Controlled Mud-Cap Drilling for Subsea Applications: Well-Control Challenges in Deep Waters
- Borre Fossli (Ocean Riser Systems AS) | Sigbjorn Sangesland (Norwegian U. of Science and Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2006
- Document Type
- Journal Paper
- 133 - 140
- 2006. Society of Petroleum Engineers
- 1.7.6 Wellbore Pressure Management, 4.2.4 Risers, 1.7 Pressure Management, 5.3.2 Multiphase Flow, 1.6.5 Drilling Time Analysis, 4.5.4 Mooring Systems, 1.10 Drilling Equipment, 5.1.2 Faults and Fracture Characterisation, 1.6 Drilling Operations, 4.6 Natural Gas, 1.7.1 Underbalanced Drilling, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.2 Separation and Treating, 4.5 Offshore Facilities and Subsea Systems, 1.6.1 Drilling Operation Management, 2.1.7 Deepwater Completions Design, 4.3.4 Scale, 3 Production and Well Operations, 1.11 Drilling Fluids and Materials, 1.7.2 Managed Pressure Drilling, 4.1.5 Processing Equipment, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.7.5 Well Control, 5.2.1 Phase Behavior and PVT Measurements, 4.2 Pipelines, Flowlines and Risers, 4.3.1 Hydrates, 1.8 Formation Damage
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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.
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.
|File Size||626 KB||Number of Pages||8|
Barker, J.W. and Gomez, R.K. 1989. Formation of Hydrates DuringDeepwater Drilling Operations. JPT 41 (3): 297-301. SPE-16130-PA.
Coker, I.C. 2004. Managed PressureDrilling Applications Index. Paper OTC 16621 presented at the OffshoreTechnology Conference, Houston, 3-6 May.
Hariharan, P.R. and Judge, B. 2004. ECD Management ToolOffers Potential for Reducing Drilling Problems and Costs . Paper OTC 16623presented at the Offshore Technology Conference, Houston, 3-6 May.
Petersen, J., Bjørkevoll, K.S., and Lekvam, K. 2001. Computing the Danger of HydrateFormation Using a Modified Dynamic Kick Simulator. Paper SPE/IADC 67749presented at the SPE/IADC Drilling Conference, Amsterdam, 27 February-1March.
Sangesland, S. 1998. Riser LiftPump for Deep Water Drilling. Paper IADC/SPE 47821 presented at theIADC/SPE Asia Pacific Drilling Conference in Jakarta, 7-9 September.
Skalle, P., Holand, P., and Sangesland, S. 2002. Evaluation of DeepwaterKicks and Future Countermeasures. Paper presented at the Pennwell Deep OffshoreTechnology Conference, New Orleans, 13-15 November.