Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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DTI MEOR Workshop: A Participant's View

Stephen Goodyear, Principal Reservoir Engineer, AEA Technology plc, gives his views on the DTI MEOR workshop. (stephen.goodyear@aeat.co.uk)

The workshop provided an excellent opportunity to discuss the current status of Microbial Enhanced Oil Recovery, with particular reference to the UKCS, and I'd like to thank the DTI and RML for putting together a well balanced and interesting programme of presentations.

There is clearly ample evidence that microbes can clean oil from beaches and release oil from core tests under controlled conditions. The question is whether we can use microbes cost effectively and with sufficient confidence to improve recovery in the "microbial jungle" of mature reservoirs. In this article I outline some of the issues that were raised during the day, that might be addressed in future work programmes, whether these are field trials or additional supporting laboratory and reservoir engineering work.

What sort of MEOR should we consider?
Iain Spark introduced the workshop to a wide range of different techniques, based on injecting microbes, stimulating indigenous microbes, use of injected carbon sources etc. Some interesting comments were made on which applications might have the best chance of success. If these could be confirmed it would help focus future MEOR development activity.

Injected or indigenous microbes
A clear impression from the meeting was that reservoirs are complicated biological systems before we intentionally try to stimulate microbial activity, and that any microbes that are introduced would have to compete with the indigenous species. While it may be possible to show beneficial effects in laboratory conditions with tailored microbial cultures, in the reservoir these are likely to be out competed by the better adapted indigenous species.

Fate of injected carbon and oxygen
Some MEOR treatments require the use of sucrose or molasses as the carbon source. Ian Vance and Egil Sunde suggested that this would be quickly consumed by microbial activity either within the injection facilities or locally to the well in the reservoir. This would make it difficult to conceive of treatments providing deep emplacement of flow diversion agents based on injected carbon sources. Of course this would not be an issue for microbes using oil as their carbon source.

A similar issue arises when considering the aerobic process presented by Egil Sunde and Franz Gößnitzer. Reservoirs are generally reducing environments and low levels of injected oxygen will be very quickly consumed, even in the absence of additional microbial activity. It is difficult to see how the aerobic process would work in the field (in contrast to laboratory studies), unless chemicals are produced by the microbes near the injector that can propagate through and affect the bulk of the reservoir (see below).

Deep emplacement
Microbial activity around the injector may make deep placement of microbes and their products difficult, unless the necessary nutrients can be "packaged" (so that they are not consumed until they are deeper in the formation) or the unwanted microbes can be suppressed. An aspiration was to identify ways for microbes to generate EOR active chemicals at low concentrations that could propagate through the reservoir. If this could be done, the challenge would be to isolate and identify the chemicals, demonstrate their effectiveness in the laboratory and understand why they would not be lost too rapidly through adsorption to be able to propagate. Results from surfactant flooding work in the past is not encouraging in this regard, but may be nature is much cleverer than we have managed to be so far!

Scale up to offshore conditions
A key practical issue is the volume of chemicals that need to be injected offshore. Egil Sunde and Grethe Kjeilen both presented laboratory work addressing this question. An area that needs further clarification is the balance between throughput of nutrients and the timescale for microbes to metabolise these, and how to scale results at the laboratory to the field scale. Dario Frigo discussed the volumes of chemicals that might be required to achieve whole reservoir treatments highlighting that applications based on carbon added systems would be logistically very difficult. A useful reference which explores some of theses issues in more detail is "Reservoir Engineering Analysis of Microbial Enhanced Oil Recovery", S L Bryant and T P Lockhart, SPE 63229.

How do we know it is working?
A large number of microbial treatments have been applied worldwide in onshore reservoirs. Often this is late in field life, in very high water cut wells with the assessment of benefit of the treatment based on a decline curve analysis before and after the treatment (as illustrated by Lewis Brown in his presentation). The difficulty is then the degree of confidence in the predicted decline curves.

This issue is illustrated by production data from part of a North Sea field. Over a four year period the data shows a steady reduction in oil rate that can be matched with decline curve analysis. In fact the data can be matched quite satisfactorily with a range of models including exponential (22%/year), hyperbolic and harmonic decline curves.

However, each of these models gives significantly different predictions for the remaining reserves. In this particular case a detailed geological and simulation model had also been constructed and matched to the overall field history. When this is compared to the predictions of the decline curve analysis it is clear that the exponential decline model is too pessimistic, with the hyperbolic and harmonic models giving better fits. It is worth noting that while there can be an underlying physical justification for applying exponential decline to reservoirs under primary depletion, this does not extend to reservoirs under waterflooding.

This shows the difficulties in interpreting results, even with a good four year baseline history, unless a clear IOR signal is present such as an increase in oil rate, rather than just a slowing in the rate of decline. The situation offshore, with fewer wells and more changes in injection and offtake rates only complicates this. If the prize from MEOR is of the order of 3% STOIIP, as suggested by David Hughes, it may be difficult to show that MEOR is working. Perhaps it would help build confidence to look for other evidence of MEOR effectiveness, for example reduced remaining oil saturations or permeabilities from infill wells after microbial treatments.

Norne
A highlight of the day was the presentation on the Norne project, the first large scale offshore MEOR project. The workshop was shown some of the early results from the laboratory and the field. One of the general comments made during the day was the suggestion that MEOR may have been held back by a lack of openness. Perhaps Statoil will be able to break the mould by making sufficient information available for other workers to investigate this interesting aerobic MEOR technique.

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