Chevron looking for clean sweep

Chevron is working on a project designed to boost the recovery rate from the UK North Sea Captain heavy oilfield to 50%. If successful, this could deliver up to 160million additional barrels of oil and boost recoverable reserves past the 450million barrels mark.

The key is the use of a suite of techniques including extended-reach drilling and multilateral wells and the use of sophisticated polymer chemicals. When first developed in the early-1990s, the recoverables figure was put at some 300million barrels based on a recovery rate of about 34%. Since then, two further panels of the large Outer Moray Firth field have been opened up – Captain B and C. More than 200million barrels have been produced from Captain to date.

It was in 2006 that a long-term development team was tasked with looking at technologies and techniques that could be applied to optimise Captain field recovery with the long-term target of 50%, which is exceptional for heavy-crude reservoirs.

They came to the view that “every 1% of increased production represents around 10million barrels”.

Results thus far have been positive. Chevron says numerous opportunities exist for multilateral technology in Captain, with the first dual lateral well successfully drilled in 2007 and another planned for this year.

Work with polymers is less advanced, with the first in-field application scheduled for 2009. The project team, led by Stewart Mitchell, is working with Chevron’s Energy Technology Company (ETC) in Houston to evaluate this approach to extended oil recovery (EOR) using a phased approach employing polymer flood followed by potential application of alkali surfactant polymers to improve the sweep efficiency in the reservoir.

Polymers in injection water essentially thicken it, pushing more oil ahead of it as it washes through the reservoir, while surfactant with alkaloids acts like “washing-up liquid on greasy dishes”, separating and mobilising more oil from the reservoir sand. Chevron says this two-phase treatment offers high potential for recovering significant additional production.

Polymer flooding does work, albeit most applications have been small-scale because it is expensive.

Chevron says lessons learned on polymer flooding, particularly from projects that have been less than entirely successful, are helping the project team refine its approach.

Incomplete reservoir intelligence and management, poor ability to model process mechanisms at field scale, dependence on unscaled laboratory experiments and unavailability of appropriate polymers or surfactants in the quantities required are all factors that have emerged as potential barriers to success.

Chevron Europe is also drawing on the experience of its IndoAsia business unit, where colleagues are working on a surfactant/polymer augmented enhanced recovery project in the giant Minas field in Indonesia, which is expected to go to limited field trial in 2011. The Minas team has carried out extensive work over the past few years looking at a range of technologies for EOR, and the Captain team are capitalising on that.

Marco Goense, former Captain subsurface petroleum engineer, now leading the EOR design team in Houston, said: “The Minas project has allowed ETC to build organisational capability in surfactant/polymer augmented EOR and Captain is able to leverage that knowledge and experience.”

The work in progress at ETC aims to enhance understanding of the process variables and requirements of scaling up from the laboratory to field trials.

The team is also working on characterising various polymers for application in Captain using core samples to gauge their effectiveness. But they have a lot to do before the 2009 pilot based on a single injection well.

Critically, Chevron is developing reservoir simulation models to predict performance of the polymers, assessing possible impact on the production process and developing prevention or mitigation measures, as well as reviewing the practical preparation required for running a trial.

Said Stewart: ‘We are working hard to get a good understanding of the scope and complexity of this project and what it’s going to take to implement it successfully, with minimal impact on base business and before production declines too far. We have a window of opportunity to aim for, otherwise we risk leaving a lot of barrels in the ground.”

Assuming the trials are a success, then the challenge becomes how to handle the logistics of manufacturing the chemicals then delivering, mixing and pumping below ground large quantities of surfactants into the Captain reservoirs.

So how much added value could full-scale polymer use generate for Chevron Europe? The upside could be high.

But Stewart warned: “‘The project is still in its early stages and there are a lot of challenges to overcome, but if we execute successfully, we are talking about adding substantial production. The prize is well worth the effort.”