Geosteering
WHAT IS GEOSTEERING?
The wellpath is adjusted, based on all available log and deviation data, to stay within the target horizon. In this case the wellpath (black and white) goes too low, which is identified by a trend in log data. 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 log data, drilling parameters and lithology. The wellpath is steered back down into the target horizon.


Geosteering




KEY POINTS:
  • Steering horizontal wells based on LWD data, MWD data, PWD data, sample/litho logical data and all other data to hand.
  • The primary aims of geosteering are to avoid danger areas, such as overlying or underlying shales, and to maximise porosity exposure.
  • The methodology of job can be tailored to the field or formation.


WHY USE GEOSTEERING?
  • Real-time adjustments to the well trajectory are essential for successful drilling. Seismic and static models provide a broad overview, but detailed review is essential to stay within target porosity.
  • The geosteerer is also the acting wellsite geologist, reducing the requirement for extra specialist personnel. Geosteerers take up very little office space.
  • Log data allows precise along-hole positioning of events, such as faulting and unit penetration, allowing for detailed and accurate predictions of surfaces to be made. 
  • Detailed review and understanding of log data allows recognition of minor faulting and discontinuities, sometimes on a sub-biozonal scale.
  • Geosteering is not significantly limited by poor or absent samples.
  • A good geosteerer can save money and make money! Geosteering mitigates against the risk of losing BHA's. It also 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 GEOSTEERING
  • To maximise the effectiveness of geosteering, field data should be made readily available. Ideally all log data, survey data and pre-existing structural interpretations should be made available. A pre-drilling data review should be carried out to standardise log picks, assess lateral variation of log response, thickness variation, likely faulting trends and collision risks. At a minimum, adjacent offset well data should be made available for correlation and mapping (if not already carried out). This task might take from hours to weeks, dependent on the pre-existing data quality.
  • In time for the pre-spud meeting, all offset data should be studied to identify potential problems before they arise.
  • Geosteering generally requires 24 hour coverage and thus a two man crew is usual. This allows immediate recognition and correction of problems. Certain jobs, particularly with a lower ROP, can be drilled with one geosteerer. 
  • Geosteerers require only the very minimum of space.
  • The geosteerer 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.


ABREVIATIONS USED:
MWD= measure whilst drilling
LWD = log whilst drilling
PWD = pressure whilst drilling
BHA = bottom hole assembly
ROP = rate of penetration.