11-art and science of geosteering

The Art and Science of

GeosteeringTom Arnold, Director of Training& Co-Founder of the GEOGAP Training Center

•Brief history of geosteering•The Science •The Art

…What We Will Cover

History

Excel Spreadsheet [1990’s]

•Data Input manually.

•Calculations provided wellbore plot.

•Mudlog data provided indication of wellbore location relative to target.

21st Century Software

•Powerful correlation and interpretation tools

•Excellent view of the geologic setting

•Provides fast turn-around for reporting

•Direct import of necessary data

The Science

When Everything Works Right….

When the gamma data is good and matches the offset, life is good. The bad news is that it is often not the case.

Lay-over correlation

The wellbore remains near the center-line of the target zone.

Woodford Shale At its Best….

Good Carbonate Signature: Mississippi

Shales provide the best signatures for geosteering, as a result of the highest concentration of radioactive minerals.

Seen above, the gamma signature has plenty of character to use for correlation.

Good Gamma Ray

When Faulting Occurs… It Gets Interesting!

start

end

major change

resultWithout the aid of software, determining the throw of the fault would require careful analysis of the drill cuttings. Even then, the exact displacement would be in question. But with good software the value is known to the foot.

When correlation falls apart, the first thing you should look for is a fault. Move the data around and see where it might fit; either up or down.

start end

Recognition of Faulting

Bed Thickening

Thickness variations happen all the time. Therefore it is important to be able to recognize them when they occur. This change greatly effects the relationship between the wellbore and the target zone.

Bed Thickening

Eroded Top: Mississippi Formation The wellbore entered

the top of the Mississippi as it emerged from the unconformity, showing a normal transition between shale and carbonate. Then after drilling several hours, the wellbore entered the eroded surface indicating shale once more.

Know your geologic setting and never loose sight of the BIG PICTURE! Otherwise this could have been miss-interpreted as a fault.

Mississippian Carbonate Signature

What appears to be noisy gamma, may indicate fractures and micro-faults within the formation.

..many are present but a few are pointed out.

Delaware Basin Turbidities

Gamma indicated significant shift in lithology. Drill cutting confirmed the same, showing extensive clastic detritus.

The Smaller the Pay Zone, the more Critical the Fault

fault

compression fold

4’ fault begins

fault ends

With small pay zones, a fault of only a few feet can make a big difference. Couple that with increased dip angles and folding, makes interpretation while drilling a nightmare. Without good software to keep the wellbore in zone and either knowledge of the area or experience to assist, the productivity of the well can come into doubt.

Even without geologic changes, other factors effect the wellbore; directional drillers, drill motors, anisotropy, other equipment, etc… These situations lead to porpoising of the wellbore or drilling out the pay zone . This can mean having difficulty during completion.

Other Factors

porpoising

The Art

Bad Data

Gamma ray data from an offset well may be damaged or suffer from poor calibration.

Bad data from either the LWD gamma ray or from the offset well moves both correlation and interpretation into the realm of Art.

Seen below are examples of data from the LWD gamma ray where the tool has failed

Often times all we have is the general shape between the LWD and offset gamma to work with.

Since carbonates have such a low gamma signature, how is correlation possible?

Geosteering in a Carbonate

offset

LWD

Other times we have nothing more than an educated guess. The ‘educated ‘ part comes from experience.

Bakken

Hunton

The Final Product: Complete Cross-Section Accurate Geosteering requires

being able to see the ‘Big Picture’. That means the ability to look beyond the LWD data. When correlations come into question, consider the lithology and gas data from the logger. Account for other information from LWD sensors like resistivity, azimuthal gamma ray, neutron density and others if available. When you add in experience, this is the point where both science and art merge.

The assimilation and analysis of all data available provides the most accurate correlations and are necessary to assure the successful completion of a horizontal well.

A Question of InterpretationAmerican Judge…..that was a

9.8

British Judge: No …it was only a 9

Cuban Judge: … 8.2It is all a question of subjective interpretation based on background and experience. Given this complexity, the more people you have evaluating the ‘performance’, the better the judging…. just like figure skating.

The more ‘eyes’ on the ‘performance’, the less chance of missing something important.

The End

Questions?