The Njord Field is one of the most complex reservoirs in the Norwegian North Sea due to a large number of faults. Seismic quality in the heavily faulted areas is poor making seismic interpretation difficult. Typically the uncertainty in top structure is in the order of 10–100 m. The well design criteria on Njord are dependent on the structural uncertainty and the availability of new technology. Njord started with relatively simple single-bore horizontal wells. This design was not suitable in the intensely faulted areas. High amplitude U-, S- and W-shaped wells have been effective to penetrate all reservoir units in uncertain areas, to minimize the risk of hole-instability and to drain oil from multiple fault compartments. The need for placing a well in the best reservoir interval, to penetrate longer reservoir section and to drain oil from sparsely located undrained compartments led to drill a tree-branched well in the recent days. A new active visualization assisted geo-steering technology coupled with biostratigraphy has been tremendously successful to steer the well path in the most complicated area. Another well has been drilled in another heavily faulted area where the pressure depletion in some of the compartments was in excess of 300 bar. The drive for a cheaper way of drilling sparsely located relatively smaller undrained pockets led to initiate a new campaign to find and drill low cost infill targets using the Through Tubing Rotary Drilling technology. Intensive research and development activities have been undertaken to resolve outstanding issues, such as, protection of X-mas tree and down hole safety valve, risk for mud loss and differential sticking, zone isolation by swell packers, etc. This paper summarizes the reasons for adopting so many different well types, and a comprehensive description on their planning, execution, challenges, successes and failures. Introduction The Njord Field is located in the Haltenbanken area of the Norwegian North Sea approximately 130 km northwest of the operations base in Kristiansund (Fig. 1). The field is situated in the southern part of block 6407/7 and straddles into the northern part of block 6407/10. However, the commercial reservoir is comprised of three main areas in block 6407/7, namely, the eastward dipping eastern area called the East Flank, the crestal part of the structure called the Central Area and the northward dipping northern part called the Northern Area (Fig. 2). No hydrocarbon discovery has so far been made in block 6407/10. The field was discovered in late 1985 by the exploratory well 6407/7–1 in the East Flank and was subsequently appraised by well 6407/7–2 in the Central Area, 6407/7–4 in the East Flank, and 6407/7–3 and 6407/7–5 in the Northern Area within a time frame of 1986–1991 (Fig. 2). Wells 6407/7–1, 6407/7–3 and 6407/7–4 encountered hydrocarbon-bearing formations of Middle Jurassic Ile at the top, Lower Jurassic Tilje in the middle and Lower Jurassic Åre at the bottom. Hydrocarbon-bearing Ile and Tilje, and water-bearing Åre were found by well 6407/7–2. Commercially viable test oil productions were observed from the Tilje reservoirs in 6407/7–1, 6407/7–2 and 6407/7–4 wells. However, well 6407/7–3 penetrated a much thinner oil pay while the area appraised by 6407/7–5 was water bearing. Therefore, no economically viable resource was immediately foreseen in the Northern Area. A plan for development and operation (PDO) was approved in February 1995 based on production of the oil resources from the Tilje Formation of the East Flank and the Central Area. However, enough flexibility was introduced in the plan to produce the additional (technical) oil resources in the Tilje Formation of the Northern Area, and oil resources in the Ile and re Formations if proved economically viable. The PDO expected stock tank oil originally in-place (STOOIP) was estimated to be 101.3 million Sm3. The expected recoverable oil reserves were 31.6 million Sm3 corresponding to a 31 percent overall recovery factor. A production plateau rate of 12480 Sm3/d was foreseen. The technical oil resources were estimated to be 39.6 million Sm3 with 4 to 6 million Sm3 of recoverable reserves.

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The Njord Field is one of the most complex reservoirs in the Norwegian North Sea. A large number of faults, and uncertainties associated with structures, fault transmissibilities, fluid contacts, rock and fluid properties pose a great challenge for reservoir management. The main development philosophy was to minimize uncertainties based on drilling and production/injection experiences. The uncertainty was managed to a great extent by limiting the number of pre-drilled wells. The early experiences, particularly from a horizontal well led to radical changes in the well designs. The concept of drilling high amplitude sinusoidal wells was very successful for safe penetration of all reservoir intervals in multiple fault compartments. It also eliminated the need for a large number of wells. An active visualization assisted geosteering together with biostratigraphy was successfully applied to drill a complex three-branched well in the most faulted area of the field. Another well was drilled in an area where pressure depletion was in excess of 300 bar in some of the fault compartments. State-of-the-art seismic interpretations have been performed on various data sets to identify and locate reservoir intervals and faults. Reservoir simulations assisted by 4D seismic have identified remaining oil pockets. A low cost solution for drilling relatively small, sparsely located infill targets using the through tubing rotary drilling technology has been considered. Systematic reservoir monitoring through collection of various types of data, such as, PLT, RFT/MDT, tracer, production/injection, bottom hole pressure, 4D seismic, etc. has been an effective tool to devise reservoir management strategies. In addition to unique well designs, selective perforation, re-perforation, gas injection, zone isolation and infill drilling have improved the oil recovery significantly. This paper summarizes the reservoir management challenges, techniques and technologies applied to evaluate and monitor the reservoir performance, strategy to manage uncertainty, practices for improved recovery, successes and failures. Introduction The Njord Field is located in the Haltenbanken area of the Norwegian North Sea approximately 130 km northwest of the operations base in Kristiansund (Fig. 1). The field is situated in the southern part of block 6407/7 and straddles into the northern part of block 6407/10. The commercial reservoir is comprised of three main areas in block 6407/7 namely, the eastward dipping eastern area called the East Flank, the crestal part of the structure called the Central Area and the northward dipping northern part called the Northern Area (Fig. 2). The Tilje Formations constitute the main reservoirs in these areas. However, there are few important prospects in the vicinity of the main field namely, the northwestern area called the North-West Flank, the southeastern Upper Jurassic prospects, and the Ile Formation in the Central Area, East Flank and the Northern Area. The field was discovered in late 1985 by the exploratory well 6407/7–1 in the East Flank and was subsequently appraised by well 6407/7–2 in the Central Area, 6407/7–4 in the East Flank, and 6407/7–3 and 6407/7–5 in the Northern Area within a time frame of 1986–1991. Wells 6407/7–1, 6407/7–3 and 6407/7–4 encountered oil-bearing formations of Middle Jurassic Ile at the top, Lower Jurassic Tilje in the middle and Lower Jurassic Åre at the bottom. Gas-bearing Ile, oil-bearing Tilje and water-bearing Åre were found by well 6407/7–2. Commercially viable test oil productions were observed from the Tilje reservoirs in 6407/7–1, 6407/7–2 and 6407/7–4 wells. However, well 6407/7–3 penetrated a much thinner oil pay while the area appraised by 6407/7–5 was water bearing. Therefore, no economically viable resource was immediately foreseen in the Northern Area. The North-West Flank was explored in late 2000 by well 6407/7–6, which encountered highly rich gas-condensate in the Tilje Formation. Three exploratory wells were drilled in block 6407/10 during 1987–1992 but no hydrocarbon discovery was made.

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.