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 Hydril and 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.
The objective of this study is to evaluate the application of dual gradient drilling in the top hole portion of a 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. We believe 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 certain hazards seen in shallow marine sediments. Some of the hazards a THDG drilling system can minimize include methane hydrates, shallow gas, and shallow water flows. 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 project represents an effort by the MMS to advance and encourage research that has the potential to become part of the Best Available and Safest Technology (BAST) for offshore oil and gas development in Federal waters.
When starting a well, the operator needs to address and mitigate the shallow subsurface geotechnical hazards that threaten the safety of the drilling unit prior to installation of the blowout preventor (BOP). The purpose of the THDG package is to maintain the required borehole pressure in a riserless (i.e., conventional drilling riser not used) drilling mode, using a rotating head with a mechanical seafloor pump and mud return line to the ship to:
- Mitigate various pressure related geotechnical hazards at shallow penetration depths (e.g., pressured water and gas sands) by imposing the optimum circulating pressure and mud rheology to improve wellbore stability and hole cleaning without increasing mud weight;
- Mitigate formation fracturing and mud loss by controlling the pressure on the wellbore, using a seafloor pump either as an annular choke (i.e., to increase wellbore pressure) or as a mud lift pump in riserless mode (i.e., to eliminate the hydrostatic pressure effect of the mud column that would be in the riser);
- Reduce the seafloor pollution and loss of mud caused by the “dump and pump method;” and
- Reduce the number and size of casing strings required during drilling operations (i.e., by extending casing setting depths to cover problem formations with fewer casings).
Benefits to MMS and Industry
MMS and the industry will have the results of a preliminary engineering study on the use and packaging of dual gradient drilling for top hole applications that could potentially improve safety by mitigating shallow gas, shallow water flows, limited margin between fracture and pore pressure gradient, and gas below the hydrate cap or sealing zone (when drilling through hydrates) in the Gulf of Mexico. This technology could also minimize the environmental impact of the current “pump and dump” practice while drilling conductor and surface holes.
By using a THDG drilling package for riserless oil and gas exploration and development drilling, a smaller and less expensive rig can be used, drilling risks can be reduced, and time on site and rig costs could be decreased. By decreasing time on location, and not using a conventional drilling riser, smaller and less expensive vessels can be used to perform the same work, resulting in lower exploration costs. Industry will also benefit by avoiding the requirement to circulate large volumes of mud onto the seafloor to handle geotechnical hazards and water flows in the top hole.
Deployment of Results
The results of the project will be conveyed to the MMS and the petroleum industry through M.S. Theses, and Ph.D. Dissertations, TAMU-MEEN design project report, conference presentations, and publication in trade journals. The MMS and industry supporters of the project will also receive project reports and an overall final report that will tie the individual theses and design efforts together and provide a comprehensive documentation describing all project results.
“Top Hole Dual Gradient Drilling System: A strategic Approach Towards Deepwater Drilling”, Sukesh Shenoy, Amol Dixit, A.S. Nandagopalan, Dr. Steve Suh, Dr. Jerome J. Schubert, IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference, April 2006.
“The Feasibility of Top Hole Drilling with Dual Gradient Technology,” Brandee Elieff, Dr. Jerome J. Schubert, IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference, April 2006.