The Varg field (PL038, Block 15/12) is located in the Norwegian Sea. The Varg reservoir is Oxfordian sandstone of Jurassic age, with an upper (1000–2000 mD) and lower (100–200 mD) sand separated by a mud rich sandstone. The field is highly compartmentalized and is located around a salt dome, and contains a number of different formation waters ranging from high salinity, higher barium (up to 280 mg/l) in the West and lower salinity and lower barium (30 mg/l) in other areas. All waters contain naturally occurring dissolved iron at concentrations up to 175 mg/l. Following scaling in several wells, a chemical re-selection and treatment optimisation programme was initiated. Extensive laboratory studies were undertaken to select optimum inhibitors, which was further complicated by environmental requirements. The presence of dissolved iron was shown to have an adverse effect on the incumbent scale inhibitor, leading to the selection of a number of alternative products. Given the highly compartmentalized nature of the reservoir and the large permeability contrast between zones, near wellbore modelling studies, examining chemical placement using both conventional (aqueous) based treatments and also viscosified treatments, were conducted. The potential for poor placement and subsequent poor lifetimes led to further detailed simulation work using up to date PLT logs to further refine the treatments in subsequent wells. Therefore, this paper describes the various challenges facing scale control in the Varg field. The paper presents results from a chemical re-selection exercise showing the controlling influence of dissolved iron, together with coreflood studies used to select the most effective non-damaging product for subsequent field trials. Extensive near wellbore modelling results are presented to illustrate the challenges faced with respect to effective chemical placement, which highlight the challenges faced. Several field trials have now been conducted with a new chemical and the results of these are also discussed.
Introduction & Challenges
The Varg field is located in block 15/12 of the Norwegian Sea (Figure 1) at a water depth of 86m, and came on stream in December 1998. The wells are tied into the wellhead platform Varg A, and the oil is processed at the FPSO Petrojarl Varg (Figure 2). The distance between the well head platform and the ship is 1 km. The oil is exported by tankers and the produced gas re-injected. The Varg reservoir is Oxfordian sandstone of Jurassic age, with upper and lower sands separated by a mud rich sandstone. The upper sand, which is mainly deposited in the south, is of good quality (permeability range 1000–2000 mD). The lower sand is of more variable quality with an average permeability around 100–200 mD with some zones significantly lower. The field has a complex fault pattern, especially close to the salt dome. There are strong tectonian forces in the area and the seismic has poor resolution due to overlaying chalk. This limits the sensitivity of the reservoir description used in the full field model and makes accurate assessment of the water sweep patterns difficult. There are different fluid properties in most wells, consistent with the highly compartmentalized nature of the field and the field consists of several PVT regions. Most of the wells are vertical or normally deviated cased, cemented and perforated wells with relatively short pay zones from different formation zones. There are also horizontal wells, the longest horizontal being about 1000m long, again producing from different zones. Most of the wells have gas lift due to low reservoir pressure. Carbonate scaling is relatively insignificant in this reservoir with the main challenges associated with barium sulphate scales both relating to self scaling issues and also mixing of reservoir formation waters with injected sea water.
