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Permalink Reply by j garrett on January 18, 2010 at 13:39

Hmmm.... No chance they are off 19 ft?

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Permalink Reply by Skip Peel - Mineral Consultant on January 18, 2010 at 14:20

j garrett, have you checked to see if Texas well files include Directional Surveys. They are mandatory in LA. and are included in the well file of all completed wells. The ones that I run across are titled "A Gyrodata Directional Survey" and give values for Measured Depth, Inclination in degrees, Azimuth in degrees, Bore Hole Bearing in degree minutes, Dogleg Severity in degrees per 100', Vertical Depth in feet, Closure Distance Azimuth in feet per degree and Horizontal Coordinates in feet. Readings are taken approximately every 100' in Measured Depth. You may need professional assistance to interpret the data but I suspect that it is quite accurate.

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Permalink Reply by TheCommish on January 19, 2010 at 8:19

Texas has an "unoffical" varience of 10% of the lease line spacing, to remain a legal location on a horizontal. For instance, if the well-to-lease spacing is 467 ft minimum, you are allowed 47 ft wiggle as long as the variance is "unintentional" and random. If you steer the hole that way, you are busted. You won't find this in the rules but it is policy per what is known as the "Schiek memo".

Louisiana does not allow one foot of variance. You miss a permitted BHL by one foot and you are required to repermit the well to reflect the actual bottom hole location. That's because they are perfect :)

Also you should know all directionals are assumed to have a cone of error, based on depth, temperature, technology (totco vs. multishot vs. gyro, etc). But most legitmate operators try to do the right thing.

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Permalink Reply by Henry on January 19, 2010 at 1:19

Electrodynamics -- GPS --Two miles underground?

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Permalink Reply by Mac Davis on November 10, 2010 at 22:00

It's Global Positioning System, not Satellite. However, lots of idiots in the media get it wrong all the time, so the wrong term is getting more and more common.

GPS doesn't work underground. A 1 GHz radio signal doesn't travel to any significant depth underground. Even leaves overhead may block the signal.

GPS may be used to determine the surface hole location and the underground measurements are relative to that point. Other surveying means could be used to locate the surface hole location as well. I believe that most or all of the underground measurements will be relative to the surface location of the well.

There are tools to measure the borehole location continuously as you drill the well. You can run a separate survey tool after drilling is finished for a more accurate measurement. Or stop drilling temporarily, pull the drill string, and run the survey tool down and up the bore, then put the drill string back down the hole and resume drilling.

You need to run a separate survey to get accurate results because an accelerometer or gyroscope always has some drift over time. You can run the tool down the hole and back up quickly vs. taking weeks as you drill the hole.

The technology used in these survey tools is amazing to me. I find it really hard to believe you can do this sort of stuff accurately with accelerometers, but apparently, you can.

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Permalink Reply by jbob5570 on January 19, 2010 at 13:57

There is an "ellipsoid of uncertainty", but certainly not likely to be an error of 18'. Most well plans will allow +/- 50' from well center and there is a 330' offset on both sides of the unit line, giving you a minimum of 660'. Crossing unit hard lines is absolutely not allowed. MWD tools get direction from magnetometers and inclination from accelerometers arranged in an "x,y,z" pattern and the information is sent via mud pulse telemetry. Most mwd tool mfg will specify an accuracy of +/- 1 degree but they're actually more like .5 degrees. Every single well I've been on in the last 1 1/2 yrs here has been drilled in a north/south direction which increases accuracy since you're drilling in line with the earth's magnetic field lines rather than across them. (east/west)

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Permalink Reply by Greg Cellos on November 10, 2010 at 5:09

An error model could be used to generate an ellipse of uncertainty with some statistical confidence assigned (typically 2 sigma or 95% in 1D) given all the details. Without specifics on the survey tool(s) run or the well profile and environment, your question can only be answered generically. For approximation purposes, MWD systems typically perform with 3- 4 parts per thousand (along hole or measured depth) of lateral error. And note this is a half-axis value. The very best gyro survey instruments perform at 1.5 - 2 parts per 1000. All lateral position uncertainties grow with depth and inclination as azimuth errors manifest.

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Permalink Reply by j garrett on November 10, 2010 at 5:20

The Operator filed an "As Drilled" plat showing they are 454' from the property line, 64' different from their drilling permit.

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