It is very important to determine a downhole location below which a pipe string cannot be pulled or "backed off". This location is called the free point.
Some free point methods use downhole electric tools to reveal conditions indicating stuck or free pipe.
Other methods involve applying a known lift force to the pipe at the surface to detect whether or not the pipe moves downhole. Casinjacs and electric wireline tools use this method.
Electric wireline tools place a sensor at various locations downhole. The sensor detects pipe move ment at these locations in a short segment of the pipe string. The sensor output varies from that of free pipe to partially stuck pipe or stuck pipe.A specified pull force applied to the pipe causes pipe movement.
Casinjacs measure the stretch of the entire portion of the pipe that stretches when a known stretch force is applied. Proper technique involves pulling the string weight and then applying three equal successive stretch forces. The amount of stretch resulting from each stretch is recorded and three free point calculations are made using the following formula:
free point = pipe wt./ft. x stretch (ins.) x 735,000
stretch force (lbs.)
Example: 17 x 25.75 x 735,000 =
7,100 ft.
45,240
45,240 = 400 psi x 113.1
113.1 = area of 4 - 6" bore jack cyls.
The three freepoint calculations are considered and the following conclusions can be reasonably drawn:
- Freepoint depths are equal or nearly so -
The pipe is stuck at one location.
- Freepoint depths are sucessively greater -
The pipe is partially stuck at intervals but the increased lift forces pull pipe through the partially stuck intervals.
- Freepoint depths are sucessively smaller -
The hole is is crooked to the extent that the pipe becomes wall stuck further up hole as the stretch forces increase.
The results of applying successive stretch forces up to the safe pull capacity of the pipe string points out a fundamental probem with freepoint techniques used by electric wireline companies.
Prior to undertaking any method of freepointing, the pipe string should be worked to free up as much pipe as possible. The pipe should be " worked down" before freepointing.
When freepointing combination pipe strings (strings with varying weights of casing) it is necessary to calculate the stretch of each interval and weight resulting from a given stretch force.
A " Free Point Manual " is in process. As soon as it is available it will be announced.
For a spreadsheet to be used to calculate three consecutive free points see Free Point Calculator. Your browser should work with one of these spreadsheets.
Also the equation can be used by inputing values in the Free Point Formula shown above.