It is no great secret
that drilling for oil and gas involves the risk of exposure to release of high
pressure from the hydrocarbon reservoir being drilled into. It is also no
unknown fact that multiple disastrous as well as catastrophic events have resulted
from drilling a well without sufficient regard for this clear and present
danger. Time and again, it has been established beyond doubt, that there is
only one secret to success in the oilfield - follow the rules. The rules here are
pretty simple. We must not allow the hydrocarbons to come to surface unless we
want it to. The application of the rule is also very simple if we stick to the
methodology developed and improved over the years in the global oilfield
scenario based on the learnings through the decades.
The only effective means of subduing high pressure in hydrocarbon reservoirs
3-4 km deep in the ground is to introduce a counterbalance to the pressure.
This counterbalance is achieved in the form of a viscous liquid that is used to
keep the well full while drilling, commonly known as the drilling fluid. This
is also the liquid that circulates out drilled cuttings from the hole. The
specific gravity (or density) of this liquid is so designed as to exert a
hydrostatic pressure at the depth of the reservoir of oil and gas equal to or
greater than the pressure of the hydrocarbon in the reservoir. So long as the
well is full of this liquid, and so long as the density of the liquid is not
diluted or reduced, there is no danger of gas or oil entering the well while
drilling. To complement the pressure barrier that is the drilling fluid itself,
oilfield standards specify the requirement of an additional barrier, as a back
up to the drilling fluid. This back up is provided at the surface by an
assembly of gates and valves, called the Blowout Preventer, or the BOP.
The final hole of an oil (or gas) well is not an uniform diameter hole from
surface to bottom, rather, it resembles a tapering telescopic tube, with succeeding
sections of smaller diameter as the hole progresses to the bottom. Each section
is consolidated by means of a metal pipe inserted in the entire section to
prevent collapse of the hole. This pipe is then cemented in place, with cement
circulated up the entire back side of the pipe (or casing, as it is called). This
cement provides the foundation for the casing string and also acts as a barrier
against any oil or gas that might permeate to surface behind the casing. All
the casings (for each hole section) terminate at the top, in what is known as
the Wellhead. On this Wellhead sits the BOP. However, the Wellhead needs to
adapt to each smaller casing as each hole section is drilled. This entails
removal of the BOP to install the Wellhead adapter for connecting the smaller
casing to the BOP. During the time between removal of the BOP and its next
installation, the surface facilities are protected from high pressure by two
barriers – the cement inside and behind the casing; and the drilling fluid. Of
these barriers, the cement can be considered a barrier only f it has
sufficiently hardened. There have been instances when gas has percolated
through the cement while it was still setting, creating permanent channels
through it. In that case, the gas flows through the channels and bubbles into
the drilling fluid. Once it is in the drilling fluid, the bubbles migrate up
the fluid column while expanding as they near the surface, knocking out the
drilling fluid from the shallow part of the well in the process. This will
result in an uncontrolled flow of oil and gas into the well and soon all the
drilling fluid will be evacuated from the well bore and the whole well will
become a gas and oil spewing dragon.
Under normal
circumstances, when the BOP is in place, this event does not call for any great
panic. Even if gas migration takes place, all the drilling fluid cannot be
knocked out in an instant. It will take time, and that time is enough to close
the BOP, which takes less than a minute to close. After that, the well can be
restored to its normal overbalanced state by pumping heavy fluid through the
BOP valves. However, if the BOP is not in place, there would be no barrier left
to control the flow of oil and gas to surface. This is why, the BOP should
never be removed before ensuring that the cement has hardened and there is no
gas migration taking place. What little information is available through media
reports, indications are that the BOP was removed in Baghjan-5 well without
ensuring the integrity of the cement barrier, and the consequence is now before
us
Historically, Mankind
has suffered heavily due to oilwell blowouts. Yet, the simple lessons that are
learned through each painful event are sometimes forgotten or ignored, on a
continuous basis, until history repeats itself with more damage, more
destruction. The blowout in well Macondo in the Gulf of Mexico, drilled by the
ultra-modern, 7th generation deepwater drillship Deepwater Horizon was also
brought about by poor cementation, and the bypassing of the golden rule of
ensuring barrier integrity before concluding operations. That catastrophic
event caused such inestimable damage to the environment, in addition to
damaging the reputation of two of the biggest companies in the oilfield - BP
and Transocean, and the credibility of oil drilling as a whole, that one would
have thought that any such human lapse would never be repeated. Yet, we see
complacency in the oilfield so prevalent among not just small companies, but
reputed multinationals as well. Some still believe that the barrier theory, the
BOP testing policy and all the good things that have come about as a result of
all historical learnings are but mere theoretical norms. In other cases, the
most common of all, companies themselves, as a matter of unofficial policy,
encourage or abet such ruinous practices to reduce the total time for the well,
and consequently the total cost. Whatever the reason, when it comes to drilling
in high pressure oil and gas reservoirs, adherence to best practices is
indispensable, and imperative. Else, we will have another Macondo, another
Baghjan-5, and God only knows how many more disasters in the future
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