Search citation statements
Paper Sections
Citation Types
Year Published
Publication Types
Relationship
Authors
Journals
This paper was prepared for presentation at the 1999 SPE Asia Pacific Oil and Gas Conference and Exhibition held in Jakarta, Indonesia, 20–22 April 1999.
This paper was prepared for presentation at the 1999 SPE Asia Pacific Oil and Gas Conference and Exhibition held in Jakarta, Indonesia, 20–22 April 1999.
The advantages of a dual gradient mud column have been well documented. Significant work has been done on the riserless drilling systems by several different companies. This paper will propose two methods to achieve the same dual gradient.Nitrogen Injection: Building from proven air drilling procedures and underbalanced techniques nitrogen can be used to cut the mud weight back in the riser above the seafloor or the cut can be made deeper by combination with a concentric riser.Floating Mud Cap: A dual activity rig can use a casing riser linked to the adjacent marine drilling riser. A submersible pump in the casing riser regulates returns that will set the mud cap level in the drilling riser. A pressure sensor in the subsea BOP allows monitoring the effective hydrostatic pressure. This approach again combines field proven procedures and hardware in the deepwater environment. Both methods will be presented with emphasis on the hardware and operational procedures required to successfully implement them. Both of the methods employ existing equipment and procedures. Introduction One of the major challenges of drilling in deepwater in the Gulf of Mexico is due to low fracture gradients and shallow abnormal pressure. The margin between the fracture gradient and required mud is often less than 1 ppg.In young formations, fracture pressures are almost equal to the overburden1,2,3. Because much of the overburden is only due to the weight of the seawater the fracture pressures are only slightly greater than ‘normal pressure’. Unfortunately, there is not a corresponding drop in deepwater pore pressures due to the immaturity of the deposits typically encountered. The water confined in the rapidly deposited clay is in part pressurized by the weight of the overlying sediments trying to wring the water out, which creates a narrow margin between the pore pressures and fracture pressures. As a result the pore pressure and fracture gradients curves almost lie atop each other. A dual gradient drilling approach will result in the effective mud weight at the previous casing shoe being less than the effective mud weight at the drilling depth. The industry's effort for riserless drilling4 is designed to take advantage of this benefit. If casing points are not limited by the mud weight of the previous casing shoe, it is possible to eliminate casing points from the well program. Figure 1 shows a common goal of the dual gradient drilling. Previous efforts only brought the mud weight back to sea water equivalent at the seabed. Even better results can be achieved by cutting the mud to less than seawater at the seabed or at the shoe. By doing this it is possible to find a combination of the two mud weights that cause the heavier mud gradient to always lie between the pore pressure and fracture pressure lines. The pressures may be underbalanced inside the cased hole or riser but the open hole is overbalanced. This can be achieved by reducing the density in the upper mud section to less than seawater or by adjusting the cut point to below the seabed. This paper proposes to achieve the dual gradient by two methods. Both of which use existing equipment and employ known drilling techniques. The first proposal is nitrogen injection where normal gasification drilling methods are applied to the riser section. 1) Nitrogen injection option This paper will outline how nitrogen injected subsea can effectively create a dual gradient by gas lifting the mud in the riser. Another operator had considered a similar approach, but had proposed to sweep the entire 21" marine drilling riser with gas. This paper proposes to combine nitrogen injection with a high pressure concentric casing riser5. This reduces greatly the gas required to cut the mud. Figure 2 illustrates an example of the nitrogen being injected into a subsea BOP stack.
Underbalanced and lightweight drilling is very promising method to solve numerous drilling problems in depleted reservoirs, mature fields as well as deep-water regions. Also, maintaining underbalance during the whole drilling operation will prevent or reduce formation impairment in various formation types. In order to properly design an underbalanced operation, good control of the downhole pressures is necessary. A technology programme to develop a dynamic underbalanced drilling simulator, for proper design of operations has been undertaken. This work has included laboratory experiments, modelling achievements as well as full scale testing. Comparison of full scale data as well as field data with simulations show that the simulator predict very well the steady state downhole pressure conditions for most gas - liquid rate combinations within the realistic operational range. Also, transient pressures during rapid operational changes can be predicted well. In parallel, an innovative separator system and methodology for utilising UBD techniques in offshore operations have been developed. These technologies have been utilised in an offshore environment, for the purpose of developing lightweight drilling offshore. This paper will present the various technology elements developed (dynamic simulator; separator system and methodology for lightweight drilling) and present verification tests and field applications of these in an offshore environment. Introduction Underbalanced drilling has been increasingly used to address many field and operational problems during the last years. Some of the advantages of applying underbalance drilling are to reduce formation damage, avoid lost circulation, minimise differential sticking and increase the rate of penetration and bit life. Successful UBD depends on a complete understanding of the reservoir as well as proper design and computer modelling of the complex multiphase flow system1. Dynamic pressures2 should be evaluated using an advanced transient model; and procedures to minimise such effects developed. However; such a model had to be developed and verified, in order to meet the needs of complex operational scenarios and fluid systems3. Most UBD operations have so far been conducted onshore. However, a few operations have been performed offshore from barges (Lake Maracaibo, Venezuela) and jack-ups (Southern North Sea). Since rig heave is not pronounced, no special modifications were necessary to perform the operations. Offshore wheather conditions however, are much more critical than in a lake, and therefore a number of challenges needed to be addressed and overcome to apply UBD technology from a floater4. Of special importance was the need to develop a more compact, efficient and cost-effective separator system. These technology elements have been tested and implemented in a real operation in the Campos Basin offshore Brazil. In the following sections the development of the technologies as well as field applications will be described. Dynamic UBD Simulator; DYNAFLODRILL. The need of a dynamic design tool for any aspect of the hydraulic, multi-phase operation during underbalanced drilling, was identified early. A R&D programme to develop such a tool was defined in co-operation between Rogaland Research and several major operators, including Petrobras.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.