Drilling of Multitarget Sinusoidal Wells to Manage Geological Uncertainty and Poor Reservoir Connectivity on the Njord Field, Offshore Norway

Abstract

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.