30
Water pressure and pump rates are maintained until the hydraulic fractures
extend outward to distances as great as 300 m (1000 feet) from the well. The growth
rates and lengths of fractures can be tracked with a geophysical technique known as
microseismic monitoring, which triangulates fracture locations by detecting the
motion of breaking rock with an array of special transducers called “geophones.”
The fractures themselves do not have to be especially large to create highpermeability flowpaths for gas in ultra-tight rocks like shale. Laboratory permeability measurements (Soeder 1988) showed that barely-visible hairline cracks were
important for gas movement in shale, because in the ultra-tight matrix a hairline
crack looks like an eight-lane freeway to a gas molecule.
Short half-life radioactive tracers such as iodine or antimony isotopes are sometimes added to the proppant to allow the height of the hydraulic fractures to be
traced in the subsurface (Smith and Montgomery 2015). These tracers are useful in
vertical wells, where a wireline gamma log can be employed to detect the top and
bottom of the propped fracture. In staged fractures along shale laterals, microseismic monitoring is a more effective technique.
Step 4. Isolating the Stage When a hydraulic fracturing stage is finished, the pressure is released and a seal is set into the production casing to close off the perforated
and fractured zone from the rest of the well. In the past the seals were typically
bridge plugs made of solid cement or a composite material that had to be drilled out
to open up the well after completion. Newer designs use a donut-like rubber cylinder called a packer that is equipped with a check valve. The valve blocks the downhole direction to keep frack pressure in the stage being treated from entering the
previously fracked stage. When the well begins production, fluid and gas flow is in
the uphole direction, and the check valves open.
Step 5. Moving to the Next Stage The perf gun is reloaded and lowered back into
the well, and another set of perforations is shot into the next stage of production
tubing. The hydraulic fracture treatment is repeated on this interval, which is then
closed off with another bridge plug or packer. The process continues stage by stage
until reaching the heel. Depending on the size of each stage, the number of stages
per lateral, and the number of wells requiring stimulation on a single pad, the typical
hydraulic fracturing job usually takes about 2 weeks to a month to complete for
each well pad location.
Step 6. Flowback and Production Shale gas is commonly “overpressured,” which
means that the initial gas pressure in the rock is greater than the hydrostatic pressure
gradient. Thus, the gas pressure is able to push the frack fluid back up and out of the
well. The operator does this with the intent of expelling as much liquid as possible,
diverting it into a holding tank or pond through a pipe called the “blooey line.” The
initial returned fluid, known as “flowback” is made up of discontinuous phases of
gas, water, and sometimes petroleum. Since the well is not on production yet, the
fluids must be stored onsite, and storage of gas is always a problem. The blooey line
is usually fitted with a flare bucket, generally a metal can filled with burning, diesel2 What Is Fracking?
Water pressure and pump rates are maintained until the hydraulic fractures
extend outward to distances as great as 300 m (1000 feet) from the well. The growth
rates and lengths of fractures can be tracked with a geophysical technique known as
microseismic monitoring, which triangulates fracture locations by detecting the
motion of breaking rock with an array of special transducers called “geophones.”
The fractures themselves do not have to be especially large to create highpermeability flowpaths for gas in ultra-tight rocks like shale. Laboratory permeability measurements (Soeder 1988) showed that barely-visible hairline cracks were
important for gas movement in shale, because in the ultra-tight matrix a hairline
crack looks like an eight-lane freeway to a gas molecule.
Short half-life radioactive tracers such as iodine or antimony isotopes are sometimes added to the proppant to allow the height of the hydraulic fractures to be
traced in the subsurface (Smith and Montgomery 2015). These tracers are useful in
vertical wells, where a wireline gamma log can be employed to detect the top and
bottom of the propped fracture. In staged fractures along shale laterals, microseismic monitoring is a more effective technique.
Step 4. Isolating the Stage When a hydraulic fracturing stage is finished, the pressure is released and a seal is set into the production casing to close off the perforated
and fractured zone from the rest of the well. In the past the seals were typically
bridge plugs made of solid cement or a composite material that had to be drilled out
to open up the well after completion. Newer designs use a donut-like rubber cylinder called a packer that is equipped with a check valve. The valve blocks the downhole direction to keep frack pressure in the stage being treated from entering the
previously fracked stage. When the well begins production, fluid and gas flow is in
the uphole direction, and the check valves open.
Step 5. Moving to the Next Stage The perf gun is reloaded and lowered back into
the well, and another set of perforations is shot into the next stage of production
tubing. The hydraulic fracture treatment is repeated on this interval, which is then
closed off with another bridge plug or packer. The process continues stage by stage
until reaching the heel. Depending on the size of each stage, the number of stages
per lateral, and the number of wells requiring stimulation on a single pad, the typical
hydraulic fracturing job usually takes about 2 weeks to a month to complete for
each well pad location.
Step 6. Flowback and Production Shale gas is commonly “overpressured,” which
means that the initial gas pressure in the rock is greater than the hydrostatic pressure
gradient. Thus, the gas pressure is able to push the frack fluid back up and out of the
well. The operator does this with the intent of expelling as much liquid as possible,
diverting it into a holding tank or pond through a pipe called the “blooey line.” The
initial returned fluid, known as “flowback” is made up of discontinuous phases of
gas, water, and sometimes petroleum. Since the well is not on production yet, the
fluids must be stored onsite, and storage of gas is always a problem. The blooey line
is usually fitted with a flare bucket, generally a metal can filled with burning, diesel2 What Is Fracking?