Formation collapse gradient is an important parameter for well design and drilling mud selection, especially in a field with history of wellbore instability problems. In modern well design, this gradient is easily neglected since its tendency to be lower than pore pressure gradient. However, some field studies in Indonesia showed that the collapse gradient may be higher than the pore pressure gradient which will result in an unstable well if the bottom hole pressure (BHP) is not well addressed.
In the event of formation collapse, the common drilling practice is to increase the mud weight (MW) being used to a level above the collapse gradient which is time consuming and very rigid operation. Meanwhile, Manage Pressure Drilling (MPD) offers a relatively more dynamic wellbore pressure control by adjusting the surface back pressure (SBP) applied to the annulus for a given MW. Instead of shifting the MW or changing drilling parameters, MPD provide adjustment to the required equivalent circulating density (ECD) or equivalent static density (ESD) based on formation collapse gradient. Additionally, in the event of mud losses due to high ECD/ESD, MPD technique simply lowers the SBP to decrease the BHP without the need to lower the MW.
This paper will utilize hydraulic simulation software to model the drilling window, considering not only pore and fracture pressure gradient, but also involving collapse gradient to optimize recommended MPD parameters including MW and SBP when drilling, pipe connection, and completing the well.