In 1996, three competing projects began aimed at mitigating many of the problems encountered in ultra-deepwater drilling, including the narrow window between pore pressure and fracture gradient, high riser loads, high deck loads, and high costs of drilling fluids, etc. These three projects were conducted by Shell on a Submersible Pump System, Transocean on DeepVision, and Hydril on the SubSea MudLift Drilling system in joint industry programs to develop the technology known as dual gradient drilling. All three projects achieved a dual gradient by placing a rotating diverter on top of the BOP stack, which diverted the mud returns from the annulus to a set of seafloor pumps. These pumps then circulated the mud and cuttings back to the rig via an external return line. The marine riser was filled with seawater rather than mud as in the conventional case. After an enormous amount of time, energy, and money were spent to develop dual gradient technology, a test well was drilled in the Gulf of Mexico by the members of the SubSea MudLift Drilling Joint Industry Project in 2001.
In all three of these industry projects, the dual gradient drilling package was designed to be emplaced after surface casing was set to manage the window between pore pressure and fracture pressure, thus, extending the length of open hole sections between casing strings, and minimizing mud costs. Although these systems lowered the riser loads with a water filled riser, the packages were still designed to be employed on fifth generation drilling vessels using the existing riser system. Because these projects focused on deeper portions of the wells, massive, elaborate, and complex systems were developed.
One dual gradient drilling application that has not been fully studied is the potential benefit of utilizing this technology in the top hole portion of the well. This would allow conductor and surface casing to be set deeper (possibly with a smaller third generation floater), which would allow safer drilling of the intermediate hole. Some of the hazards a THDG drilling system can minimize include methane hydrates, shallow gas, and shallow water flows.
From the results of Phase I of this project, it is believed a Top Hole Dual Gradient (THDG) drilling package would improve drilling safety by providing a mud circulating system which has limited pressure control while drilling in a managed pressure drilling mode. THDG systems can lead to better technology than the current “pump and dump” process currently utilized in top hole drilling. An additional benefit of this technology is setting conductor and surface casing deeper in HTHP deep gas well applications as well as in ultra deep water applications. By focusing more attention on the top portion of the wellbore, smaller lighter and simpler drilling equipment and facilities could be used to drill deeper water wells.
This report documents the results of our work on Phase I and includes one Master thesis, one conceptual design report on the equipment that could be utilized, and three preliminary reports on different concepts for equipment that could potentially be utilized.