Characterization of dense zones within the Danian chalks of the Ekofisk Field, Norwegian North Sea

<p>The Ekofisk Field is a giant field which has been producing at a high level for more than forty years and, since 1987, this production has taken place with the support of sea-water injection. The Danian-aged chalk deposits of the Ekofisk Formation and the Maastrichtian Tor Formation form the main reservoir units in the Ekofisk Field. The Ekofisk Formation principally consists of porous resedimented chalks intercalated with relatively thin and lower porosity beds, called dense zones. A multi-scale study of dense zones, from scanning electron microscopy to wells and seismic impedance data, has allowed the characterization and mapping of these deposits. Five main dense zone lithotypes have been identified: (1) argillaceous chalk; (2) chalk with abundant flint nodules; (3) chalk beds cemented with silica/nano-quartz; (4) calcite-cemented chalk; and (5) stylolitized chalk. The different types of dense zones tend to cluster in certain stratigraphic intervals, such as the EE and EM reservoir units at the base and in the middle part of the Ekofisk Formation. Dense zones have different mechanical properties compared to porous chalks and, depending on the connectivity of their fracture networks, they can act as preferential conduits or baffles for the reservoir fluids. An increased understanding of the distribution, characteristics and geological factors at the origin of the dense zones is fundamental to better define the reservoir architecture and ultimately identify unswept zones for future infill drilling targets. </p

MassFLOW-3D as a simulation tool for turbidity currents

Turbidity currents are the most important mechanism for the dispersal and deposition of sand in the deep-sea setting and thus the main phenomenon leading to the formation of oil and gas reservoirs in deep water deposits. The flow characteristics of turbidity currents are difficult to observe and study from the modern environment and their experimental approximations in the laboratory are typically limited by scaling issues, unrealistic flume geometries and short durations. Computational fluid dynamic (CFD) analysis, realised as numerical simulations, has been developed to fill the gap between the small and large scale, integrating data from theory, nature and experiments. CFD can also shed light on flow parameters which are so far impossible to deduce from experimental and field studies, such as detailed density and turbulent kinematic energy distributions. The deterministic process modelling CFD software MassFLOW-3D™ has been developed and used successfully to construct a three-dimensional model for the simulation of turbidity currents. All principal hydraulic properties of the flow (e.g. velocity, density, sediment concentration, apparent viscosity, turbulence intensity and bottom shear stress) and its responses to topography can be monitored continuously in three dimensions over the whole duration of the turbidity current. In this paper, comparisons made between the numerical output of MassFLOW-3DTM and the physical experiments are presented. In addition, the code is used to model the spatial characteristics, velocity structure and deposits of high-density turbidity currents and the flow dynamics of low-density turbidity currents in a sinuous channel. The numerical simulations show close fit to experimental sandy turbidity current dynamics for flows with sediment concentrations up to 27%. However, despite this initial success, on-going customisation and validation of these models, together with implementation of improved subroutines aimed at sediment transport and deposition, is essential in improving the computational code and our understanding of the natural phenomena