Unit 12 - Directional Surveying

Question 1

Why do we need directional surveying?

Answer 1

• when we drill a directional well, the actual trajectory of the well must be regularly checked to ensure that it is in agreement with the planned trajectory

→ this is done by surveying the position of the well at regular intervals

Question 2

What are the informations you need to determine the coordinates of all points in the well trajectory?

Answer 2

• position of initial refernece point (Rotary Table)
• measured depth (AHD) of survey station
• direction (Degrees from North) in which wellbore is oriented at survey station
• inclination (degress from vertical) of wellbore at survey station
• mathematical model of wellbore trajectory

Question 3

What are the three azimuth reference systems for directional surveying?

Answer 3

• Magnetic north
• True (geographic north)
• grid north

Question 4

What was the magnetic single shot used for ?

Answer 4

• Measuring inlcination and direction of a well

Question 5

Magnetic single shot consists of 3 section… Name them!

Answer 5

• An angle unit consisting of a magnetic compass and an inclination measuring device
• A camera section
• A timing device or motion sensor unit

Question 6

• the angle unit of a magnetic single shot consists of what?

Answer 6

• magnetic compass and plumb bob

Question 7

How does a magnetic single shot work?

Answer 7

• when tool is in correct position, compass is allowed to rotate and align itself with the earth’s magnetic field
• plumb bob hangs in vertical position no matter how the instrument is deviated in the hole
• when in the correct position, lightbulbs are illuminated and a photograph image of the plumb bob is superimposed on the compass card

Question 8

What is the difference between a magnetic single shot and magnetic multi-shot

Answer 8

• magnetic multi shot has works on same principle but has a special camera unit → a roll of film is automatically exposed and wound on at pre-set intervals

Question 9

Why is the multishot only taken, when the drillpipe is tripped out of the hole?

Answer 9

• because it must remain in the non-magnetic collar→ otherwise inaccurate measurements

Question 10

Why can you only do magnetic surveys in open holes?

Answer 10

• because steel casing also becomes magnetized

Question 11

Principle of a gyro single shot?

Answer 11

• a gyroscope is rotated by an electric motor at approximately 40,000 rpm
• on surface gyroscope is lined up with a known direction (True North) and as the tool is run in hole, the axis of the tool should continue to point in the direction of true North
• a compass card is attached to, and aligned with, the axis of the gyroscope and acts as the reference direction from which all directional surveys are taken
• Once the tool has landed in the required position in the drill collars the procedure is very similar to that for the magnetic single shot
• since compass card is linked to the axis of the gyroscope it records a True North bearing which does not require correction for magnetic declination

Question 12

Why do gyroscopes have to be retrieved from a wireline?

Answer 12

• gyroscopes are very sensitive to vibration

Question 13

What are the two particular sources of error, that occur when using photographic instruments?

Answer 13

1. Instrument error
2. Reader error

Question 14

What is an instrument error?

Answer 14

• due to inaccuracy of the device itself → infrequent calibration and damage caused to the instrument

Question 15

What is a reader error?

Answer 15

• developed film is easily mis-read

→ some discs may have to be magnified to be read properly

• readings should be verified by another person ( but is rarely the case on rig)

Question 16

What is the disadvantage of a gyroscope ?

Answer 16

• tendency to drift off line
• risk of damage due to vibration during drilling

Question 17

What is the function of a magnetometer?

Answer 17

• measures intensity and direction of earth’s magnetic field

Question 18

What is the function of an accelerometer?

Answer 18

• measures earth’s gravitational field

Question 19

What is a steering tool?

Answer 19

• kind of tools specifically designed to orientate deflecting tools and monitor the well’s progress during a correction run

Question 20

What is the problem with steering tools?

Answer 20

• they are on a wireline and drillstring can not rotate while you send signals → can only be used when a mud motor is being used to make the correction run

Question 21

How do you call the downhole component of the steering tool?

Answer 21

• a probe

Question 22

What are the advantages of using a steering tool instead of using a photographic instrument for orienting and surveying?

Answer 22

• Saves rig time due to:
• sending results to surface more quickly
• fewer attempts required to get orientation correct
• allows a correction run to be completed in shortest possible time
• . Better directional control of well path due to continuous monitoring
• Able to monitor the orientation of deflection tool during drilling.

Question 23

What is the main disadvantage of using a steering tool?

Answer 23

• due to the wireline, steering stools cannot be used in conventional rotary drilling – only with a mud motor