26
Modern drill rigs use a hydraulically-driven bit in a bottomhole assembly to drill
vertically down from the surface, build the heel curve and create the horizontal
borehole all in a single operation without having to pull out of the hole and change
components. The final string of casing inserted and cemented into the completed
well is called the production casing, and it runs from the depth of the target formation (or the toe of the lateral) all the way back to the surface. To produce the oil or
gas from the rock, holes known as “perforations” are created in the production casing to allow hydrocarbons to enter the well.
The holes are made using a perforating gun or “perf gun.” In the old days, actual
bullets were employed, hence the name. Modern perf guns use shaped demolition
charges consisting of up to 60 g of RDX, AMX, or HNS, all of which are militarygrade high explosives. The guns consist of a remotely-operated detonator connected
to demolition charges inside a downhole carrier unit that is designed to contain the
explosive debris. The blasts create holes in the casing between 6 and 20 mm in
diameter (¼ to ¾ inch), with a depth into the rock from 10 cm (4 inches) to more
than a meter, and there are generally 12–36 holes created per meter of length (4–12
holes per foot). Successive shots are turned at an angle of about 60° from the previous shot to spiral the perforations around the casing.
As part of the preparations for hydraulic fracturing, a massive, high-pressure
wellhead known as a frack gate (Fig. 2.4) is installed at the surface, just above the
main casing and connected to the production tubing. It is designed to allow equipment and materials to pass while controlling the entry and exit of fluids. The main
wellhead pressure valves at the top of the production casing are left wide open during the frack, because the proppant sand being pumped downhole and returning
afterward would abrade any obstruction in its path (these valves can be seen in
Fig. 2.4 immediately below the frack gate). Abrasion by moving sand is a concern
on all hydraulic fracturing stimulations, but especially in horizontal wells. Although
production casing is typically made from half-inch (1.25 cm) thick, high-tensile
strength steel pipe that meets American Petroleum Institute (API) standards, there
have been rare cases where a hole was abraded in the heel by proppant sand particles racing through the turn.
In a conventional vertical well, the hydraulic fractures extend outward from
either side of the wellbore as vertical cracks called “wings” in the direction of maximum horizontal compressive stress. The wings may extend as far as 1000 feet
(300 m) in either direction from the borehole (Ahmed et al. 1979). Fractures break
in the maximum stress direction because the only way the walls can move apart to
create the crack is in the minimum compressive stress direction at right angles to
this. For example, imagine compressing a walnut in a nutcracker. The nut will crack
in the direction of maximum compression on a line between the jaws. However, the
two sides of the crack will move apart in the direction of minimum stress, or perpendicular to compression, and the shell fragments will fly out the sides of the
nutcracker.
In a related, similar stress issue, if a frack is attempted at a depth that is too shallow, the net overburden pressure will be less than the strength of the rock, and the
rock will not break vertically. Compressive stress downward is needed to force the
2 What Is Fracking?
Modern drill rigs use a hydraulically-driven bit in a bottomhole assembly to drill
vertically down from the surface, build the heel curve and create the horizontal
borehole all in a single operation without having to pull out of the hole and change
components. The final string of casing inserted and cemented into the completed
well is called the production casing, and it runs from the depth of the target formation (or the toe of the lateral) all the way back to the surface. To produce the oil or
gas from the rock, holes known as “perforations” are created in the production casing to allow hydrocarbons to enter the well.
The holes are made using a perforating gun or “perf gun.” In the old days, actual
bullets were employed, hence the name. Modern perf guns use shaped demolition
charges consisting of up to 60 g of RDX, AMX, or HNS, all of which are militarygrade high explosives. The guns consist of a remotely-operated detonator connected
to demolition charges inside a downhole carrier unit that is designed to contain the
explosive debris. The blasts create holes in the casing between 6 and 20 mm in
diameter (¼ to ¾ inch), with a depth into the rock from 10 cm (4 inches) to more
than a meter, and there are generally 12–36 holes created per meter of length (4–12
holes per foot). Successive shots are turned at an angle of about 60° from the previous shot to spiral the perforations around the casing.
As part of the preparations for hydraulic fracturing, a massive, high-pressure
wellhead known as a frack gate (Fig. 2.4) is installed at the surface, just above the
main casing and connected to the production tubing. It is designed to allow equipment and materials to pass while controlling the entry and exit of fluids. The main
wellhead pressure valves at the top of the production casing are left wide open during the frack, because the proppant sand being pumped downhole and returning
afterward would abrade any obstruction in its path (these valves can be seen in
Fig. 2.4 immediately below the frack gate). Abrasion by moving sand is a concern
on all hydraulic fracturing stimulations, but especially in horizontal wells. Although
production casing is typically made from half-inch (1.25 cm) thick, high-tensile
strength steel pipe that meets American Petroleum Institute (API) standards, there
have been rare cases where a hole was abraded in the heel by proppant sand particles racing through the turn.
In a conventional vertical well, the hydraulic fractures extend outward from
either side of the wellbore as vertical cracks called “wings” in the direction of maximum horizontal compressive stress. The wings may extend as far as 1000 feet
(300 m) in either direction from the borehole (Ahmed et al. 1979). Fractures break
in the maximum stress direction because the only way the walls can move apart to
create the crack is in the minimum compressive stress direction at right angles to
this. For example, imagine compressing a walnut in a nutcracker. The nut will crack
in the direction of maximum compression on a line between the jaws. However, the
two sides of the crack will move apart in the direction of minimum stress, or perpendicular to compression, and the shell fragments will fly out the sides of the
nutcracker.
In a related, similar stress issue, if a frack is attempted at a depth that is too shallow, the net overburden pressure will be less than the strength of the rock, and the
rock will not break vertically. Compressive stress downward is needed to force the
2 What Is Fracking?