BIOSTEERING
What is Biosteering?
The wellpath is adjusted, based on microfossils, to stay within the middle target horizon. In this case the wellpath (black and white) goes too low and is picked up on fossils. The bit inclination is raised, but shortly after re-entering the target the well hits a down-thrown reverse fault, placing the wellpath above the target. This is recognised on fossil data and the wellpath is steered back down into the target horizon.


Biosteering

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 avoid danger areas, such as overlying or underlying shales, and to maximise porosity exposure.
  • 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.
  • 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 it's effectiveness (think of it as an extra down-hole logging tool, not in isolation).

WHY USE BIOSTEERING?
  • 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 50ft behind 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.
  • Sometimes 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 GO ABOUT BIOSTEERING
  • 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 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 liases with the company representatives, engineers, directional drillers and wellsite geologist in order 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.


Abbreviations Used:
MWD = measure whilst drilling
LWD = log whilst drilling
BHA = bottom hole assembly
ROP = rate of penetration.