97
A broader study of wellbore integrity problems from a wide range of countries
including Australia, Austria, Bahrain, Brazil, Canada, the Netherlands, Poland, the
U.K., and the United States found that published data on the integrity of well barriers are highly variable (Davies et al. 2014). Rates of well failure ranged from about
2% to as high as 75%. About 6.3% of the Marcellus Shale wells inspected in
Pennsylvania between 2005 and 2013 were found to have wellbore integrity-related
violations, including cement or casing failures, blowouts, and gas venting. Of the
143 actively producing wells in the U.K. included in the study, only one (0.7%) was
found to have evidence of a well integrity failure (Davies et al. 2014).
Nevertheless, the question of why Ingraffea et al. (2014) found the risk of wellbore integrity problems in shale gas wells to be six times greater than conventional
wells remains unanswered. Because the vertical topholes of horizontal shale wells
and conventional wells are constructed and completed in more or less the same
manner, the cause of greater wellbore integrity problems in shale wells is probably
not related to construction practices or materials alone. Taking a step back to look at
the bigger picture suggests that at least one major difference between conventional
and unconventional wells is the amount of fracking.
Conventional wells penetrate the target zone vertically, and although some do not
require fracking to be productive, many do. Vertical wells are typically fracked only
once in the zone of interest (refer back to Fig. 2.3). The laterals of unconventional
wells, on the other hand, are fracked dozens or even hundreds of times, depending
on their length. The hydraulic fracturing fluid fills the production casing all the way
to the surface, and the repeated pressurization and release during the frack stages
may create cracks in the cement that is used to isolate the different casing strings.
Because cement is strong under compression but weak in tension, the pressure fluctuations may result in the cement de-bonding from the steel casing, creating a small
fracture called a microannulus. These can extend long distances vertically up a
borehole and create a flowpath for gas migration (Soeder 2017). When a flowpath
like a microannulus is present, modeling studies have shown that natural gas from
depth can reach the surface in less than 2 days (Schwartz 2015). The formation of
such microannuli may be responsible for the higher rates of wellbore integrity failure found by Ingraffea et al. (2014). Laboratory and field tests are required to define
this problem and determine a solution.
Gas migration might also occur if a hydraulic fracture intercepts a pre-existing
vertical flowpath like a fault or an abandoned well, providing a path upward for gas
to reach shallow aquifers. Although such incidents are rare, the probability is not
zero. A case was documented of a Marcellus Shale frack affecting an abandoned
well in Tioga County, Pennsylvania in 2012. The incident occurred after East
Resources had drilled and fracked a Marcellus Shale well on the Guindon farm in
Union Township (Detrow 2012). The driller was aware that an old gas well was
located less than a kilometer away. It had been completed in February 1932 by the
Morris Run Coal Company, who drilled it to a depth of 5385 feet (1641 m) on a
farm owned by a Mr. W.J. Butters. East Resources thought the old Butters well had
been properly plugged with cement before being abandoned as required by state
“plug and abandon” (P&A) regulations. It was not.
6.1 Water Quality and Stray Gas
A broader study of wellbore integrity problems from a wide range of countries
including Australia, Austria, Bahrain, Brazil, Canada, the Netherlands, Poland, the
U.K., and the United States found that published data on the integrity of well barriers are highly variable (Davies et al. 2014). Rates of well failure ranged from about
2% to as high as 75%. About 6.3% of the Marcellus Shale wells inspected in
Pennsylvania between 2005 and 2013 were found to have wellbore integrity-related
violations, including cement or casing failures, blowouts, and gas venting. Of the
143 actively producing wells in the U.K. included in the study, only one (0.7%) was
found to have evidence of a well integrity failure (Davies et al. 2014).
Nevertheless, the question of why Ingraffea et al. (2014) found the risk of wellbore integrity problems in shale gas wells to be six times greater than conventional
wells remains unanswered. Because the vertical topholes of horizontal shale wells
and conventional wells are constructed and completed in more or less the same
manner, the cause of greater wellbore integrity problems in shale wells is probably
not related to construction practices or materials alone. Taking a step back to look at
the bigger picture suggests that at least one major difference between conventional
and unconventional wells is the amount of fracking.
Conventional wells penetrate the target zone vertically, and although some do not
require fracking to be productive, many do. Vertical wells are typically fracked only
once in the zone of interest (refer back to Fig. 2.3). The laterals of unconventional
wells, on the other hand, are fracked dozens or even hundreds of times, depending
on their length. The hydraulic fracturing fluid fills the production casing all the way
to the surface, and the repeated pressurization and release during the frack stages
may create cracks in the cement that is used to isolate the different casing strings.
Because cement is strong under compression but weak in tension, the pressure fluctuations may result in the cement de-bonding from the steel casing, creating a small
fracture called a microannulus. These can extend long distances vertically up a
borehole and create a flowpath for gas migration (Soeder 2017). When a flowpath
like a microannulus is present, modeling studies have shown that natural gas from
depth can reach the surface in less than 2 days (Schwartz 2015). The formation of
such microannuli may be responsible for the higher rates of wellbore integrity failure found by Ingraffea et al. (2014). Laboratory and field tests are required to define
this problem and determine a solution.
Gas migration might also occur if a hydraulic fracture intercepts a pre-existing
vertical flowpath like a fault or an abandoned well, providing a path upward for gas
to reach shallow aquifers. Although such incidents are rare, the probability is not
zero. A case was documented of a Marcellus Shale frack affecting an abandoned
well in Tioga County, Pennsylvania in 2012. The incident occurred after East
Resources had drilled and fracked a Marcellus Shale well on the Guindon farm in
Union Township (Detrow 2012). The driller was aware that an old gas well was
located less than a kilometer away. It had been completed in February 1932 by the
Morris Run Coal Company, who drilled it to a depth of 5385 feet (1641 m) on a
farm owned by a Mr. W.J. Butters. East Resources thought the old Butters well had
been properly plugged with cement before being abandoned as required by state
“plug and abandon” (P&A) regulations. It was not.
6.1 Water Quality and Stray Gas