Harald Johansen

The petroleum characteristics and filling history of the Frøy field and the Rind discovery, Norwegian North Sea

Abstract The Brent Group reservoirs of the Froy field and the Rind discovery, Norwegian North Sea, show complex distributions of petroleum and aqueous fluids. Multiple fluid contacts are present in the Rind discovery, and the fields show various extent of compartmentalisation due to faulting. Geochemistry data of core extracts indicate that the two fields have different petroleum sources (Bhullar, Karlsen, Holm, Backer-Owe & Tran, 1998). Late diagenetic modifications of the reservoir quality consist of quartz cementation followed by carbonate and kaolinite cementation and dissolution of K-feldspar. Fluid inclusion results indicate that filling of the reservoirs occurred after the main phase of quartz cementation at pressure–temperature conditions close to the present. Differences in fluid properties between fault compartments in both fields have been identified. Fluid inclusion data have proved especially valuable for evaluation of the sealing capacity of faults through time, and given evidence for the fault lineament north of both fields to be a time-integrated pressure seal. Except for a lower gas condensate zone in the 25/2–13 compartment of Rind, the present reservoir fluids are volatile oils close to gas saturation. The previous presence of an oil in the lower reservoir zone of 25/2–13 has also been demonstrated. The fluid inclusion data indicate that the condensate in 25/2–13 represents a second episode of migration/filling rather than phase separation.

Flow Along Fractures in Sedimentary Basins

Sedimentary basins are large depressions in the earth’s crust containing from a few to more than 15 km of sediments. At deposition, sediments may contain in excess of 50% water; huge volumes of fluids are thus buried in deep basins. Fluid is also produced by water-releasing reactions in the sediments and positive topography surrounding the basin may drive groundwater into it under the influence of gravity. Fluids in basins will move relative to the sediment grains, transporting heat and solids in solution; they can thus influence sediment temperatures and cause diagenetic reactions. The extent to which this occurs will depend on the velocity of the fluid, its temperature and the solutes dissolved in it. In basins a myriad of factors will control fluid flow and fluid-rock interaction, consequently only rather rough estimates of fluid movements and their effects are in general achievable.

Models for Calcium Carbonate Precipitation in the Near-Well Zone by Degassing of CO2

Abstract: Calcium carbonate scale formation is a well known problem for water producing wells. Although there are several types of scale forming processes, we investigate the case of calcium carbonate precipitation when the degassing of CO2 causes the calcium equilibrium concentration to decrease towards a production well. We study a simplified system of carbonate chemistry, which allows for analytical expressions for the porosity loss as a function of time. The precipitation process normally goes from flow-limited away from the well to precipitation-limited close to the well. We derive an expression that estimates the transition zone between these two regimes. Furthermore, we present analytical estimates for the porosity reduction at a given radius as a function of time, including an estimate for each of these precipitation regimes. These analytical results are tested against numerical solutions for the porosity loss, which account for the full set of equations of the model. The analytical models give an accurate estimate of the linear porosity reduction with time, until at least half the porosity is lost. Examples of scale formation are given for the two regimes. Reasonable values for the precipitation kinetics indicate that most production operations have a kinetics-limited regime close to the well. The models also show that this type of scale formation takes place very close to the wells, typically within a few well radii from the walls of the well.