The analysis of land–atmosphere feedbacks requires detailed representation of land processes in atmospheric models. The focus here is on runoff–infiltration partitioning and resolved overland flow. In the standard version of WRF, runoff–infiltration partitioning is described as a purely vertical process. In WRF-Hydro, runoff is enhanced with lateral water flows. The study region is the Sissili catchment (12 800 km2) in West Africa, and the study period is from March 2003 to February 2004. The WRF setup here includes an outer and inner domain at 10- and 2-km resolution covering the West Africa and Sissili regions, respectively. In this WRF-Hydro setup, the inner domain is coupled with a subgrid at 500-m resolution to compute overland and river flow. Model results are compared with TRMM precipitation, model tree ensemble (MTE) evapotranspiration, Climate Change Initiative (CCI) soil moisture, CRU temperature, and streamflow observation. The role of runoff–infiltration partitioning and resolved overland flow on land–atmosphere feedbacks is addressed with a sensitivity analysis of WRF results to the runoff–infiltration partitioning parameter and a comparison between WRF and WRF-Hydro results, respectively. In the outer domain, precipitation is sensitive to runoff–infiltration partitioning at the scale of the Sissili area (~100 × 100 km2), but not of area A (500 × 2500 km2). In the inner domain, where precipitation patterns are mainly prescribed by lateral boundary conditions, sensitivity is small, but additionally resolved overland flow here clearly increases infiltration and evapotranspiration at the beginning of the wet season when soils are still dry. The WRF-Hydro setup presented here shows potential for joint atmospheric and terrestrial water balance studies and reproduces observed daily discharge with a Nash–Sutcliffe model efficiency coefficient of 0.43.

SPE members Abstract Well 30/6-C-26A was drilled to 9327 m measured depth in January 1995 from the Oseberg C platform in the North Sea. The well has a horizontal reach of 7853 m which is a new world record in Extended Reach Drilling. The last 2100 m were drilled horizontally in the reservoir 6 – 8 m vertically above the oil water contact. This paper will describe the planning phase as well as operational challenges experienced during drilling of this well. Introduction The Oseberg Field was discovered in 1979. To develop this 27 × 5 km giant field, two platforms were located 15 km apart. To drain the oil between the platforms two subsea wells were drilled and completed (Figure 1). For the last three years Norsk Hydro together with the partners in the Oseberg Field have put up an overall goal to increase the recoverable reserves by 50 million SM3 (316 million barrels). This corresponds to a final recovery factor of 64 %. Horizontal drilling is one of the most important factors to achieve this goal. The horizontal drilling programme on the Oseberg Field is one of the most comprehensive to take place in the North Sea. The first horizontal well in the Oseberg Field was drilled in 1992. Since then a total of 17 horizontal wells have been successfully drilled and completed. The general trend during this period is that both the length of the horizontal reservoir section as well as the total depth for the wells have increased (Figure 2.). New equipment and technology as well as general field experience played an important role when deciding to go ahead with C-26A. The Oseberg Field Reservoir The reservoir units on the Oseberg Field consist of several sand units within the Middle Jurassic Brent Group (Figure 3). The main reservoir unit is the Oseberg Formation which consists of medium to coarse grained fan-delta sandstones of excellent reservoir quality. The vertical thickness is about 20–60 m and the reservoir qualities are excellent. The Etive Formation overlies the Oseberg Formation. The upper unit in the Brent Group is the Tarbert Formation which is of fair to poor quality and thickness in the south, but is a significant unit in the northern part of the field where the vertical thickness can exceed 40 m. The Ness Formation separates the Tarbert and the Oseberg formations. The Ness Formation consists of delta plain channel sandstones interbedded with overbank fine-grained sediments and coal beds. The channel sandstones are difficult to map from seismic and well data due to their size and scattered occurrence. P. 131

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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.