Subsea Mudlift Drilling: Planning and Preparation for the First Subsea Field Test of a Full-Scale Dual Gradient Drilling System at Green Canyon 136, Gulf of Mexico

Abstract

Abstract Currently the deepwater drilling industry uses single gradient drilling: one single mud weight from the surface to total depth (TD) to control the thin margins between fracture gradient and pore pressure gradient. In this conventional system, a single mud column extends from the rig floor to TD, resulting in a hydrostatic bottomhole pressure. Pore, fracture and mud pressure gradients are referenced to the rig floor. Dual Gradient Drilling (DGD) provides the same bottomhole pressure with a combination of two fluid gradients: slightly heavier mud from the mudline to TD, and seawater from the mudline back up to the rig floor. All gradients are referenced to the mudline rather than the rig floor. With DGD, the margins between fracture gradient and pore pressure are significantly greater while drilling the well. The benefits of a dual density system are well documented, but in general, lower cost wells can be drilled more safely and with more completion flexibility than with single gradient systems. Since 1996, the SubSea MudLift Drilling (SMD) Joint Industry Project (JIP) has been developing a dual gradient drilling system and all the associated drilling and well control techniques and procedures. This paper is the second in a series of papers presented on SMD technology (see SPE 71357 and SPE 71359). It describes the planning and preparation for the world's first dual gradient well. The well will be a partnership collaboration between Texaco, Diamond Offshore and the SMD JIP. It is a Gulf of Mexico Green Canyon well test in a water depth of 910-ft (Figure 1). This location has an intermediate water depth with a known pore pressure environment. It provides an excellent opportunity for extensive testing prior to the final development and delivery of a dual gradient drilling system for use in ultra-deepwater (beyond 6000 ft).