Making calculations on fault planes with deviated boreholes

[Edward Hakanson]

Hi I have been exploring the Utilites module of Rockworks and the different calculators that it has in the Widgets and have not yet found out how to use rock works to do the following calculation. If I have a fault plane of known strike that I want to assign a dip and determine at what measured depth a well trajectory crosses the fault plane. A vertical well would be simple because I simply take the MD and TVD when I get to the fault trace bur as the dip increases I consequently increase or decrease the distance required to drill to get to the fault. This is similar to a true dip or apparent dip problem but I am not clear on how to do the operation. Basically what I need to get out of the program is the distance that the fault plane moves with depth at a constant dip and then take that distance and use it in the borehole survey report to determine the total measured depth required and the xyz coordinates where I can expect to intersect the fault. Any suggestions?

[JimReed]

Hi Edward,

As you've discovered there are no widgets to solve this problem. In theory, you could do it within the Borehole Manager, but that's a lot of hassle. Instead, I propose that we create a new widget along these lines:

Input:

- Collar X

- Collar Y

- Collar Z

- Borehole Dip Direction

- Borehole Dip Angle

- Fault X (any point along fault)

- Fault Y

- Fault Z

- Fault Dip Direction

- Fault Dip Angle

Output:

- Intersection X

- Intersection Y

- Intersection Z

- Distance From Collar To Intersection

- Vertical Distance From Collar To Intersection (Phrased such that it accommodates drilling upwards for underground applications).

If this sound right, we'll add it to RockWorks17 and get you an early beta/trialware release as soon as possible (say two or three weeks). If that doesn't work for you and you've only got one hole, just send me the input data and I'll send you the answer.

Best regards,

Jim Reed.

[Alison]

Another option (although maybe not quite as exact as you were looking for) would be to plot the 3D well trace with depth labels, along with the fault plane. You can create the fault plane as a grid through the Grid / Initialize program or as a ribbon through the Fault menu in the Utilities. The depth labels should give you a ballpark estimate of the measured depth at which the well trace crosses the plane.

[Edward Hakanson]

Hi Jim,

Thanks for your comments. Do you think that a program could be written such that it would be interactive and based on actual survey data or on a planned borehole path. The wells that we drill start out vertically down to the KOP (kick off point) depth and then we begin to construct angle up to a given maximum inclination which we hold on tangent to the end of the well. It seems that your suggestion would only work for a borehole with constant dip from the surface.

Maybe this could be achieved if you include the following parameters in the Widget: KOP, BUR (build up rate), azimuth (instead of borehole direction) and maximum inclination (instead of borehole dip). I also think that the reference x, y, z coordinates do not necessarily need to be collar coordinates but rather any x, y, z coordinates along the borehole path so that on the fly calculations like "how much further do we need to drill to get to the fault plane?" can be made easily.

Just out of curiosity, how would this be done from within the borehole manager, as you mentioned?

Regarding Alison's comment I like the idea because in addition to visualizing the length of the borehole needed to intercept the fault plane it would allow you to click on the 3D model at the intersection to get the corresponding x, y, z coordinates, although approximate. However I think that Alison's suggestion is best suited for creating a model once the initial calculations have been, say from within a Widget. Is there a way to add thickness to the fault trace so that it is visualized as a slab rather than a plane? I ask because generally faults have a zone of influence that varies depending on the magnitud of the fault.

[JimReed]

Hi Edward,

Doh! I was assuming that you were originally talking about a straight hole (think mining). Someday I'll learn to read more carefully.

Anyway, this is not a widget thing (although I still intend to add the program that I outlined). Instead, I'm thinking that this particular program merits a more serious program that we'd add to the Utilities / Survey menu.

Input:

- Downhole survey (planned or actual) file.

- Type of target object:

- Point: X,Y,Z

- Line: X1,Y1,Z1

- Plane (Fault): X,Y,Z,Dip Direction, Dip Angle

Output

- For each point within the well (say 1-foot increments);

- XYZ for well

- Bearing to closest intercept with object

- Distance to closest intercept with object

- XYZ for object intercept

Sound right?

Alison essentially covered the borehole manager solution that I was alluding to (i.e. doing it graphically).

Best regards,

Jim.

[Edward Hakanson]

Sounds like it will work Jim. Thanks for your attention to this matter.