Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 418

Hydraulic fracturing or fracking is permitted by regulatory agencies
for producing natural gas and oil from shale (see Box 9.6.2 ). Hydraulic
fracturing is, however, prohibited for most other injection wells. Because
hydraulic fractures can be hard to control and can alter the properties of
the formations, fracking may allow potential migration of the disposed
fl uid into protected groundwater resources. To protect these groundwa-
ter resources, regulatory agencies require fl uid pressure to remain below
the so-called frac gradient pressure. We will be discussing this aspect
further in the section on potential impacts of geological sequestration.
Scale-dependence of permeability
The permeability of rocks not only varies over many orders of magnitude,
but also depends on the scale of the rock sample. This scale-dependence
of permeability originates from heterogeneity in rocks and the way per-
meability is measured. The permeability is obtained from the fl ow rate for
a given pressure gradient. Flow paths of different length scale will be
accessible to fl uids depending on the length scale of measurement. If the
pressure gradient is placed over a long distance, more fl ow pathways will
be accessed, and the likelihood of intersecting a long fast-fl ow path is
higher. Because the fl ow rate will refl ect the highest permeability
pathways sampled, analogous to an electric current with various
resistors positioned in parallel rather than in series, the measured perme-
ability increases when the length scale of the measurement increases.
Box 9.6.2 Hydraulic fracturing (or “fracking”)
The ability to create high-permeability vertical fractures is the basis for the relatively
new approach to extracting hydrocarbons called hydraulic fracturing (or “fracking”).
Hydraulic fracturing is allowing the production of large amounts of natural gas and oil
from otherwise very low-permeability shale rocks. In hydraulic fracturing, long horizon-
tal wells are drilled through the shale formations that contain either natural gas (primar-
ily methane) or oil in economically useful quantities. Fluid at very high pressure is then
injected into the horizontal wells to create sub-vertical fractures in the shale. Proppants
(e.g., sand) and other chemical agents in the injected fl uid fl ow into these fractures and
serve to hold them open even after the injection over-pressure is removed. These open
fractures provide the permeability needed to economically extract hydrocarbons from
the rock for fuel.