Managed pressure drilling can be defined as a closed path drilling technique. Unlike the conventional drilling where the mud is flowing to atmospheric pressure, in managed pressure drilling, the mud flows out of the annular space with a back pressure which is provided by MPD choke. The bottom hole pressure in managed pressure drilling is a sum of the hydrostatic pressure HP of the column of drilling fluid, annular frictional pressure AFP and the back pressure BP. The bottom hole pressure BHP with this combination can be effectively controlled by manipulating the mud flow rate or the back pressure and this lead to better well pressure management.
BHP = HP + AFP+ BP
Constant bottom hole pressure CBHP is considered as one of the most used managed pressure drilling - MPD - techniques which is based on maintaining the bottom hole pressure within a given interval under both static or dynamic conditions. Using this method helps to keep the bottom hole pressure BHP (which is composed of back pressure BP, hydrostatic pressure HP and annular frictional pressure AFP) greater than the pore pressure PP and less than the fracture pressure FP.
In most cases, the hydrostatic pressure HP provided by the mud weight used in such method is less than the pore pressure PP and the back pressure BP is regulated to bring the bottom hole pressure BHP inside safe window above the pore pressure. Among the objectives of this strategy is to overcome the kick-loss issues when drilling narrow windows environments.
Among the issues which are related to this technique is how to shift from static situation (no circulation) to dynamic situation (with circulation) or from dynamic to static without losing control on operations - facing loss or gain situation-. The tip behind this strategy is applying a back pressure BP to compensate the annular pressure AFP (loss of AFP when shutting the pumps off) while making connections or surface equipment issues. Shifting from one situation to another is performed by gradually opening or closing the MPD choke while simultaneously increasing or reducing the mud pumps speed.
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| Fig 1. Mud Pump Speed vs Back Pressure |
This principle of keeping the wellbore pressure constant has many benefits such as avoiding drilling problems related to mud weight changes, drilling through tight pressure windows and reaching the total depth of drilling in optimized conditions.
Under this category of constant bottom hole pressure, two different methods can be applied to achieve the target: back pressure BP application and continuous mud circulation CCS. The back pressure BP method is based on using surface equipment such as MPD pump and choke (Manual or automatic) to keep the well bore pressure above the pore pressure. The CCS system is based on using a continuous circulation system that provides mud circulation when making or breaking connections so the wellbore is kept circulated during all the operations which helps to stay within the required pressure profile.
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| Fig 2. MPD System Layout |
The automatic choke used for the back pressure application is considered more precise when drilling through narrow pressure windows. The semi-automatic or manual are less accurate and needs to be used in more or less sufficient pressure windows.
Many hydraulics models have been developed to be integrated in computer control system designed for automatic chokes. These models involves many parameters and correlations for bottom hole pressure profiles to predicts the required surface back pressure. The use of MPD choke has been supported by the MPD pump to make a surface circulating loops to maintain a continuous circulation through the choke to get to precise annular surface pressure.
The continuous circulation system was developed to help keeping the circulation during make up or break connections and consequently steady bottom hole pressure which is chosen to be inside the designed pressure profile ( between pore pressure and fracture pressure). This system can be used to prevent the drilling problems which are caused by the frequent switching on or off of the mud pumps.
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| Fig 3. Continous Circulating System |
The continuous circulating system is composed of many parts. The three important parts are: the coupler, mud diverter and the hydraulic power unit. This system is also consisted of three preventers: upper pipe rams, middle blind rams and lower pipe rams. The principle is based on creating an isolated chamber around the connection by closing around the drill pipe. Then this chamber is pressurized by CCS mud circulation path to allow for making or breaking joints. Above the continuous circulation system, there is a snubber that helps to control the upward movement of the pipe due to the upward force generated after pressurizing the chamber. During making or breaking connection, the bottom hole pressure is kept steady and the equivalent density is not allowed to fall below the pore pressure. After making the connection, the closed chamber is depressurized and the CCS can be opened to recover operations.
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