Managed Pressure Drilling (MPD) - 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 (IADC definition).
During MPD operations the Bottom Hole Pressure (BHP) can be defined as follows;
BHP = Static mud weight + ECD component + Applied Surface Backpressure (using an MPD choke manifold and RCD)
The well bore is sealed using a Rotating Control Device (RCD). A rotating sealing element / diverter which allows drill pipe to enter and exit the well bore whilst maintaining pressure in the annulus, also allows rotation of the drill pipe whilst containing annulus pressure. The sealing elements rotate with the drill string, as they are mounted in a bearing assembly. They divert flow from the well to the MPD system, via a flow spool under the RCD.
Returns are then diverted to an MPD Automated Choke Manifold: a set of high-pressure valves and associated piping that includes two automated chokes, arranged such that one automatic choke may be isolated and taken out of service for repair and refurbishment while well flow is directed through the other one. It also includes a pressure gauge upstream and a Coriolis flow meter on the manifold. Both chokes are automated and hydraulically controlled by a Programmable Logic Controller PLC system using real-time control system software, surface pressure, temperature and flow rate data and the MWD BHP to maintain tight control over the BHP. The chokes are designed for continuous use while MPD, with cuttings and mud returns.
The MPO Total Control Driller systems then also uses a Mud Gas Separator (MGS). This is an MPD MGS device that removes air or gases from drilling liquids. This works by increasing the surface area available to the mud so that bubbles escape (through the use of various cascading baffle plates). Gas is routed away from the rig up to the top of the derrick via the vent line on an offshore installation, or to a flare pit on a land rig. The MGS allows MPD operations to continue with back ground gas, or minor connection gasses, in the returned mud flow.
MPD as a technique is being quickly adopted in the industry globally, and is a less risky operation than Underbalanced Drilling, which has also seen increasing application. In terms of equipment and operating principles from the incoming mud flow (i.e. exit from the flow spool) to return of fluids to the rig mud system there is very little difference between MPD operations and Underbalanced operations. Use of automated choke systems which were first applied as far back as 1998 for drilling on the Arun field in Indonesia, are now being implemented by several service companies for MPD operations. While there are some variations in operating principle between the various systems, they all are automated which removes the substantial scope for human error.
The benefits in enhanced control of BHP whilst drilling is the main driver for implementing MPD in most fields.
MPD is a drilling optimisation tool, that if applied and implemented correctly can save 20 to 40% of well costs. The higher well cost savings being on high spread rate HPHT and Deep Water projects.