Deflection Detection in Horizontal Drilling and Geosteering

Deflection Detection in Horizontal Drilling & Geosteering

It is well known that when the inclination of the motor in a horizontal drill string approaches dip, there can be a tendency for the inclination to follow dip. This can be aided when drilling in a soft zone bounded by either one or two hard zones. In such cases the BHA can get locked into a certain stratigraphic position. Typically, the bit will be deflected off of hard zones within the overall transition. It may dig into the hard zone and bounce off at various gamma readings. These will usually be the same ledges but can vary quite a bit. The changing gamma in a long soft/hard transition that exhibits deflection can be difficult to correlate as these changes can give the appearance of moving through the section. Deflection is further compounded by local changes in the (continuous) inclination as the bit is deflected. As a result, the avg. inclination calculated over a given interval can be inaccurate. This may manifest as short-term changes in dip – usually up to a few degrees. These changes can be big or small depending on how much the bit is being deflected. Another factor is that the gamma may express differently depending how close to dip the inclination is running. From my experience the hard (lime) stringers in shale will generally read the same or cleaner when the inclination is closer to dip. This can also make exact correlation in the lateral difficult. It can even trick some geologists into believing faults have been crossed.

Although this is speculative it seems that wells in which  there are many turns (backturns to save VS and/or side turns to space laterals on multi-well pads) are most prone to getting locked into or between deflection zones. Actually this may be the case only in those wells where the overall torque and drag tends to aid the process of deflection.

In the past, I had worked on a well utilizing Schlumberger’s Power Drive Rotary Steerable tool. They were also equipped with a continuous inclination readout. In this well there was a hard lime stringer within the upper part of the target interval. Our usual plan was to break through that and drill in the center of the target interval. Unfortunately after many tries with the Power Drive we could not break through it. However, what we did see is the change in the continuous inclination readout as the bit was deflected off of the hard lime zone. Inclination before it hit was aimed down – lower than dip, and inclination after it was hit was aimed high – higher than dip. This occurred multiple times and the evidence was rather irrefutable that this happened. We tried a few times to come up three or four feet above the zone and slam through it with a lower inclination. It nearly worked but not this time. This is the usual procedure for such situations and it usually works – but not this time. The end result was that the entire well was drilled above the lime stringer – within the target interval but not in the preferred spot within the target interval. This zone was thin enough that it was not really considered to be a frack barrier but that is a possibility.

In any case, geosteerers should be aware of the possibility of deflecting off of hard zones if:

1)    the inclination is very close to dip and

2)    there is a hard zone and soft zone interface and

3)    there is torque and drag in the hole due to multiple turns

Example of SES “look” when bit deflects (in this case off of low side of lime stringer (up in green, then deflect down in yellow, then back up through the zone in blue. In reality the dip from 9530 MD to 9600 MD is more constant than gamma matching shows. Such a zig zag configuration after gamma is matched is thus a “signature” of deflection. 

                           
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