28
work backward toward the heel. Each stage receives a set of perforations followed
by the hydraulic fracture treatment. It is then sealed off while the next stage is
treated. The seals are removed for production after all stages have been fracked.
The descriptions of the reservoir stimulation steps that follow are those that were
developed by Mitchell Energy for the successful production of gas from the Barnett
Shale in Texas during the late 1990s that started the shale gas revolution. Drilling,
completion, stimulation, and production techniques are constantly evolving as companies seek ways to improve efficiencies and reduce costs, and this book would be
obsolete before the ink was dry if the latest trends were included. For example, one
technique said to be fairly common on newer wells at this writing is to hold the frack
pressure on a zone for extended periods of time to “let it soak.” In a month or a year,
the “latest thing” will be something else. Nevertheless, the historical steps developed by Mitchell for successful hydrocarbon production from shale provide useful
lessons in shale gas engineering and a good overview of the issues encountered
when stimulating an ultra-tight rock.
Step 1. Prep and Cleanout The hydraulic fracturing process starts by cleaning the
perf holes using a 15% solution of muriatic or hydrochloric acid (HCl). Perforating
casing with high explosives tends to force pieces of steel and pulverized cement into
the formation, and these must be removed. While the acid is cleaning out the perforations, the hydraulic fracturing system undergoes pressure testing and all the equipment is calibrated.
Electronic instrumentation is used to collect real-time measurements of pressure
data at the wellhead, downhole, and in the annulus behind the production casing. A
flow meter on the blender measures the volume of fluid pumped downhole, and a
densometer measures the amount of sand in the fluid. Engineers closely watch the
wellhead, annulus, and bottomhole pressures, pump rate, fluid density and material
parameters throughout the frack.
The high- and low-pressure systems on a hydraulic fracturing operation are
plumbed separately, so fluid from one cannot get into the other unless the operator
allows it. The working parts of the pumps used to generate the frack pressure consist
of positive displacement pistons inside high-strength steel cylinders. The rate at
which these pistons advance can be controlled very precisely to maintain a specific
flow volume and/or pressure. The migration of frack fluid into the formation is
known as “leak-off” and the pumps have to be precise enough to make up for this
volume loss while maintaining pressure. Safety cutoffs are in place if pressure or
volume parameters are exceeded, and the high-pressure parts of the system also
have relief valves to prevent critical components from blowing out.
Step 2. Fracture Initiation The well is filled with water containing a frictionreducing chemical additive called polyacrylamide, which creates an extremely slippery liquid known as “slickwater.” Slickwater is used to reduce pressure losses due
to friction as the frack fluid is pumped from the surface to the formation down a long
string (often several kilometers) of production casing. Downhole pressure losses
can be as much as 50% without this treatment. The frack fluid is under a hydrostatic
2 What Is Fracking?
work backward toward the heel. Each stage receives a set of perforations followed
by the hydraulic fracture treatment. It is then sealed off while the next stage is
treated. The seals are removed for production after all stages have been fracked.
The descriptions of the reservoir stimulation steps that follow are those that were
developed by Mitchell Energy for the successful production of gas from the Barnett
Shale in Texas during the late 1990s that started the shale gas revolution. Drilling,
completion, stimulation, and production techniques are constantly evolving as companies seek ways to improve efficiencies and reduce costs, and this book would be
obsolete before the ink was dry if the latest trends were included. For example, one
technique said to be fairly common on newer wells at this writing is to hold the frack
pressure on a zone for extended periods of time to “let it soak.” In a month or a year,
the “latest thing” will be something else. Nevertheless, the historical steps developed by Mitchell for successful hydrocarbon production from shale provide useful
lessons in shale gas engineering and a good overview of the issues encountered
when stimulating an ultra-tight rock.
Step 1. Prep and Cleanout The hydraulic fracturing process starts by cleaning the
perf holes using a 15% solution of muriatic or hydrochloric acid (HCl). Perforating
casing with high explosives tends to force pieces of steel and pulverized cement into
the formation, and these must be removed. While the acid is cleaning out the perforations, the hydraulic fracturing system undergoes pressure testing and all the equipment is calibrated.
Electronic instrumentation is used to collect real-time measurements of pressure
data at the wellhead, downhole, and in the annulus behind the production casing. A
flow meter on the blender measures the volume of fluid pumped downhole, and a
densometer measures the amount of sand in the fluid. Engineers closely watch the
wellhead, annulus, and bottomhole pressures, pump rate, fluid density and material
parameters throughout the frack.
The high- and low-pressure systems on a hydraulic fracturing operation are
plumbed separately, so fluid from one cannot get into the other unless the operator
allows it. The working parts of the pumps used to generate the frack pressure consist
of positive displacement pistons inside high-strength steel cylinders. The rate at
which these pistons advance can be controlled very precisely to maintain a specific
flow volume and/or pressure. The migration of frack fluid into the formation is
known as “leak-off” and the pumps have to be precise enough to make up for this
volume loss while maintaining pressure. Safety cutoffs are in place if pressure or
volume parameters are exceeded, and the high-pressure parts of the system also
have relief valves to prevent critical components from blowing out.
Step 2. Fracture Initiation The well is filled with water containing a frictionreducing chemical additive called polyacrylamide, which creates an extremely slippery liquid known as “slickwater.” Slickwater is used to reduce pressure losses due
to friction as the frack fluid is pumped from the surface to the formation down a long
string (often several kilometers) of production casing. Downhole pressure losses
can be as much as 50% without this treatment. The frack fluid is under a hydrostatic
2 What Is Fracking?