96
years for frack chemicals to reach shallow aquifers (Gassiat et al. 2013). Overpressure
occurs in a rock when the pore system is isolated from hydrostatic pressure. The
hydrostatic pressure gradient (water) is typically about half that of the lithostatic
pressure gradient (rock), so overburden pressure is always greater than pore pressure. However, it is not unusual for the pore waters in deep formations to become
separated from the water column to the surface. When this occurs, the pore pressure
increases from hydrostatic to lithostatic and the rock is said to be overpressured.
Gas shales are typically found to be overpressured when first drilled, but gas or oil
production from the fracked well reduces this pressure to hydrostatic or lower fairly
quickly. Once the well begins production, the pressure gradients in the formation
direct fluid flow toward the wellbore, not toward the surface.
Although they might agree that it is not likely for frack liquids to migrate upward,
O&G opponents have raised repeated alarms that stray gas might do so because of
its buoyancy. Again, with fractures bending over to the horizontal at depths shallower than 2500 feet, there are few available flowpaths for vertical gas migration.
Nevertheless, there does appear to be at least an empirical correlation between an
increased frequency of stray gas incidents and the presence of shale gas wells (Li
et al. 2016).
An analysis of 75,505 environmental compliance reports for 41,381 conventional
and unconventional oil and gas wells in Pennsylvania found that shale gas wells
have a six times greater risk of wellbore integrity problems compared to conventional wells (Ingraffea et al. 2014). This work showed a statistically-valid correlation between well type (conventional vs. unconventional) and the probability of
cement/casing failure, but it did not go into details about the possible causes of this
failure. It is also important to note that “wellbore integrity problems” do not necessarily translate directly into “stray gas.” Gas migration requires a source for the gas,
a flowpath to the surface, and a pressure gradient to drive flow. Stray gas sources are
often either the produced gas from the target formation, or gas occurring naturally
in the shallower geologic units penetrated by the well.
Several modeling studies have shown that drilling the vertical top hole of a shale
gas well using compressed air instead of a liquid drilling mud may affect pressure
gradients within aquifers. Compressed air at pressures of up to 350 psi (2.4 MPa)
entering confined or semi-confined aquifers can drive groundwater flow at rates that
will entrain and mobilize existing methane gas, potentially causing stray gas migration (Geng et al. 2014; Zhang and Soeder 2016). Although compressed air drilling
is faster and creates cleaner holes, many drillers have switched to liquid drilling
fluids to reduce pressure effects on aquifers.
Canadian studies in northern British Columbia on gas migration from wells that
were drilled horizontally but not fracked found that well construction quality was
the most important determining factor for gas migration. Poorly constructed wells
using inferior materials were much more prone to leakage and gas migration than
wells that were installed properly (Sandl et al. 2019). Other studies have also noted
that the presence of “problem wells” from poor construction seems to correlate
more strongly than fracking with the occurrence of stray gas (Brantley et al. 2018).
6 Fracking and Water
years for frack chemicals to reach shallow aquifers (Gassiat et al. 2013). Overpressure
occurs in a rock when the pore system is isolated from hydrostatic pressure. The
hydrostatic pressure gradient (water) is typically about half that of the lithostatic
pressure gradient (rock), so overburden pressure is always greater than pore pressure. However, it is not unusual for the pore waters in deep formations to become
separated from the water column to the surface. When this occurs, the pore pressure
increases from hydrostatic to lithostatic and the rock is said to be overpressured.
Gas shales are typically found to be overpressured when first drilled, but gas or oil
production from the fracked well reduces this pressure to hydrostatic or lower fairly
quickly. Once the well begins production, the pressure gradients in the formation
direct fluid flow toward the wellbore, not toward the surface.
Although they might agree that it is not likely for frack liquids to migrate upward,
O&G opponents have raised repeated alarms that stray gas might do so because of
its buoyancy. Again, with fractures bending over to the horizontal at depths shallower than 2500 feet, there are few available flowpaths for vertical gas migration.
Nevertheless, there does appear to be at least an empirical correlation between an
increased frequency of stray gas incidents and the presence of shale gas wells (Li
et al. 2016).
An analysis of 75,505 environmental compliance reports for 41,381 conventional
and unconventional oil and gas wells in Pennsylvania found that shale gas wells
have a six times greater risk of wellbore integrity problems compared to conventional wells (Ingraffea et al. 2014). This work showed a statistically-valid correlation between well type (conventional vs. unconventional) and the probability of
cement/casing failure, but it did not go into details about the possible causes of this
failure. It is also important to note that “wellbore integrity problems” do not necessarily translate directly into “stray gas.” Gas migration requires a source for the gas,
a flowpath to the surface, and a pressure gradient to drive flow. Stray gas sources are
often either the produced gas from the target formation, or gas occurring naturally
in the shallower geologic units penetrated by the well.
Several modeling studies have shown that drilling the vertical top hole of a shale
gas well using compressed air instead of a liquid drilling mud may affect pressure
gradients within aquifers. Compressed air at pressures of up to 350 psi (2.4 MPa)
entering confined or semi-confined aquifers can drive groundwater flow at rates that
will entrain and mobilize existing methane gas, potentially causing stray gas migration (Geng et al. 2014; Zhang and Soeder 2016). Although compressed air drilling
is faster and creates cleaner holes, many drillers have switched to liquid drilling
fluids to reduce pressure effects on aquifers.
Canadian studies in northern British Columbia on gas migration from wells that
were drilled horizontally but not fracked found that well construction quality was
the most important determining factor for gas migration. Poorly constructed wells
using inferior materials were much more prone to leakage and gas migration than
wells that were installed properly (Sandl et al. 2019). Other studies have also noted
that the presence of “problem wells” from poor construction seems to correlate
more strongly than fracking with the occurrence of stray gas (Brantley et al. 2018).
6 Fracking and Water