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.
