37 / 56
I
t is often stated that ‘perception is reality.’ With
perception, humans use sensory processes to make
decisions with little thought. This is how we function
in life. Without perceptive shortcuts, life would slow to a
crawl. However, if a workforce develops a habit of working
only from perception during precarious oilfield operations,
either due to inadequate training, lack of information, or an
absence of critical thought, research has shown this creates
the potential for serious safety and mechanical failures.
Confirmation bias is the technical term for the tendency
to perceive a new situation to be the same as all other
similar situations. In the workplace, this translates to an
ethos of ‘we have always done it this way.’
In opposition to perception, which can be faulty,
applying critical thinking to an operation enables one to
analyse the context of surrounding circumstances and
clearly understand the complete scenario. In short, the
solution in a potentially dangerous procedure is to stop and
think critically.
Casestudy
A five-year old well, in its initial years, had been produced
via casing. In the second year, 2 3/8 in. tubing was installed
to increase capillary action. As the reservoir pressure
continued to deplete, the production engineers decided
to install a gas lift system in an attempt to improve
production. A workover crew and rig were contracted to
remove the production tree, install a blowout preventer
(BOP), and then pull the existing tubing and tubing anchor.
The crew was required to install new tubing with a packer
and a gas lift system.
Initial observations
The crew arrived on location and observed 350 psi tubing
pressure (SITP) and 350 psi casing pressure (SICP). The
pressures were reported and the decision was made to
continue, since the well was known in the past to produce
considerable amounts of gas. This probably caused
the pressures to read at a higher level due to the lower
hydrostatic pressure in the well. In this particular field,
a reverse circulation to kill a well with low formation
pressures was usually carried out with 9.2 ppg field water.
Calculations showed that 133 bbl of water were needed.
Process
The crew began pumping and noted that the pump
pressures seemed abnormally high. They also saw that
the casing gauge remained at 350 psi while pumping,
indicating a non-functioning gauge. There were probably
some restrictions in the tubing or tubing anchor downhole.
The supervisor noted this and the decision was made to
continue as long as the well would circulate.
After pumping 120 bbl, field water was observed
flowing from the well and the crew considered the reverse
circulation complete. With the pumps turned off, the tubing
pressure showed 0 psi. The inoperable casing gauge still
showed 350 psi. The crew installed a backpressure valve
(BPV) in the tubing as a secondary barrier.
Examples of confirmation bias, or assumptions based
on perception, in this scenario included the beliefs that:
Ì
Since wells in this field had always been killed with
reverse circulation, that would also work on this well.
Ì
The reverse circulation could be achieved with the
pump gauge, so there was no need for an operable
casing gauge.
Ì
The circulation was complete since kill fluid had
returned to the surface; therefore all hydrocarbons had
been removed from the well.
However, after removing the production tree and while
lifting the tubing hanger, gas began to rapidly escape from
the well, probably due to a damaged tubing hanger seal.
Rather than risk injury or life, the crew wisely evacuated
since there was no known method of well control that they
could apply. Gas vapours reached an ignition source and an
explosion with sustained fire ensued.
Response
A firefighting package and personnel were mobilised by
Wild Well. Additional operations personnel were then sent
to the location to assist with well control after the fire
was extinguished. At this point, since the fire was initially
gas followed by oil, a decision was made on whether to
immediately extinguish the flames or let it burn. Sometimes
a fire is allowed to burn. If the fire is extinguished and a
significant amount of time is needed to kill the well, an
environmental spill could occur. To mitigate environmental
damage, the fire would probably be allowed to burn until
operators were prepared to kill the well.
A hard line was still attached to the casing valve,
whereby kill fluid could be pumped directly into the
annulus. As soon as the appropriate kill fluid arrived
on location the fire was extinguished with a foam fire
suppressant. A solution of calcium chloride at 10.5 ppg was
reverse circulated. When the well was under control with
no further leaks the well was then forward circulated to be
certain there was no remaining gas in the annulus.
Steve Richert, Wild Well Control, USA,
explains how erroneous
perception and confirmation bias have the potential to cause serious
workover well control events in the oilfield.
|
35