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Dual gradient drilling systems
Perhaps one of the most important ventures in the area of high-cost technologies for deepwater challenges is the development of dual gradient drilling systems (DGDSs). DGDS is often referred to as riserless drilling.
It is generally accepted that DGDS is required in water depths of > 5,000 ft. There have been a number of unpublished examples, however, in which application of the technology was needed in water depths as shallow as 3,000 ft. The need for DGDS is caused by the reduced fracture gradient of formations below the mudline, resulting from the reduced weight, or gradient (0.5 vs. 1.0 psi/ft), which, itself, is a result of water above the mudline as viewed from a drillship operating at sea level. The various systems shown in Fig. 1, in one manner or another, isolate the borehole pressure gradient below the mudline from the drilling mud gradient above. In all but the Maurer Technology, Inc. DGDS, isolation is achieved mechanically by valves and pumping. The Maurer approach seeks to achieve the same benefit by pumping lightweight solid additives (LWSAs) from the drillship into the riser at mudline. This concept allows minimum equipment and intervention risk on the seafloor. The LWSAs investigated to date consist of hollow glass spheres and polymeric beads.
Advantages of dual gradient drilling
The advantage gained by these systems can be noted by comparing Figs. 2 and 3. For the conventional drilling case (Fig. 2), the gradient in the wellbore is relative to the drillship in all cases, because the mud column is hydraulically continuous from the bottom of the hole up the riser to the drillship. This results in additional pressure being applied at the mudline (mud density minus seawater density times water depth times a units constant). The increased “backpressure” at the mudline has the effect of minimizing the drilling distance between casing points. The pressure at the bottom of the hole over a particular interval is usually referred to as equivalent circulating density. The equivalent circulating density from the mudline to total depth for conventional riser systems is always greater than for subsea systems in which the pressure (both circulating and static) required to get the mud from the mudline to the drillship is hydraulically isolated from the borehole or greatly reduced in density at the mudline.
Fig. 3 demonstrates that isolation of the pressure caused by the drilling mud above the mudline results in a borehole gradient that allows significantly longer openhole sections before reaching the depth at which casing must be set to avoid exceeding the fracture pressure.
Types of DGDS
Fig. 1 is an attempt to show some of the DGDS concepts being considered. The Subsea Mudlift development program produced the only prototype DGDS successfully field tested to date; however, deployment cost has proved problematic to market penetration.
Consideration is being given to the DGDS approach (Fig. 4) proposed by Maurer to reduce the cost of achieving a dual gradient drilling capability. Maurer is leading a consortium to look into the feasibility of injecting LWSA at the mudline to control gradient in the riser. The strength of the approach is that it has the potential to simplify the equipment installed at the mudline and to reduce the cost of the DGDS. In addition, the LWSAs are well behaved in the riser; they maintain constant shape and do not migrate significantly when pumping is stopped for a reasonably long period of time. To date, both hollow glass spheres and polypropylene beads have undergone testing for use as LWSAs. However, an investigation of alternatives is ongoing.
Fig. 5 shows a side-by-side comparison of the results of using DGDS compared with conventional single gradient drilling. As noted in the figure, the setting depth of all casing strings is significantly increased. This is achieved, in effect, by isolating the borehole pressure from the weight of the seawater above the mudline. Most systems achieve this isolation through rather complex combinations of pumps and cuttings processing equipment at the mudline. The Maurer DGDS achieves a similar effect by lowering the fluid density in the riser well below 8 lbm/gal. Regardless of the system considered, the most notable benefits of DGDS technology are that it has the potential to enhance the capability of drilling to even deeper targets in ultradeep waters of the GOM, and it allows active control of borehole mud gradient. It should be noted that the latter benefit could also be a significant safety consideration.
- Smith, K.L., Gault, A.D., Witt, D.E. et al. 2001. SubSea MudLift Drilling Joint Industry Project: Delivering Dual Gradient Technology to Industry. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisianna, USA, 30 September–3 October. SPE-71357-MS. http://dx.doi.org/10.2118/71357-MS.
- Eggemeyer, J.C., Akins, M.E., Brainard, R.R. et al. 2001. SubSea MudLift Drilling: Design and Implementation of a Dual Gradient Drilling System. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisianna, 30 September–3 October. SPE-71359-MS. http://dx.doi.org/10.2118/71359-MS.
Noteworthy papers in OnePetro
J.P. Schumacher et al. 2002. Planning and Preparing for the First Subsea Field Test of a Full-Scale Dual-Gradient Drilling System, SPE Drilling & Completion Volume 17, Number 4. 80615-PA. http://dx.doi.org/10.2118/80615-PA
J.C. Eggemeyer et al. 2001. SubSea MudLift Drilling: Design and Implementation of a Dual Gradient Drilling System, SPE Annual Technical Conference and Exhibition, 30 September-3 October 2001. 71359-MS. http://dx.doi.org/10.2118/71359-MS
Cohen, John H. 2013. Dual Gradient Drilling – 101. https://webevents.spe.org/products/dual-gradient-drilling-101