Fishing Technology Manual
Determining the Amount DOK Required to Reach the Fish
Determining the Amount DOK Required to Reach the Fish
Overview
You will need to determine the amount down on the kelly (DOK) required
to reach the fish in order to design your fishing string. Because you can’t
always rely on the string figures furnished by the customer, it is best to
personally check the figures before beginning operations.
to reach the fish in order to design your fishing string. Because you can’t
always rely on the string figures furnished by the customer, it is best to
personally check the figures before beginning operations.
The In and Out Method
The “In” and “Out” method is a good way of checking the string figures for
possible mistakes. The formulas used to calculate the total out and total in
and then the amount DOK required to reach the fish are shown below.
possible mistakes. The formulas used to calculate the total out and total in
and then the amount DOK required to reach the fish are shown below.
Note: The total “out” must always exceed the total “in” to use this formula.
Example
The following is an example using the “In” and “Out” method for
calculating the depth of the top of the fish.
The customer twisted off while drilling at 4,000 ft. They recovered
3,895.73 feet, including the length DOK. The customer then made a bit trip
and tagged the fish with 37 feet. DOK. To calculate the difference between
the bit run and the following overshot run, subtract the (in) from the (out).
The difference will be the length DOK with overshot run.
See the calculation of the depth of the top of the fish for this example in
Table 1-2
calculating the depth of the top of the fish.
The customer twisted off while drilling at 4,000 ft. They recovered
3,895.73 feet, including the length DOK. The customer then made a bit trip
and tagged the fish with 37 feet. DOK. To calculate the difference between
the bit run and the following overshot run, subtract the (in) from the (out).
The difference will be the length DOK with overshot run.
See the calculation of the depth of the top of the fish for this example in
Table 1-2
Determining the Kelly Bushing (KB) Measurement
Calculating the original elevation
The standard oilfield measurement to any depth downhole is recorded from
the wellhead, plus the distance to the kelly bushing (KB) on the rig that
drilled the well. This is called the original elevation and is recorded in the
well file.
It is important for everyone to be talking about the same depth. That is why
the total depth is always reported from the original rig elevation. If the
original rig has been replaced by a smaller workover rig, you will need to
account for the difference in this rig’s elevation from the original elevation.
the wellhead, plus the distance to the kelly bushing (KB) on the rig that
drilled the well. This is called the original elevation and is recorded in the
well file.
It is important for everyone to be talking about the same depth. That is why
the total depth is always reported from the original rig elevation. If the
original rig has been replaced by a smaller workover rig, you will need to
account for the difference in this rig’s elevation from the original elevation.
Determining the true well depth
The difference between the original rig elevation and the current rig
elevation is added to or subtracted from the current pipe tally in order to
establish the true well depth. This is called the kelly bushing (KB)
measurement.
For example, assume that the original elevation of a well is 30 feet. The
current elevation is 6 feet. If you start running pipe from the current
elevation, you are already 24 feet into the well. Therefore, in order to reach
a fish that is at a depth of 4,500 feet, you will need to run pipe 24 feet
shorter than 4,500 feet because the depth is measured from the original
elevation (24 feet higher).
elevation is added to or subtracted from the current pipe tally in order to
establish the true well depth. This is called the kelly bushing (KB)
measurement.
For example, assume that the original elevation of a well is 30 feet. The
current elevation is 6 feet. If you start running pipe from the current
elevation, you are already 24 feet into the well. Therefore, in order to reach
a fish that is at a depth of 4,500 feet, you will need to run pipe 24 feet
shorter than 4,500 feet because the depth is measured from the original
elevation (24 feet higher).
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