Answers to test on Directional Surveys

ANSWERS TO A TEST ON UTILIZING DIRECTIONAL SURVEYS

1) A well is deviated 5^o from surface to bottom hole. Its measured depth is 10,000'.

Question A) What is the well's TVD?

a) 9961
b) 9826
c) 9653
d) 9215

9961' only a 39' difference.

Question B) What is the total offset for this well?

a) 120'
b) 234'
c) 578'
d) 871'

871'

2) If you are currently working on a 3D interpretation, what is the projection the 3D project was loaded in?

You're asking me?

2) What is the definition of True (Geodetic) North?

True north is the direction of the meridians (lines of longitude) of the earth. They converge at the north and south poles where the earth's rotational axis intersects the earth's surface.

3) What is the definition of Grid North?

In a map projection, Grid North is parallel to the direction of the Y axis.

4) Is there any place in the Gulf of Mexico where True North and Grid North directional survey coordinates (X-Y offset values) are the same?

Yes, along the central meridian of a map projection. Elsewhere they are not the same. The amount of difference between the Grid and True North coordinates will vary according to map projection, distance from the central meridian, and latitude. These difference can be significant.

5) You are a geologist doing an interpretation on a block in High Island SE. A well with a 5000' of closure does not tie with the seismic data. You are ready to have it out with your geophysicist. Your mapping expert suggests that you check the north reference for the directional survey. You find that the north reference is not specified.

Question. For this well, what is the possible magnitude of error in location at bottom hole, if the north reference is not assumed correctly?

a) 2-3'
b) 20-30'
c) 200-300'

Depending upon where you are, you could be off by as much as 200 - 300 feet.

6) In the Gulf of Mexico today, most contractors adjust the survey azimuth and X-Y coordinates to:

a) True North
b) Grid North
c) Magnetic North

b - Grid North. Today this is the dominant reference. Historically however, up to 20% of the surveys are referenced to True North.

7) For the onshore United States, most contractors adjust the survey azimuth and X-Y coordinates to:

a) True North
b) Grid North
c) Magnetic North

a - True North. This is the opposite of offshore areas. Most international data is referenced to True North also. This data must be converted to Grid North before if can be used by most mapping programs.

8) You are mapping horizontal drilling in the Austin Chalk using Z-Map. You need to load the directional surveys. Would they be loaded:

a) As is.
b) Require some corrections prior to loading.

b - Onshore directional surveys are usually referenced to True North. Mapping programs however, require Grid North data. As a result the data will most likely need to be converted from one reference system to the other.

9) What percent of wells to be loaded into an onshore Louisiana 3D seismic project will require modification (if any)?

Most wells will probably need to be adjusted. 3D seismic projects use a Grid North reference system and most onshore wells are initially referenced to True North.

10) What is the definition of magnetic declination.

It is the angular difference between the geodetic meridian (True North) and the local magnetic meridian. The magnetic pole and the rotational pole of the earth are not coincident. The magnetic declination varies from place to place and with time.

10) In the Gulf of Mexico, the magnetic declination is:

a) East
b) West

Today most areas of the GOM are east (+, positive value). East of Viosca Knoll the magnetic declination is west (-, negative value). In 1960 the entire GOM was east.

11) You have a 3D project overlapping the High Island Southeast and West Cameron South areas. Since each side has its own map projection, what projection should you use for the 3D project?

a) Use Texas South Central on the Texas side and Louisiana South on the Louisiana side.
b) Use Texas South Central.
c) Use Louisiana South.
d) Use UTM zone 15.
e) Either b,c,d.
f) All of the above
g) None of the above.

e - All of the data must be in the same projection. It doesn't matter which projection you use as long as you use only ONE projection.

13) If you used Louisiana South, do any of the directional surveys need adjusting prior to loading the 3D project?

a) All of the surveys.
b) Louisiana surveys.
c) Texas surveys.
d) None of the surveys.

c - The Texas surveys are probably referenced to the Texas South Central, State Plane Coordinate Zone, and will have to be converted to the Louisiana South, State Plane Coordinate Zone.