Drilling engineers have been calculating hydrostatic pressure greater than reservoir pressure in almost all conventional drilling applications. This has resulted in various drilling challenges, such as reduced rate of penetration, differential sticking, mud losses, formation damage, wellbore stability, and formation fracture and associated cost. Managed pressure drilling (MPD) and underbalanced drilling (UBD) have played a vital role to significantly minimize these drilling problems onshore as well as offshore. Since most of the wells being drilled today include measurement-while-drilling (MWD), logging-while-drilling (LWD), and rotary steerable system (RSS) tools, the interface between these tools with MPD and UBD technologies has become extremely critical not only to understand but also to execute. These logging and steering tools have huge proven combined benefits in both MPD and UBD environments. The performance of all these tools in MPD and UBD environments is influenced by different challenges in terms of data transmission (mud pulse telemetry), hydraulics, steerability, and maintenance.

The well 34/8-A-6 AHT2 was drilled from the Visund Field Floating Production and Drilling Unit (FPDU) in the North Sea, and set on production in October 2005. The well was drilled to 9082 m/29796 ft measured depth and has an Along Hole Depth (AHD) reach of 7593 m/24911 ft, which is a world record for Extended Reach Drilling (ERD) from a floating installation. The 34/8-A-6 AHT2 is also the longest Down hole Instrumentation and Control System (DIACS) installation worldwide, with the lower isolation packer set at 8560 m / 28084 ft measured depth. The well includes three hydraulically operated flow valves, which are used as down hole chokes to optimize the production from the separate zones in the reservoir. Subsea developments in combination with ERD wells can increase oil production and lower total development cost. The drilling progress was 108 m/day from seabed to total depth, according to the Rushmore drilling performance definition, and the payback time for this well was less than two months. Experiences gained on this well indicate that even longer wells can be drilled from subsea locations in the near future. Introduction The Visund field is located in block 34/8 in the North Sea 150 km west of Norway (Figure 1, 2, 3). The field was discovered in 1986 and production started in 1999. The Visund field is an oil & gas field, with a water depth of 335 m (1100 ft). The depth of the main reservoir is between 2900–3000 mTVD, with a maximum pore pressure of 434 bar. The field is 24 Km long and 4 Km wide. With this shape of the field, ERD wells drilled both to the North and to the South will increase drainage area and thereby the total recovery from the field. The Visund Floating Production and Drilling Unit (FPDU) is located centrally on the field. The Visund North satellites consist of two wells tied back to the FPDU with a 9 km long subsea pipeline. The well in this case history is a world record ERD well drilled from a floating installation. In the early pre-planning phase, the well was planned as a separate costly subsea development, drilled by a separate semi-submersible rig. A new technical and economical study showed that this well could be drilled more economically from the existing Visund FPDU, using existing subsea systems. The total depth of the well 34/8-A-6 AHT2 is 9082 m. The horizontal reach (slot to TD) is 7484 m and the along hole depth (AHD) reach is 7593 m, - a world record reach from a floating installation. (Figure 4, 5, 6) Low friction factors in relation to torque were experienced by the use of an optimum well profile. Good hole cleaning was obtained with the use of 180 RPM on the drillstring together with maximum allowable flow rate. The ERD well has a Down hole Instrumentation and Control System (DIACS) completion with tree separate zones, operated by three hydraulically controlled flow valves. This is the longest DIACS completion in the world, with the lower isolation packer set at 8560 m. The well is produced at a rate of 2500 Sm3/day (15700 bbl/day) with production from all zones. Production from the upper zone A would not have been possible without a controlled production from the other zones, hence adding value to the DIACS completion design Experiences from this well show that even longer wells can be drilled from subsea locations in the near future. Optimal pre-planning with use of all service companies involved in detail planning and risk identification workshops are a critical factors for success. In the operational phase the work in the subsurface team was optimised through using 3D visualisation tools. These 3D tools facilitated in getting a common understanding in the whole team, which was used to optimize the reservoir pay zone drilling of the well. Subsea developments in combination with ERD wells can increase oil production and lower total development cost, in comparison to costly additional subsea systems that need to be installed prior to drilling a new well.

Over the last decade, new technologies and economic strategies have enabled operators to give new life to mature fields and old platforms. Production and economic optimization are main goals of reentry campaigns. With time, the industry has seen growing opportunities for reentry wells as mature fields and platforms are becoming older and less productive. Fully utilizing reentry technological capabilities and achieving successful operations require effective well planning and execution. Cutting and pulling of old completions and casings, wellbore cleanouts, plug and abandonment operations, section milling, mud systems, well integrity, cased-hole and open hole sidetracks, whip stocks, cutting/swarf handling, surveying tools, well collisions, and existing rig capabilities on platforms are the major challenges to the growth of reentry business. However, development of rotary steerable systems, logging while drilling, modern surveying tools, and under-reaming technologies have given impetus to the reentry well drilling market. From the concept phase of plug and abandonment to well delivery and production, seamless planning and communication is required among all the stakeholders. Modern surveying tools such as continuous north-seeking gyros and gyro while drilling have revolutionized the surveying industry in high magnetic interference environments, giving ease to planning sidetracks and accurate wellbore positioning in high well-density environments. Drilling close to the motherbores is becoming a common and attractive way of exploiting the reserves as no detailed logging and characterization is required. This has resulted in complexities of directional drilling and well collision risks. Risk of well collisions with producing wells is one of the biggest challenge in reentry wells.

TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractThe well 34/8-A-6 AHT2 was drilled from the Visund Field Floating Production and Drilling Unit (FPDU) in the North Sea, and set on production in October 2005. The well was drilled to 9082 m/29796 ft measured depth and has an Along Hole Depth (AHD) reach of 7593 m/24911 ft, which is a world record for Extended Reach Drilling (ERD) from a floating installation.The 34/8-A-6 AHT2 is also the longest Down hole Instrumentation and Control System (DIACS) installation worldwide, with the lower isolation packer set at 8560 m / 28084 ft measured depth. The well includes three hydraulically operated flow valves, which are used as down hole chokes to optimize the production from the separate zones in the reservoir.Subsea developments in combination with ERD wells can increase oil production and lower total development cost. The drilling progress was 108 m/day from seabed to total depth, according to the Rushmore drilling performance definition, and the payback time for this well was less than two months. Experiences gained on this well indicate that even longer wells can be drilled from subsea locations in the near future.

Rathole elimination (RHE) BHA comprised of hydraulics and ball-drop underreamer has been used in different North Sea fields. RHE success has been very important for operators in completion, cementing, casing running optimization, and handling contingencies by running intermediate liners and casings. One of the major benefits of the RHE system is time saving because the rathole is removed in the same drilling run and the dedicated underreaming run is not required. In today's field operations, this time saving has become considerably important on the older platforms as well. An RHE system has been successfully run on the North Sea continental shelf (NCS) with five different operators and in six challenging fields, resulting in significant time and cost saving for each operator. BHA and hydraulics design are the basis of these successful RHE operations. Starting from a vertical well to whipstock exits and 2D wells followed by 3D complex wells have established that the RHE system is fully directional and steering capability is not compromised by BHA complexity. Suites of applications, including the integrated dynamic design and analysis 4D platform was used for BHA optimization and stability, underreamer and bits cutting structure selection, and directional response prediction. Both push-the-bit and point-the-bit rotary steerable systems (RSS) have been used in these operations. RHE showed great success and technology maturity on NCS in various borehole sizes. This paper presents the historical run summaries and the technology maturity life cycle in the North Sea.