Applying dual gradient drilling in complex wells, challenges and benefits

Master's thesis in Petroleum engineeringDual gradient drilling is a manage pressure drilling technique that may extend viability of deep and ultra- deep water drilling and production, in a manner that may be classified as safe or safer, than what can be achieved by conventional means. DGD is especially beneficial in wells with difficult wellbore targets, often characterized by narrow pressure windows. Dual gradient offers the potential to reach the target depth using fewer casing strings, potentially ending up with a hole better suited for completion. This is made possible by utilization of two or more fluid gradients from rig floor to total depth of the well, compared to the conventional single gradient. Majority of the work completed through this thesis has been assigned around one dual gradient system in particular, the EC-Drill & CMP, which utilize a pumped riser principle. Introducing a subsea pump to take the well returns through an external conduit back to surface, the need for a continuous mud column back to surface is reduced. This fact leaves the mud level in the riser to be manipulated to fit the operation at hand, effectively mitigating wellbore controllability concerns. This system introduces a novel method for improved loss and kick detection, using the rpm/power output to detect unbalance to the system. To describe how the operability of the EC-Drill & CMP system may be plausible, a Matlab script was created. Further introducing a PI-controller code, which to a large degree renders the system autonomous, even when imposing changes to any of the manipulated variables, mainly topside and subsea pump flow rate, the goal is to maintain as constant a bottom hole pressure as possible. To support the EC-Drill & CMP work, relevant literature covering hydraulic control has been reviewed. Through extensive case studies provided by BG-Group, dual gradient application areas was outlined and discussed.2015-06-1