Investigating the Effects of Operating Parameters on the Increase of the Casing Milling Efficiency in Deep Drilling Operations

摘要

Tightness of oil/gas boreholes is essential from the economic and environmental point of view.Therefore,casings(steel pipes)are placed into the boreholes and cemented in place to ensure the isolation of the geological zones,the stability of the borehole,the prevention of lost circulation,etc.However,sometimes it happens in deep boreholes that existing casings have to be removed.From an economical point of view,it is often more effective to rebuild an existing pipework than to drill a new borehole.Generally,this is achieved by milling the pipes.This means that there are sometimes milling sections of considerable length.Nowadays,the milling method is very well advanced.However,the main challenge in a milling process is to achieve fast milling progress with continuous and complete chip removal.In practice this is very hard to accomplish,because the effects of the milling process have not yet in detail been fully understood.The main common problems during casing milling are for example the growth of chip nests around the drill string,the undesired wear pattern of the mills(conical wear of the milling tool),and the production of large casing sheet-like fragments.These all can lead to an uncontrolled milling process,and as a result to increased costs of the milling process.The main objective of this article is to investigate and study the effects of operational parameters and in-sert types on the milling process in the laboratory,in order to effectively avoid chip nests and casing fragment-forming issues and to provide a controlled milling process.In the frame of a DGMK project(DGMK project 759),a small model scale casing milling test rig equipped with sophisticated measurement technology was designed and built up at the Institute of Drilling Engineering and Fluid Mining(IBF)of the Technical University Bergakademie Freiberg,Germany,in order to study and conduct basic research on the casing milling process in boreholes.Tens of casing milling tests have been successfully conducted with the casing milling test rig using conventional tungsten carbide inserts and special inserts developed by the scientists of the Technical University Bergakademie Freiberg called FHP-inserts(ultra-hard material inserts).The tests were conducted with different combinations of rotational speeds and forward velocities.The available test results showed that lon-ger chips are formed at high rotational speeds.Furthermore,shorter chips were observed at lower rotational speeds,but the insert wear trend was observed to be relatively higher at lower rotational speeds compared to higher rotational speeds.Additionally,it was observed that the trend of the insert wear increased by increasing the forward velocity for all rotational speeds.The FHP-insert with a cobalt content of 10% seemed to be wear resistant com-pared to the conventional insert.It can be concluded that in the frame of the first research project phase,a modern test rig with great possibilities to simulate a wide range of milling operation conditions was successfully developed and tested to study and investigate the process of casing milling in boreholes in the laboratory.Moreover,the test rig offers opportunities to test innovative and new ideas,and to study various insert types and insert geome-try effects.The results and findings are encouraging.It is highly recommended to continue with the project to perform further laboratory and field tests covering wider operation conditions,and to study the effects of various insert types and geometry designs,in order to statistically validate and to make safe proof of the first phase findings and results.As a result,a milling guideline has to be developed.