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Aubrey Whymark BSc MSc FGS
Wellsite Geologist, Geosteerer & Biosteerer
GEOSTEERING
What is Geosteering?
Geosteering is the science, or art, of maintaining a near horizontal well bore within a pre-defined and often thin geological layer. The geosteerer 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 geosteerer must fulfil the client’s specific trajectory requirements, which may have dogleg or inclination limitations, or hard ceilings / floors defined by fluid contacts.
 
Geosteering can be divided into three sections: 1) planning stages, 2) the build-up and landing section, 3) the horizontal section. All are of equal importance for the success of a well. There are two basic methodologies in geosteering, one that can be applied where significant uncertainty exists and one, more scientifically, that can be applied when reasonable predictability exists.

​To learn about geosteering in greater depth, please download my Basic Guide to Geosteering:
FREE Basic Guide to Geosteering
Rev. 1.1
Figure 1:  The wellpath is adjusted, based on all available log and deviation data, 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 based on LWD data, MWD data, sample / lithological data and all other data to hand.

  • The primary aims of geosteering are to maximise exposure to optimal target porosity and avoid potential problems like over-pressured shale, watered-out horizons, tight formation, etc.
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  • The methodology of job can be tailored to the field or formation.

Why use Geosteering?
  • ​Geosteering maximises the time spent in the optimum target porosity. Seismic and computer models offer a broad guide but without geosteering, relying on models alone, only thick targets can be successfully followed. If attempting to drill a well in a 3 ft or 10 ft target the chances of success without a quality geosteerer would be low.

  • ​Geosteering reduces drilling time as more time is spent in fast target ROP, less time in tight layers with slow ROP.

  • ​Geosteerers mitigate against issues in the well, reducing the risk of stuck or lost BHA's, reducing the need to sidetrack, reducing the potential for penetration of water-bearing layers, mitigating against fault risk, etc.

  • The specialist geosteerer usually doubles up as a wellsite geologist, offering value for money and ensuring all geological tasks are accomplished to a high standard.

  • ​Geosteerers are a focal point for the all data acquired during drilling. They assimilate, interpret and communicate these data. An experienced geosteerer extracts the maximum value from the data acquired.

  • The cost of 1 or 2 geosteerers is minimal in comparison to the cost of a well or even the cost of LWD acquisition. Their presence will significantly contribute towards a successful well drilled in target and on time. Failure to have the highest standards of geological input can very significantly add to the cost and time taken to drill a well. For instance, a lost million-dollar tool, the sidetracking of a well, a well drilled in the wrong layer, abandoning a well through lack of understanding are just three examples of issues I've seen where geological standards are sub-optimal.
How to Geosteer?
  • ​For more depth, please refer to my free Basic Guide to Geosteering.

  • ​When setting up a field for geosteering, one needs to review the geology, gain as much structural understanding as possible and plan wells in a fashion that maximises success (usually this means broadly drilling along the strike of the bedding). One needs to review geological data and choose the LWD data required to do the job of steering (each field and target is unique in this aspect) whilst at the same time minimising cost (some tools are costly to run but may add little value). The RSS and bit need to be chosen with geological considerations in mind. If necessary, pilot studies may need to be performed, that might review lithologies or core samples to assist geosteering.

  • ​For each well drilled, planning is critical. It can be done by the operations geologist, wellsite geologist or be computer assisted to save time. Offset wells must be reviewed. Layer thicknesses and distances to the target top from numerous markers must be calculated. Issues in offset wells must be evaluated. Well plans must be reviewed. Potential problems must be identified.

  • The well must be successfully landed. This requires careful monitoring and projection. A cross-section must be built up, either manually or using specialist geosteering software. The structure is understood, and the landing point is adjusted to land in the optimum porosity.

  • The horizontal section is where the geosteerer really comes into play. As drilling proceeds the geosteerer is making a cross-section and continually calculating apparent bedding inclination. These data, combined with modelled and geophysical data enable the geosteerer to make logical decisions on the way ahead: The likely position of the target and the likely apparent bedding inclination ahead. The geosteerer is constantly adjusting the desired bit inclination to stay within the desired target porosity. 

  • At the end of a well, a report is written, learning points established and evaluations performed.