Pressurized Mud Cap Drilling (PMCD) is a drilling technique that has been used for over seven years in many wells where conventional drilling proved impossible or uneconomic1–5 . PMCD is typically used to drill fractured carbonates, where the pore and fluid loss gradient are virtually the same, resulting in total mud losses and kicks in the same hole section. However, it is not restricted to just carbonates, it can be used on any fractured rock that is very competent in respect to wellbore stability, or formations prone to severe to total loss with good wellbore stability characteristic.
Traditional PMCD typically requires the periodic injection of sometimes large volumes of weighted mud into the annulus in order to maintain a reliable mud cap. The process is highly cyclical, often unpredictable, and can be prohibitively expensive. The process also requires highly skilled and experienced people to run it properly, an increasingly rare commodity these days.
Use of the Micro-Flux Control (MFC) method with PMCD could permit the process to be automated and controlled to a much higher level of accuracy, allowing less experienced people to confidently run the system. While continuously injecting across the wellhead, and using delta flow as the control variable, the casing back-pressure would be monitored. Casing pressure increases, indicating a loss of mud cap fluid downhole, would be automatically controlled by precise adjustment of the continuous delta flow, together with pressure monitoring, rather than by periodic injection of large volumes of fresh mud cap fluid.
This paper will briefly describe the MFC method, rig up details and operational issues. It will also describe how to apply the procedure to automate and operationally enhance PMCD, explaining the benefits when compared to the use of current procedures. PMCD case histories will be reviewed, with illustrations of costs associated with the current process and a description of how the MFC is capable of simultaneously reducing risk and costs. This increased efficiency should allow PMCD to be used when economic factors restrict its use in accessing significant hydrocarbon reserves.