MudLift Drilling System Operations (1998)

Abstract MudLift Drilling is a term used to describe a deepwater floating drilling system where the drilling mud is pumped from the seafloor to the drilling vessel to created the effect of dual pressure gradients in the returning mud column. The dual gradient system better matches natural formation pore pressure and fracture pressure gradients, thus, minimizing the need for casing strings. When the MudLift system is used with a conventional marine riser, the riser is filled with seawater and the mud is pumped up return lines. When used with independent return lines, the MudLift system permits "Riserless" drilling. The Riserless term refers to lack of a conventional marine riser. Smaller return lines require much less tension, may be faster to deploy and contain much less mud volume. Riserless drilling significantly reduces the loads on a drilling vessel. The subsea MudLift pump provides the means for quickly adjusting annular mud column pressure to permit faster penetration rates, to control a kick and/or to evade or escape from differential pressure sticking. Introduction MudLift Drilling is a deepwater floating drilling technique that uses a subsea booster pump to lift mud returns to the drilling vessel. Mud exerts one pressure gradient from the bottom of the borehole to the seafloor and the subsea booster pump maintains another pressure gradient from the seafloor to the vessel. Figure 1 shows a concept drawing for the MudLift system. The MudLift system includes a subsea pump and diverter system in addition to the conventional subsea BOP stack. Return lines replace the conventional marine riser for "riserless" drilling. This greatly reduces riser tensioning loads and mud storage requirements for a drilling vessel. Lopes and Bourgoyne,1 Gault 2and Snyder.3 have explained how the dual gradient system better matches natural formation pore pressure and fracture pressure gradients in deep water, thus, minimizing the need for casing strings. Figure 2 shows for an example well: formation pore pressure, fracture pressure, single pressure gradient lines for several mud weights and the MudLift dual gradient-low Mud Weight (MW) and dual gradient-high Mud Weight (MW) lines. The effects of annular circulating pressures and pressure surges due to pipe movement are ignored here for simplicity of illustration. The dual gradient-high MW is the actual mud density used with the MudLift system. The MudLift booster pump reduces the pressure in the returning mud column to, in effect, create the dual gradient system. In this example, the MudLift pump maintains seawater pressure at the mudline for a dual-low MW equivalent to seawater. Figure 2 shows that a dual gradient mud column overbalances pore pressure without exceeding fracture gradient for a much longer section of hole than a single gradient mud column. Figure 3 shows formation pore pressures and fracture pressures at 5000 feet, 7500 feet and 10,000 feet of water depth. The advantages of a MudLift system becomes progressively greater as water depth increases. Several questions are frequently asked about this concept.How should the dual gradient mud system be achieved: gas lift, pumping or other means?If seawater gradient is maintained from the sea floor to the vessel will that require a mud weight in the borehole that is too high to be achievable?How much pump pressure and horsepower are required to achieve the MudLift?How much pressure must the subsea diverter system hold?