Analysis of Riserless Drilling and Well-Control Hydraulics
- Jonggeun Choe (Seoul Natl. U.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1999
- Document Type
- Journal Paper
- 71 - 81
- 1999. Society of Petroleum Engineers
- 1.3.2 Subsea Wellheads, 2.1.7 Deepwater Completions Design, 1.12.1 Measurement While Drilling, 4.1.2 Separation and Treating, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.1.4 Gas Processing, 5.4.2 Gas Injection Methods, 1.7 Pressure Management, 1.7.1 Underbalanced Drilling, 3 Production and Well Operations, 1.10 Drilling Equipment, 4.2.4 Risers, 3.1.6 Gas Lift, 4.1.5 Processing Equipment, 5.6.4 Drillstem/Well Testing, 5.3.2 Multiphase Flow, 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 1.11.5 Drilling Hydraulics, 4.2.3 Materials and Corrosion, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.7.5 Well Control
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Riserless drilling is an unconventional technique using a relatively small diameter pipe as a mud return line from the sea floor instead of a large diameter marine riser. The schemes were developed in the late 1960s to reduce wear on blowout presenters and to make drill pipe re-entry easier by balancing internal and external subsea well pressures. However, these concepts were not implemented at that time because water depths were shallow, and technology was not available. In the Gulf of Mexico, exploration attempts have been made in areas which have more than 7,000 ft water depth. A conventional large diameter riser requires a vessel with huge weight and space capacities, large mud volumes to circulate through a riser, and numerous casing points because of the relatively low separation between the formation pore pressure and fracture pressure, especially in the Gulf of Mexico. These problems may be reduced significantly by applying riserless drilling. Therefore, riserless drilling is one of the attractive alternatives for economically exploring oil fields in deep water. The concept of riserless drilling currently has many unsolved problems such as system configuration and well control. This paper presents basic concepts of riserless drilling and a brief review of problems associated with conventional marine riser drilling for deep water applications. The paper also presents hydraulics and well control considerations for riserless drilling with comparison to conventional riser drilling.
As proven petroleum reserves decline through continued production, exploration for new oil and gas resources will extend into environments which present significant economic risks and technical hurdles. A detailed study using multicompany data1 disclosed about 8-10 billion bbl of oil equivalent in place for deep water areas of the Gulf of Mexico (GOM) outer continental shelf. There are also more than 15 known discoveries in water depths between 3,000 and 7,500 ft in the GOM2 and there is great interest in accelerating the development of the known deep water discoveries. Leases have been obtained in water depths up to 10,000 ft with a requirement that they be drilled within the next decade. With new technologies and hardware, it may be possible to produce oil and gas economically from reservoirs in deep water areas in the future.
One of the basic and most challenging problems in deep water operations is the use of a marine riser. A marine riser has been used to provide a connection between the drilling vessel and the wellhead. This serves as a guide for the drill pipe into the hole and as a mud return path to the vessel. It also supports control cables and choke and kill lines. Floating drilling operations in deep water presently involves the use of a 21 in. outer diameter (OD) marine riser.
In the early stage of offshore development, especially for shallow water, it was possible to solve problems associated with water depth increase by increasing the size of both marine riser and subsea wellhead. The first wellhead was 13-5/8 in. in diameter and a common size today is 18-3/4 in. As the diameter and length of a marine riser increases, the riser with the mud that is contained in it becomes quite heavy. The riser wall thickness has to increase to control expected internal pressures, to handle high stresses in the riser, and to attach auxiliary buoyancy units. These factors substantially increase the unit cost of the riser and require a large vessel to hold the riser. Therefore, it is very difficult to use a large diameter marine riser and wellhead to drill much beyond 6,000 ft water depth. Almost all the developments in the GOM have water depth less than 4,000 ft except for a few areas such as the Mensa field (5,400 ft) operated by Shell.
Many alternatives to the use of the conventional marine riser system for deep water drilling have been investigated. One is injection of gas at the blowout preventer (BOP) level in order to reduce the effective density in a marine riser down to seawater density. This is similar to a gas lifting operation and some results of feasibility studies to date are available.3 Another is elimination of the conventional large diameter marine riser which is called riserless drilling (RD). Schemes for drilling without a marine riser were developed in the 1960s and Watkins4 first patented RD concepts in 1969 to reduce rotating BOP (RBOP) wear and to make drill pipe re-entry easier by balancing subsea internal and external well pressures.
Riserless drilling also has many other benefits for deep water applications such as reduction of casing points, less weight and space requirements, and easier station keeping. But it also has many unsolved problems such as system configuration, development of necessary equipment, procedures, and well control. This paper presents basic concepts of riserless drilling and a brief review of problems associated with conventional marine riser drilling for deep water applications. The paper also presents detailed studies on RD hydraulics, kick detection methods, and well control with comparison to conventional methods. The studies are from the results of Phase I research sponsored by the riserless drilling joint industry project. System configuration and detailed engineering are outside of this paper's scope.
Riser Problems in Deep Water
Although marine risers have been used successfully for water depths in excess of 7,000 ft, it is impractical to extrapolate current technologies with a marine riser to 10,000 ft water depth. Table 1 shows default data for a comparison study of RD and conventional riser drilling (CRD). The riser size is 21 in. OD, water depth is 10,000 ft, and total depth is 30,000 ft.
For a 21 in. OD [inner diameter (ID)=19.5 in.] marine riser, internal capacity, and net steel weight are about 370 bbl and 160 kip for every 1,000 ft of length, respectively. The buoyed riser weight in seawater with 15.5 ppg mud in it will be 250 kip per 1,000 ft of length which gives 2,500 kip for 10,000 ft water depth. The riser weight will further increase because of choke and kill lines attached, and the riser couplings. Therefore, it will require huge buoyancy units which result in an increase of riser OD and cause riser handling problems. Only fourth or fifth generation semisubmersibles may have adequate space and weight capacities to handle these requirements. Composite materials can be used to reduce the weight requirement.
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