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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.

The Njord field offshore Norway produces oil from a highly faulted and heterogeneous sandstone reservoir where both vertical and lateral communication is severely restricted. This paper describes how novel multitarget sinusoidal well designs have been successfully drilled and completed on Njord in order to increase production and reserves. Introduction The Njord field is located 130 km off the coast of Norway in the Haltenbanken area. The field is developed with a semi-submersible rig with Production, Drilling and Quarter (PDQ) facilities. The water depth in the area is 330 m. The Njord sandstone reservoir, located 2600–3100 m below the seabed, is a complex faulted half-dome structure with a dense fault pattern in the central and western part of the structure, but with relatively few faults in the eastern part of the field (Fig. 1). The dense "chessboard" fault pattern in the central part of the field leads to poor lateral reservoir connectivity, as well as poor seismic quality and large uncertainty in top structure depth. The reservoir formation is subdivided into four main units with high permeability contrasts and none to very limited vertical reservoir connectivity. In the original field development plan the fault density in the central part of the field was underestimated. The planned horizontal well designs were later found to be unable to drain the area efficiently. Multi lateral drilling was investigated as an alternative, but found to be economically unattractive. Instead multitarget sinusoidal wells have successfully been drilled. The objective of these wells was to penetrate the various fault blocks in a sinusoidal well path. Reservoir sections close to 3000 m combined with several True Vertical Depth (TVD) fluctuations up to 280 m have been drilled. This gives 3 to 4 wells "for the price of one". Reservoir description The Njord field is an oil field with multiple reservoir zones of Jurassic age. The main reservoir is the Tilje Formation, but some additional resources exist in the Ile Formation. A cross section through the reservoir is shown in Figure 2. Tilje Reservoir The Tilje Formation is subdivided into four reservoir units, Tilje-1–4, separated by continuous shales. The depositional environment changes from a fluvial dominated delta front at the base to tidal flats towards the top. The reservoir is heavily stratified and laminated giving little or no vertical communication. The average porosity is in the range of 18–20% whereas the permeability varies strongly laterally and vertically from a few millidarcies to several Darcies. The formation thickness is in the range of 70 to 200 m. Faults The Njord structure is characterized by a large amount of normal faults oriented in two main directions. The major faults are striking southwest-northeast, whereas a secondary trend north-south is recognized particularly in the central part of the structure. The field is divided into three main structural parts: Central Area, East Flank and North. The heavy tectonic activity has resulted in a certain degree of compartmentalization of the reservoir, which has a major influence on the reservoir performance. The structural complexity is high in the Central Area and North, whereas the degree of compartmentalization is decreasing on the East Flank. The best seismic quality and the least faulted area appear to be on the East Flank. Poor seismic quality in the Central Area and North makes prediction of top structure difficult. Tilje Reservoir The Tilje Formation is subdivided into four reservoir units, Tilje-1–4, separated by continuous shales. The depositional environment changes from a fluvial dominated delta front at the base to tidal flats towards the top. The reservoir is heavily stratified and laminated giving little or no vertical communication. The average porosity is in the range of 18–20% whereas the permeability varies strongly laterally and vertically from a few millidarcies to several Darcies. The formation thickness is in the range of 70 to 200 m. Faults The Njord structure is characterized by a large amount of normal faults oriented in two main directions. The major faults are striking southwest-northeast, whereas a secondary trend north-south is recognized particularly in the central part of the structure. The field is divided into three main structural parts: Central Area, East Flank and North. The heavy tectonic activity has resulted in a certain degree of compartmentalization of the reservoir, which has a major influence on the reservoir performance. The structural complexity is high in the Central Area and North, whereas the degree of compartmentalization is decreasing on the East Flank. The best seismic quality and the least faulted area appear to be on the East Flank. Poor seismic quality in the Central Area and North makes prediction of top structure difficult.