Accurate free point calculation method and its application

In the process of drilling, formation factors or misoperation can lead to incidental pipe sticking, which not only extends the drilling cycle, but also may cause secondary failure and even pipe burying and other accidents, resulting in great economic loss. At present, the pulling method adopted on site to calculate free points doesn’t consider the influence of friction drag applied on downhole pipe strings, which increases the calculation error of free point depth with the increase of the friction drag, so it is hardly applicable to high-friction vertical wells and high-angle deviated wells. In this paper, the free point depth calculation formula which can eliminate the influence of the friction drag applied on the pipe string inside the casing was derived by making use of the performance that the friction drag applied on the downhole pipe string under the same hook load in the process of pulling and lowering is equal in value and opposite in direction, according to the free point depth calculation principle and the Hooke’s Law. What’s more, the operation method and implementation step corresponding to this calculation formula were put forward. The practice proves that this method can calculate the free point depth accurately and has a great practical value.