Controlled Mud-Cap Drilling for Subsea Applications: Well-Control Challenges in Deep Waters

Abstract

Summary This paper describes a new drilling-riser concept and drilling methodology for deepwater operations that will remove some of the well-control challenges and limitations currently experienced when handling kicks and deep gas influxes in deepwater regions, with the following results:Providing improved and more flexible well-control procedures.Reducing the potential of hydrate plug formation during well-control operations.Allowing for drilling longer hole sections than normally considered feasible when using conventional drilling methods, thus reducing the number of casing strings required in the well.Allowing for improved drilling performance in depleted formations. The main elements in the system are based on using a small, high-pressure drilling riser [14-in. outer diameter (OD), 12.5-in. inner diameter (ID)] with a split surface/subsea blowout preventer (BOP) and a subsea mud-lift pump connected to the drilling riser and a separate mud-return line. During drilling and well-control operations, the mud level in the riser is maintained considerably below sea level to create a mud/air interface (i.e., a "mud cap") that can be continuously adjusted up or down by the mud-lift pumping system. As a consequence, the bottomhole hydrostatic pressure will be controlled. One of the main purposes of this system is to mitigate the inherent problems with a conventional 21-in.-OD marine drilling riser during well-control scenarios in deepwater operations. The system will compensate for frictional pressures resulting from circulation and adjust the bottomhole pressure (BHP) accordingly. Introduction Experiences from deepwater drilling operations in geopressured environments such as the Gulf of Mexico (GOM) have shown that the upper layers of the subsurface have low fracture strengths close to the hydrostatic pressure of seawater. The resulting small margin between the pore pressure and the formation strength typically requires four to six or more casing strings to be set below the surface casing when drilling with a conventional marine riser system. When drilling in high-pressure/high-temperature (HP/HT) fields, or through salt intrusions, small windows between pore pressure and formation strength can be experienced. In some of these cases, after drilling only a short interval, the incremental BHP caused by circulation [i.e., the equivalent circulating density (ECD) effect] is high enough to require setting a casing string to maintain adequate well-control margins. Lost circulation is a problem experienced frequently when conventionally drilling in deepwater areas, HP/HT areas, highly faulted and fractured formations, and in depleted formations. The remedial process can be costly (in both time and money). When drilling with a conventional large riser system during a well-control event, the kick is circulated out through the chokeline. This line has a small diameter, and in deepwater wells, the friction in this line can be a significant factor while circulating out a kick, even at low pump rates. As a consequence, more than 75% of all deepwater kicks experience formation ballooning, partial losses, and other downhole problems (Skalle et al. 2002). In severely depleted reservoirs, drilling and well-control operations are often conducted within the small pressure region between formation fracture and wellbore instability (collapse). The resulting challenge can restrict the ability to drill underbalanced unless the BHPs can be controlled in a fast, safe, and effective manner. The inherent limitations of conventional well-control procedures can, as a consequence, cause severe lost-circulation or hole-stability problems, which are extremely costly in deepwater operations. In deep water, the low mudline temperature and high pressure may lead to hydrate formation, if gas is present. Hydrate plugs can cause delay in operations and can cause severe well-control challenges (Barker and Gomez 1989). In this paper, three different methods of pressure control will be discussed. The first method is the conventional way of controlling pressure in an open system with a high-pressure riser and a surface BOP. The second method is the closed-loop method of managed-pressure drilling (MPD) with a surface BOP, a rotating control device (RCD), and a pressurized riser, and the third is the method referred to as the "controlled mud cap" (CMC) with a split BOP between seabed and surface.

Development and application of transient gas‐liquid two‐phase flow model considering sudden density change

“…The two‐phase flow model was validated by measured data from a full‐scale experimental well at Louisiana University . The basic parameters of the experimental well are shown in Table .…”

Managed-Pressure Drilling: Using Model Predictive Control To Improve Pressure Control During Dual-Gradient Drilling

“…Density variation can result from different dilution-based approaches, such as injection of lightweight solid additives to reduce the density, proposed by Maurer Technology (Cohen and Deskins 2006), and injection of lightweight fluids into the riser to lower the efficient fluid density above the injection point, proposed by Louisiana State University (Lopes and Bourgoyne 1997). Diversion-based concepts, where the mud returns do not travel through a riser, but are either dumped at the seafloor or circulated back to the rig through a separate return line, have been proposed by Deep Vision (Forrest et al 2001), SubSea MudLift Drilling Joint Industry Project (Schumacher et al 2002), Shell (Gonzalez 1998), AGR (Brown et al, 2007), and Ocean Riser Systems (Fossli and Sangesland 2006). Common for all DGD concepts is that they use mud with higher-thannormal density.…”

Automatic control of managed pressure drilling

“…Density variation can be based on different dilution-based approaches, such injection of light-weight solid additives to reduce the density, proposed by Maurer Technology (Cohen & Deskins, 2006), and injection of light-weight fluids into the riser to lower the efficient fluid density above the injection point, proposed by Louisiana State University (Lopes & Bourgoyne, 1997). Diversion-based concepts, where the mud returns do not travel through a riser, but are either dumped at the seafloor or returned back to the rig through a separate return line have been proposed by Deep Vision (Forrest & Bailey, 2001), SubSea MudLift Drilling Joint Industry Project (Schumacher et al, 2002), Shell (Gonzalez, 1998), AGR (Brown et al, 2007), and Ocean Riser Systems (Fossli & Sangesland, 2006). Common for all DGD concepts is that they use mud with higher than normal density.…”