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Aubrey Whymark BSc MSc FGS
Wellsite Geologist, Geosteerer & Biosteerer
BIOSTEERING
What is Biosteering?
Biosteering is the science, or art, of maintaining a near horizontal well bore within a pre-defined and often thin geological layer utilising occurrence / absence, abundances and ratios of different species of microfossils. In effect it is a specialist form of geosteering, or geosteering 'with an extra string to the bow'. The biosteerer must micro-adjust the well trajectory from the original plan, such that it is maintained within the pre-defined target for the greatest horizontal length possible. At the same time, the biosteerer must fulfil the client’s specific trajectory requirements, which may have dogleg or inclination limitations, or hard ceilings / floors defined by fluid contacts.
 
Biosteering requires no (or fairly rudimentary) specialist equipment and a miniscule sample. It does, however, require a pilot study for evaluation and the use of a skilled micropalaeontologist / nannopalaeontologist or palynologist. The decision to utilise biosteering is a decision to enhance target exposure and mitigate against risk. It should be thought of in the same sense as adding an image log to the BHA, for instance. It is an extra tool, tailored for a particular field / target. In the correct circumstances it can be a very powerful and cost effective tool.

Figure 1:  The wellpath is adjusted, based on abundances / ratios and presence / absence of microfossils, to stay within the target horizon. In this case the wellpath (blue) initially goes too high, which is identified by a trend in log data. The bit inclination is dropped, and the base of the target is identified on log data. Bit inclination is gently raised, and it is attempted to follow the apparent bedding inclination, but a fault is then crossed.  It is necessary to raise the inclination reasonably aggressively. A second fault is crossed, and it is then necessary to drop the inclination aggressively to regain the target. 
Key Points
  • Steering horizontal wells primarily based on the microscopic fossils in the rock. Other data, such as LWD data, are routinely combined with the palaeontological data before steering decisions are made. The primary aims are to maximise exposure to the target porosity and to mitigate against issues such as faulting, unpredictable structure, and exposure to problematic features such as over-pressured shales.

  • Fossils allow determination of stratigraphic position (above or below target porosity). 

  • Biozonation schemes are based on species occurrence / absence, abundances and ratios between different species. Schemes usually work on a sub-evolutionary timescale and are controlled by environmental changes, which may in part be controlled by astrogeological climatic forcing.

  • Usually incorporates other geological information such as microfacies and lithology.

  • Usually allows units and formations do be subdivided to a much higher resolution than would be possible based on log or lithology data alone.

  • Should be used in conjunction with all available MWD and LWD data to maximise its effectiveness (think of it as an extra down-hole logging tool, not in isolation).

Why use Biosteering?
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  • It is relatively inexpensive compared with having a larger suite of logging tools in the hole to determine the stratigraphic position.

  • Can provide answers prior to the area being crossed by the LWD tool (which may be significantly offset from the bit).

  • In slim-hole wells, LWD tools may be limited to gamma only (with a pseudo idea of density/porosity derived from ROP data).

  • Microfossils can be considered as an extra tool from which to interpret stratigraphic position relative to the target.

  • An effective tool to identify faulting, magnitude of faulting and to re-evaluate target position.

  • If image logs / directional LWD data are not available, perhaps the only solution if log responses appear identical when exiting the target from both the top and the base.

  • Only a tiny sample is required. Often less than a thimble-full of sample, a few grams, is sufficient. Some areas may require marginally more, say 20 grams. This would be determined in the pilot study. 

  • Save $$$$$$$$$$, Make $$$$$$$$$$! Helps mitigate against the risk of losing BHA's. Ensures maximum possible exposure time in target porosity. This hopefully boosts production and saves drilling time, by drilling in fast ROP and reducing the number of sidetracks.
How to Biosteer?
  • ​​First carry out a pilot study (ideally at least 3 months in advance of drilling) to assess viability. This will use previously drilled vertical and horizontal wells, cuttings and core samples (if available). The target interval, together with the units above and below the target will be studied. Without a pilot study the success of initial wells will be limited.

  • The pilot study will determine which microfossil discipline (or combination of) is most appropriate for the area. Disciplines commonly used in industry include palynology, nannopalaeontolgy and micropalaeontolgy (thin section and washed residue). Not all areas are suitable for biosteering. Problems occur when no fossils are found, no clear and consistent changes are observed, or cyclic units are encountered.

  • On completion of the pilot study, a biozonation scheme is produced that will constantly be enhanced as further wells are drilled.

  • Biosteering generally requires 24-hour coverage and thus a two-man crew is usual. Without constant monitoring the lateral may go astray when the biosteerer is sleeping. Furthermore, in some areas, interpreting the stratigraphic position can prove problematic without monitoring of the entire succession.

  • Biosteerers require only the minimum of space. A sink to wash samples and a desk (1 metre) for a computer and microscope. In some cases, such as thin section preparation, marginally more preparation space (+1 metre) is required.

  • Dependent on the job, biosteerers can work with a wellsite geologist or remove the necessity for a wellsite geologist (with the biosteerer also carrying out the wellsite geologists tasks), thus saving space and money.

  • The biosteerer actively liaises with the company representatives, engineers, directional drillers and wellsite or operations geologist to make the correct steering decisions (which are not always purely geologically based).

  • Success and newly acquired data are constantly reviewed and recorded. This builds on collective experience and ensures learning is carried forward to enhance future wells.