Steam flood, optimized well placement
could unlock stranded, North Sea heavy oil
Steam flooding is one of the most ef- fective ways of boosting recovery of heavy oils. Steam injection has been applied very successfully onshore in California, Indonesia, and Oman and
offshore in Bohai Bay and in Congo.
The Steam Oil Production Company in
London is now seeking to apply steam flooding, to the Pilot, Elke, and Narwhal fields in
the UK North Sea, three undeveloped heavy-oil discoveries with large volumes of in-place
crude. Steam flooding involves injecting steam
continuously into dedicated injection wells: as
the steam condenses in the reservoir both the
rock and oil are heated up, and the now mobile
oil is swept toward the producers by a bank of
fresh water. The combination of high temperatures and the fact that the reservoir is being
swept by a gas rather than a liquid reduces the
residual oil saturation from typically 25% for a
cold water flood to just 5% in the steam zone.
This and the reduced viscosity has been proven to boost recovery to 50-80% of oil in place.
Although early steam floods deployed close
arrays of vertical injectors and producers, later
experience has shown that horizontal wells
can be more effective, and this is the proposed
technique for the Pilot steam flood project. Developing Pilot with a conventional patterns of
vertical well penetrations around 100 m (328
ft) apart would require literally hundreds of
wells, but long horizontal wells penetrating
the reservoir can bring well numbers down
by an order of magnitude and achieve a better sweep. Also, if the formation has a high
permeability, injection rates into the horizontal
wells can be high enough to ensure that heat
losses in the wellbore are not a major problem.
The Steam Oil Production Company expects to recover around 60% of Pilot’s in-place
oil via a steam flood of the reservoir. But the
key to the project’s economic viability is the
volume of steam that has to be injected to recover each barrel of crude. The plan is to test
different well spacings, injection rates and
well positions relative to the oil-water contact
in order to optimize the balance between the
recovery factor and the steam/oil ratio. Co-injection of steam and methane (or propane)
is another option that could increase recovery further and improve the steam/oil ratio,
cutting fuel requirements.
There are concerns over the efficiency of
steam flooding reservoirs where the oil zone
is underlain by a significant water zone, and
experience shows that recovery is lower in
these cases. However, following investigations
the company has developed an innovative approach to this problem which should lead to
high recovery factors. Proving that offshore
steam works, and testing technologies that enable steam flooding at reservoir depths greater than 3,000 ft (914 m), could make a material
difference to maximizing the UKCS’ remaining economic oil recovery, with potentially
2 Bbbl to be recovered via enhanced steam
flood from other UK heavy-oil projects.
When The Steam Oil Production Company
talks to the industry about its plans for its UK
offshore fields, however, the general reaction
is skeptical. That is not how the industry develops heavy oil in the North Sea: the standard-bearers were the teams that in the early
1990s put together development plans for
Alba, Captain, Harding and Gryphon in the
central North Sea, the last wave of heavy-oil
developments on the UK continental shelf.
Those were stunningly successful projects,
none more so than Harding, which is reputed
to be achieving a 74% recovery factor from
the Harding Central reservoir. Those fields
were undoubtedly developed in the best way
possible based on the available technologies
at the time. This author led the BP team that
prepared the Harding development plan in
the 1990s which in the end delivered even
more than our most optimistic forecasts.
Based on these returns, many in the industry today believe there is a standard formula for developing heavy oil in the North
Sea. It is pretty simple: maximize well productivity via long, horizontal production
wells, put them as high in the reservoir section as possible, and sweep as much water
around the system as you can.
The plans for the current Mariner, Kraken
and Bentley developments in the UK northern
North Sea, operated respectively by Statoil, En-Quest, and Xcite Energy, do not just pay homage to the fundamental nature of the reservoir
depletion programs for these early projects:
they copy them. Admittedly, the well density
has been tightened, the water handling capacity
increased and the well designs take advantage
of new technologies such as multi-lateral junctions; but fundamentally, what is being done to
these reservoirs is just the same as was done to
Alba, Captain, Harding, and Gryphon.
But the heavy oilfields of the 1990s were
(Captain excepted) very different from the
heavy oilfields that are being developed to-
The Steam Oil
Production Company Ltd.
Pilot/Elke location in the UK central North Sea.