During under-reaming operations the annular friction varies as you go deeper. Since you are enlarging the wellbore the annular velocity decreases hence the friction in the section under-reamed, but at the same time the total or cumulative friction increases since the total length of the drillstring is larger, no mention the surge effect which in some cases can be significant. Although sometimes this friction is neglected during the under-reaming operations, there are some cases when these operations must be planned carefully to avoid any surprises when the pore pressure is close or the operative window is narrow. The picture below depicts an under-reamer and the blue line show the annulus velocity, is clear that when the wellbore is enlarged the annular velocity drops, but as you go deeper the cumulative friction is greater.
Remember Keep your ECD constant during reaming operations apply the surface back pressure accordingly. Drillsoft make it easy!
In this video show how Drillsoft can simulate this complex operation easily with a few clicks.Leave a reply