TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractThis paper will describe the concept study, pre-planning and drilling of test-well BETA BHB-11 and Njord well 6407/7-A-9 AH, which is the world's first Through Tubing Rotary Drilling (TTRD) well drilled from a floating platform, where specific technology developments for a TTRD Bottom Hole Assembly (BHA) were required.

Minimum requirements for kick tolerance (KT) in Maersk Oil corporate standards drove implementation of analysis using a dispersed model instead of a single-bubble model to achieve more realistic KT values for a planned high pressure and high temperature (HPHT) exploration well with oil-based drilling fluids. The large uncertainty bands of predicted pore and fracture pressures in an HPHT exploration well offshore Norway had driven the casing design, which had very narrow margins and little flexibility in setting depths for the casing strings. KT analysis using a single-bubble model indicated allowable kick volumes of less than the required 50bbl minimum. To comply fully with the 50bbl KT, additional casing strings would be required, with several liners and/or unconventional casing sizes, adding further complexity, cost, and risk to the well and the operations. The development of a well control bridging document, including HPHT procedures, competency assurance of the crew, and use of real-time pore pressure prediction service was considered inadequate to fully manage the associated well control risk. The benefit of accepting the less than 50bbl KT would be to enable drilling of the HPHT exploration well with the planned five-string casing design, which was already more robust than in most of the offset wells, which utilized a four-string design. With the five-casing-string approach, there is little room for a contingency casing/liner string without possible compromise of some of the well objectives. An internal dispensation process was initiated due to the noncompliance with Maersk Oil corporate standards. To secure dispensation from such corporate standards, transient drilling modeling software was used to calculate updated KT on the basis of the dispersed kick (gas) model. In addition, a plan was put in place to update the well model and recalculate KT using real-time well information. This case history presents the story of KT analysis of the subject well, including transient drilling modeling to investigate the sensitivity of input parameters. Furthermore, a description of the software's use to assure safe drilling operations is discussed. Lessons learnt from use of this software during planning and well construction will also be discussed.

TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractThis paper will describe the concept study, pre-planning and drilling of test-well BETA BHB-11 and Njord well 6407/7-A-9 AH, which is the world's first Through Tubing Rotary Drilling (TTRD) well drilled from a floating platform, where specific technology developments for a TTRD Bottom Hole Assembly (BHA) were required.

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Summary The densely-faulted Njord reservoir in the Norwegian North Sea is considered one of the most complex reservoirs in the world. The field is developed from a semi-submersible platform with 15 subsea-completed wells drilled in a pre-drilling campaign in 1996 to 1997 and two major platform drilling campaigns, one in 1997 through 2000 and the other in 2002 to 2003. Drilling of two conventional sidetracked oil producers in the last campaign was challenging and costly. As the field matures, the need for a cheaper way of drilling sparsely located smaller undrained compartments became essential. This led to initiate an ambitious campaign called the low-cost infill targets (LIFT) for identifying and drilling those targets using a cheaper drilling technique called the through tubing rotary drilling (TTRD). TTRD is a hugely demanding task especially, from a floating platform as any economic rationale will be lost if completion accessories and well integrity are compromised through TTRD. To the best of our knowledge, no TTRD operations have previously been executed from a floater. The severity of depletion, especially with depletion and re-pressurization (Huff'n Puff) of parts of the reservoir provides a significant technical test and challenge for TTRD on Njord. The relative movement of the floater also presents extra operational challenges, which requires accurate measures to prevent damage to the tubing hanger, Christmas tree (XMT), downhole-safety valve, and existing completion string. Issues related to bottomhole assembly design to meet drilling and production needs, mud rheology, equivalent circulating density (ECD) management, rock mechanics, and completion techniques are critically analyzed and risk-reducing or eliminating measures are put in place through extensive research and development for each of the prospective targets. This paper is intended to give a comprehensive description on the technological challenges of the TTRD technology from a floating platform, research and development activities to qualify the technology on Njord, screening of drilling targets and the drilling experiences from two TTRD wells on Njord. Introduction The Njord Field is located in blocks 6407/7 and 10 in the Haltenbanken area of the Norwegian Continental Shelf approximately 130 km northwest of the operations base in Kristiansund. The field was discovered in late 1985 and went on production on 30 September 1997. Considering deep water (330 m) and limited area distribution of the reserves (6 km in diameter), the Njord Field was developed by a semi-submersible platform with production, drilling, and living quarters (PDQ) located directly above the subsea completed wells. The subsea-completed wells are connected to the platform via flexible risers. The produced oil is stored in a floating storage and offloading unit 2.5 km away from the production platform (Fig. 1). The commercial reservoir comprises the Lower Jurassic Tilje and Middle Jurassic Ile Formations in the three main areas in block 6407/7 namely, the East Flank and the Central- and Northern Areas (Fig. 2). However, the Tilje Formations constitute the main reservoirs with 89% of the total in-place oil volumes. The current in-place oil estimate for the Tilje reservoirs is 108.4 MSm3. A total of 17.9 MSm3 of oil has been produced by January 2005, which constitutes an overall oil-recovery factor of only 16.5% for this formation. The reasons for this kind of low-recovery factor are mainly two fold: depletion drive is the preferred production mechanism for the Central- and the Northern Areas, and the reservoir is heavily faulted leaving some of the fault compartments undepleted. Because of this low recovery factor, the need for improving the overall recovery factor is paramount.