The constant bottomhole pressure variant of MPD is a technique most defined by IADC as "an adaptive drilling process used to precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly". Typically, downhole pressure variations are interpreted to be governed strictly by the friction losses given changes in the rig pumps rate. While this is a correct understanding of MPD applications in most of the fields where MPD has proven to be successful, the next step is to understand that there are still additional factors that may impact the downhole pressure management even when keeping parameters stable at surface.
Such additional factors are consequences of procedures outside of rig pump flow rate variation such as taking surveys, sending downlinks, working the string, inconsistent rheology properties, fluid compressibility and heterogeneous mud systems. Additionally, annular cuttings load and solids control performance can affect the downhole pressure behavior, with the effect becoming stronger as the solids concentration increase in the mud system. Those situations are rarely considered in the full MPD scope; particularly when wellbore strengthening material or LCM are incorporated leading to a more complex and challenging mud system.
Having control over the challenges that result from these additional factors' impact on the downhole pressure behavior, leads to an even further optimized MPD operation. This is especially important in more demanding environments of shallow offshore, deepwater and HP/HT wells.