75
The Late Mississippian Barnett Shale in the Fort Worth basin is named for a
“typical exposure” of the unit at Barnett Springs, about 6.5 km (4 miles) east of the
town of San Saba, Texas (Bruner and Smosna 2011). Like other black shales, obtaining economical amounts of gas from vertical wells in the Barnett was challenging.
Over a period of about 18 years, Mitchell Energy tried several different drilling
techniques and reservoir stimulation methods on the Barnett including massive
hydraulic fracture stimulations. These produced significant flows of gas but at a
very high cost. George Mitchell looked into other types of innovative technology to
produce the gas, and stubbornly refused to quit. Nearly everyone suggested that he
do so. His brother and partners pleaded with him to walk away from the Barnett.
Others in the oil and gas business thought Mitchell’s obsession with shale was an
old man’s eccentricity. Even his own employees considered the Barnett Shale to be
a waste of time and money (Kinley et al. 2008).
By 1997, George Mitchell had found two technologies that appeared to work in
shale. The first was horizontal, or more correctly, “directional” drilling developed
for deepwater offshore platforms. The second was a more cost-effective hydraulic
fracturing technique known as a “light sand” frack and the use of a downhole friction reducer called “slickwater.”
The high cost of moving semi-submersible, tension leg platforms anchored in
kilometers-deep water far offshore led the major oil companies to invest a substantial amount of money into improving directional drilling technology in the late
1980s. Directional drilling had been around since the 1930s, but it was difficult to
turn a borehole too sharply without breaking the drill pipe, and it was also challenging to know the location of the hole deep underground. Being able to drill directional boreholes from an offshore platform meant that multiple wells could tap into
different reservoirs from a single location.
Two innovations greatly improved the technology of directional drilling. The
first was the “bottomhole assembly” that used a downhole motor to turn the drill bit
without having to rotate the entire drill string from the surface. The hydraulic pressure of drilling mud pumped down through the drill pipe from the surface ran the
motor. Stationary drill pipe can make tight turns much more readily than drill pipe
that has to rotate. The second innovation was improved borehole directional location, downhole navigation, and telemetry of data back up to the surface known as
“measurement while drilling” (MWD) and “logging while drilling” (LWD). A
whole new geologic profession called “geosteering” has sprung up to interpret the
downhole data and provide guidance for directional boreholes.
Mitchell realized that with the horizontal boreholes or laterals, he could make
much greater contact with the shale, staying within the formation for kilometers
instead of a few dozen meters at most if he penetrated it vertically. The horizontal
well would also allow him to frack in multiple stages at different locations along the
length of the lateral, instead of the single stage fracks that were typical of vertical wells.
Because the runs of production tubing in horizontal wells were significantly longer than in vertical wells, a friction-reducing additive called polyacrylamide was
introduced to the frack fluid as a lubricant, creating an extremely slippery liquid
4.3 The Unconventional Solution
The Late Mississippian Barnett Shale in the Fort Worth basin is named for a
“typical exposure” of the unit at Barnett Springs, about 6.5 km (4 miles) east of the
town of San Saba, Texas (Bruner and Smosna 2011). Like other black shales, obtaining economical amounts of gas from vertical wells in the Barnett was challenging.
Over a period of about 18 years, Mitchell Energy tried several different drilling
techniques and reservoir stimulation methods on the Barnett including massive
hydraulic fracture stimulations. These produced significant flows of gas but at a
very high cost. George Mitchell looked into other types of innovative technology to
produce the gas, and stubbornly refused to quit. Nearly everyone suggested that he
do so. His brother and partners pleaded with him to walk away from the Barnett.
Others in the oil and gas business thought Mitchell’s obsession with shale was an
old man’s eccentricity. Even his own employees considered the Barnett Shale to be
a waste of time and money (Kinley et al. 2008).
By 1997, George Mitchell had found two technologies that appeared to work in
shale. The first was horizontal, or more correctly, “directional” drilling developed
for deepwater offshore platforms. The second was a more cost-effective hydraulic
fracturing technique known as a “light sand” frack and the use of a downhole friction reducer called “slickwater.”
The high cost of moving semi-submersible, tension leg platforms anchored in
kilometers-deep water far offshore led the major oil companies to invest a substantial amount of money into improving directional drilling technology in the late
1980s. Directional drilling had been around since the 1930s, but it was difficult to
turn a borehole too sharply without breaking the drill pipe, and it was also challenging to know the location of the hole deep underground. Being able to drill directional boreholes from an offshore platform meant that multiple wells could tap into
different reservoirs from a single location.
Two innovations greatly improved the technology of directional drilling. The
first was the “bottomhole assembly” that used a downhole motor to turn the drill bit
without having to rotate the entire drill string from the surface. The hydraulic pressure of drilling mud pumped down through the drill pipe from the surface ran the
motor. Stationary drill pipe can make tight turns much more readily than drill pipe
that has to rotate. The second innovation was improved borehole directional location, downhole navigation, and telemetry of data back up to the surface known as
“measurement while drilling” (MWD) and “logging while drilling” (LWD). A
whole new geologic profession called “geosteering” has sprung up to interpret the
downhole data and provide guidance for directional boreholes.
Mitchell realized that with the horizontal boreholes or laterals, he could make
much greater contact with the shale, staying within the formation for kilometers
instead of a few dozen meters at most if he penetrated it vertically. The horizontal
well would also allow him to frack in multiple stages at different locations along the
length of the lateral, instead of the single stage fracks that were typical of vertical wells.
Because the runs of production tubing in horizontal wells were significantly longer than in vertical wells, a friction-reducing additive called polyacrylamide was
introduced to the frack fluid as a lubricant, creating an extremely slippery liquid
4.3 The Unconventional Solution