Identification of Key Fracture Effects on Fluid Flow and Iterative Approach for Effective Permeability Modeling of a Matrix-Dominated Carbonate Reservoir, Offshore Abu Dhabi

Abstract

Abstract Modeling effective permeability in carbonate reservoirs is a challenge in many cases, especially in subtly-fractured matrix-property dominated reservoirs, where the presence of tectonic fractures and/or matrix-related high permeability streaks can considerably enhance the effective reservoir permeability. Small-scale faults and fractures, especially when located within ‘layers’, zones or ‘corridors’, can affect water movement in the reservoir: these can be identified from core and image log more readily than from seismic data. Often neglected, diffuse fractures concentrated in stratigraphically predictable ‘fractured layers’ at reservoir-dense contacts can play important roles in fluid flow in such reservoirs. Permeabilities derived from well test data often do not match with thickness-weighted core permeability averages, therefore, core based permeability values need to be modified to match the well test data, applied according to the fracture susceptibility of the stratigraphic zonation of the reservoirs, which reflects a mechanical layering effect. Different data media are useful in determining the most applicable drivers, with respect to the different fracture styles present (‘fractured layers’, corridors & fault zones), e.g. curvature analysis of 3D seismic derived reservoir surfaces are effective indicators of possible sub-seismic scale fault zones or fold flexure structures, such lineaments defining water movement pathways in the reservoir. Lateral fracture density variation may relate to such reservoir surface attributes as an indirect indication of strain magnitude within the reservoir. Additionally, fracture density on a per foot basis, from core observations, can also represent a fracture contribution of effective permeability in the vertical profile. A conceptual model incorporating all three fracture styles summarizes the non-matrix fluid flow system present. In this particular example, the characteristics of the reservoir are controlled by both matrix properties and fractures. Achieving realistic effective permeability, by integrating core- and seismic-based fault/fracture information is a complex process that requires successive iterations, which can be both time-consuming and laborious when giant fields are considered. The presentation illustrates an iterative workflow to integrate an existing geological model (i.e. matrix-dominated model) with the core fracture and seismic-based data to construct a reliable effective permeability model for the reservoir that best fits the dynamic data from the field.