Pressure Relief Management Philosophy for MPD Operations on Surface Stack HPHT Exploration Wells

Managed pressure drilling (MPD) is an adaptive drilling technique used to improve the economics and to mitigate risks associated with drilling high pressure and high temperature (HPHT) exploration wells where the drilling window is often narrow. The technique involves the combination of surface back pressure and fluid hydrostatic column to provide the required bottom hole pressure for safe drilling. Typical MPD equipment spread includes rotating control device (RCD), chokes, high pressure lines and gate valves with Pressure relief valves (PRVs) incorporated. The primary purpose of the PRV is to protect the MPD surface equipment and the formation from being overpressured. The relief valve achieves this by bypassing the normal fluid flow path for MPD operations and relieving the system pressure to the rig Mud gas separator (MGS) through a dedicated line. Each time a PRV is activated the resulting loss of surface back pressure increases the risk of taking a kick. On the other hand, when a PRV is not activated, an excessive increase in surface pressure raises the risk of formation fracture leading to losses. Therefore, the performance of the PRV has an immense impact on assessing the risk of a well control situation, which may be caused by either loses due to formation breakdown and consequently a kick from loss of the hydrostatic pressure component of the equivalent surface density (ESD) or an influx as a result of loss of surface back pressure component of the ESD due to loss of integrity of surface equipment). Pressure Relief Management philosophy generally covers decisions such as which parts of the well system (surface and subsurface) are to be preferentially protected by the PRVs, selection of activation pressure for high level alarms, types, number and setpoints of PRVs in the MPD system during different phases of the drilling operations - drilling, connections and tripping, and MPD choke full-opening pressure. These values are dependent on formation integrity test (FIT), mud weight, drilling window, annular friction pressure and operating envelope of RCD. The set points require adjustment depending on the hole size and flow rate and may be different during completion and well control operations. This paper describes the Pressure Relief Management philosophy for a HPHT well drilled in the Niger delta. It looks at factors that drive the high-pressure alarm setting values, choice of PRV types, placement and the part of the well system being protected, PRV tripping and reset values, and MPD choke full opening pressures. It also describes the challenges and risk assessment that influenced the selection of set points (single or dual setpoints) for different phases of the drilling operations.