids would be trapped in the pore systems of overlying
rocks by capillary imbibition as they migrated slowly upward. The modeling study
concluded that in theory it might be possible for frack fluid to migrate upward and
contaminate shallow drinking water aquifers, but to do so would require the convergence of a high number of unlikely circumstances (Birdsell et al. 2015).
Even with the presence of a high-permeability fault, significant overpressure in
the shale, and fracking in the shale close to the fault, it would take nearly a thousand
6.1 Water Quality and Stray Gas
have been carried out in all of the major tight oil and shale gas producing regions of
the United States, but the majority of studies were focused on the Marcellus Shale
region, presumably because of the higher population density at risk of exposure compared to other plays.
6.1 Water Quality and Stray Gas
The perceived risk that gas and chemical additives injected into the ground during
fracking operations would migrate upward and contaminate drinking water aquifers
from below has been largely debunked (Fisher and Warpinski 2012; Hammack et al.
2014). It defies the laws of physics, because fractures simply don’t break that way,
and fluids don’t move upward against gravity unless pushed. A hydraulicallyinduced fracture requires the overburden pressure to exceed rock strength if the
fracture is to break vertically upward. In most cases, this occurs at a minimum depth
of 2500 feet (800 m). Any hydraulic fracturing attempted at shallower depths will
create fractures that propagate horizontally instead of vertically in a process known
as “pancaking” that is very inefficient for O&G recovery.
Fracking is a very specialized and competitive business. Referring to the microseismic monitoring data presented back in Fig. 1.3, it is obvious that the vertical
extent of the hydraulic fractures is limited, and they do not approach anywhere near
the freshwater aquifers. Service companies carefully monitor the placement of
hydraulic fractures within the target zone to limit costs and maximize hydrocarbon
recovery.
The main thing working against frack fluids moving upward is gravity. A frack
would literally have to be pumped way beyond design specifications to reach depths
that even begin to bring it near shallow, freshwater aquifers. Once the pumps are
stopped, fluids cease moving upward and gravity brings these down toward the horizontal well bore.
Although it has never been directly observed, a group of hydrologists decided to
model every possible way hydraulic fracturing fluid might be able to move upward
toward shallow aquifers (Birdsell et al. 2015). The migration mechanisms identified
include topographically-driven flow, overpressured shale gas reservoirs, permeable
pathways such as faults or leaky wellbores, increased formation pressures due to
frack fluid injection, and the density contrast of the freshwater frack fluid with
denser surrounding brine. The studies found that without a fast permeable pathway
to the surface like a fault or an unsealed wellbore, the frack fluid would never reach
shallow aquifers. These fluids would be trapped in the pore systems of overlying
rocks by capillary imbibition as they migrated slowly upward. The modeling study
concluded that in theory it might be possible for frack fluid to migrate upward and
contaminate shallow drinking water aquifers, but to do so would require the convergence of a high number of unlikely circumstances (Birdsell et al. 2015).
Even with the presence of a high-permeability fault, significant overpressure in
the shale, and fracking in the shale close to the fault, it would take nearly a thousand
6.1 Water Quality and Stray Gas