Drillinginfo’s process for creating directional drilling surveys was detailed in Directional Drilling Surveys from Paper to the Web. Since it’s such a long, winding path to take directional drilling surveys from paper reports to the interactive map application on DrillingInfo’s 2.0 website, I thought a few aspects of our process deserved a deeper dive.
To map a directional drilling survey accurately we must glean the crucial information from the header of that well survey and apply it to the measurements that trace the well path from surface location to bottom hole.
Three Measurements: Depth, Inclination, and Azimuth
Measured Depth is usually reported (and thus processed) in feet. Inclination (angle) is in degrees, where 0° would be a completely vertical path, and 90° would be truly horizontal.
The third measurement, Azimuth, is the direction in degrees, but this value is dependent on the North Reference of the survey. This is where the header information comes into play and where we distinguish True Vs. Grid Vs. Magnetic North.
Image from https://www.scribd.com/doc/53131905/0105MAC05HO
True Vs. Grid Vs. Magnetic North
Roughly 80% of our gathered surveys were created using True North, so this is the standard we correct to. Only about 0.1% were recorded in Magnetic North, and our research and conversations with surveyors indicate that these reports are already corrected to True North. This is helpful because Magnetic North is variable, as it changes with time.This leaves us with 20% of well paths that are recorded using Grid North. So how do we get from Grid to True? True North is the direction to the north pole, a reference that is always … True! But Grid North begs the question: What Grid?
Coordinate Systems
Looking back at the headers will indicate the coordinate system used. For instance, the above survey report uses NAD 1927, State Plane, Texas South Central.
So in this example, we can use that State Plane zone when calculating our Grid Convergence (the difference between that zone’s Grid North and True North). But the directional drilling surveys are not reported consistently, often indicating just “State Plane”, or “NAD27”, or somehow lacking crucial information. Luckily, using GIS tools, we can spatially join the Latitude/Longitude of the survey to a map of the State Plane zones and properly assign a coordinate system.
Grid Convergence Angle & Rotation
The Grid Convergence Angle is a header detail that should tell us how to correct to True North, but is unfortunately another detail that can be incomplete, missing, or just plain wrong. So we calculate it ourselves.
It’s crucial that we know the rotation method used, one of the pieces of information that is typically missing from the survey report. If Graphic rotation is used, the angle is calculated counterclockwise, but if Geographic rotation is used, the angle is calculated clockwise. (There’s also a third method, Arithmetic, not detailed here.)
image photoshopped/modified by jsw from scribd.com link above
Let’s say the Grid Convergence is reported at +3° from True North, using Geographic rotation. In the image above, our Grid North would be the angle to the right of True (GN1), and we would adjust it to the left to reach True North.
But what if we didn’t know the rotation method, or it was reported wrong? If the survey was made using Graphic rotation at +3° (GN2) , and we corrected it assuming it was Geographic, we’d move even farther to the left and end up with an error of 6°.
Results
So what are the results of calculating the convergence correctly? Here we have DrillingInfo’s Landtracs layer in gray, representing lease coverages. Notice how the directionals lines travel right up to the lease-line.
Now, imagine that our bearings were off by a few degrees. We’d have surveys crossing lease boundaries and misrepresenting the true well path.
We have taken thousands of survey reports from countless reporting companies and established a method to analyze these surveys. We retain and clean the information we need, throw out the information that is not sound, and calculate our own measurements when necessary.
This important step in our process ensures that an operator’s precise drilling is accurately reflected in DrillingInfo’s directional drilling surveys product.
Your Turn
What do you think? Leave a comment below.
Justin Scott Winn
Latest posts by Justin Scott Winn (see all)
- DI Geodata Services – Connecting GIS Professionals to Mapping Solutions - January 20, 2015
- True North – Getting Our Bearings Straight When Mapping Directional Drilling Surveys - October 23, 2014
- Respecting Offset Well Clauses and Rule 37: Protecting Operators When Well Locations Change - August 5, 2014
