Rotating Control Devices

PMCD Background

Pressurized Mud Cap Drilling (PMCD) provides a safe and efficient method for drilling wells with severe lost circulation. This technique is used very successfully for the drilling of highly fractured or vugular carbonates. Karst features are often associated with these types of reservoirs and represent a major economic and operational obstacle for drilling. Karst features are comparable to tunnel structures underground. Although the presence of large cavities is possible, the Karst feature in terms of drilling should be thought of as a cave structure when formed, that has now filled with fractured or highly permeable sedimentary material. Although cavities have been observed during drilling operations, their size is normally limited to 2 – 3 ft due to the overburden. The important aspect of Karst features is their connectivity. A small, isolated Karst feature will be filled by continued injection of fluid. A large, well connected Karst feature is difficult to plug or fill with continued injection and therefore is dealt with in a different manner. It is highly unlikely that common place LCM plugging materials will be effective on large Karst features.





Mud cap drilling has two techniques, one is the Floating Mud Cap and the second is Pressurized Mud Cap. In the Floating Mud Cap operation the annular fluid’s hydrostatic pressure is greater than reservoir pressure. This results in the annular fluid column falling to some point below the wellhead. Observation of the fluid column is difficult and therefore indications of wellbore influx and gas migration are hidden. Floating Mud Cap drilling is a blind procedure that relies on either the continuous injection of fluid or the injection of fluid at predetermined time intervals. When drilling offshore in subnormal pressure environments, seawater is commonly used as the annular fluid. This process commonly uses seawater in offshore wells. The annular injection rate is determined by the gas percolation rate observed in the wellbore. Typically annular velocities above 5,000 fpm are required to prevent gas percolations. This is the most common technique used when drilling sub-normally pressured carbonate sections.






The second technique, Pressurized Mud Cap, is the one which will be employed on this project. Pressurized Mud Cap uses an annular fluid which exerts a hydrostatic pressure less than the reservoir pressure. This imbalance of pressure results in annulus surface pressure and therefore some form of an annular seal, such a rotating head (RCH) or annular preventor, is always used. The benefit of PMCD is the ability to monitor the annulus surface pressure. This surface pressure provides indications of gas migration and bottom hole pressures. PMCD also requires less total fluid injection which helps to minimize drilling costs and cleanup time after completion.

Bit Lian Engineer Operate RCD in Malaysia