Review of Identification, Characterization and Modeling of different types of Fault & Fracture Systems in a Giant Offshore Carbonate Reservoir, UAE
Authors: Shekhar Ravi , Edwards Ewart , Obeta Chukwudi 
Published: November 2019

The subject field of this study is a geologically complex Lower Cretaceous carbonate system of stacked reservoirs and dense seals, which have been affected by extreme diagenetic modification, including various types of faults & fractures. The purpose of this paper is to summarize several years of comprehensive study of identification, characterization and modeling of different fault/fracture systems within the reservoir stack, integrating different data types, conceptual models and geomodelling approaches for individual, or groups of, reservoirs. Timely identification and appropriate characterization of the fracture-fault systems is extremely important for correct well placement and completion strategy.

The field has been producing for more than 30 years under a water-flood recovery strategy. Numerous amounts of different types of data including seismic, cores, image logs, tracers, interference tests, production/injection data etc. has been collected and studied carefully throughout those years. The integration of different data sets combined with regional structural study suggest existence of four different types of fault and fracture systems, with predictable and consistent orientations, that affect the reservoirs: Seismically mapped faults, fracture corridors, diffuse natural fractures, and thermally generated/artificially enhanced fractures. Their impacts on field performance vary, being dependent upon scale (length/throw) and stratigraphic positioning. For example, faults with large throw (30-60ft) can create inter-reservoir communication due to fault juxtaposition whereas diffuse fractures that are generally located at the top and base of the reservoirs (due to mechanical contrast between dense and reservoir rock strength) act as intra-reservoir permeability enhancement. Fracture corridors that can be either restricted to, or cross, reservoirs are most difficult to detect due to small offset and (when oriented NNE) possibly impose greatest impact in terms of flow heterogeneity. Thermally enhanced fracture effects are observed where newly drilled wells enter the water-flood affected area. These fractures can be seen on the image logs, especially where they fracture most brittle dense intervals limestones and their impact in reservoir is noted most from production water cuts.

Modeling different types of fractures for simulation studies and field development planning are always very challenging due to limited availability of the critically appropriate data. As a result, an integration of different data types was key to identify and model different faults and fracture systems. Numerous faults were mapped from seismic but only those mapped with high confidence, supported by drilling reports and LWD log evidence, and which show impact on production performance, were included in the model – all faults are considered for other purposes (e.g. well planning). Diffuse fractures were modeled combining traditional methods (fracture density from cores) with ant-track attribute from seismic to determine directionality and distribution away from cored wells; and span the scale range to through-going fracture corridors. A new method has been introduced to assign effective permeability to fractures from seismic attributes by scaling up and down to the well test permeability. The displacement functions for fractures are estimated as effective pseudo-functions, representing the diffuse fracture and matrix media and conditioned to the performance of the well observing such fracture effects. Fracture corridors, whenever observed, were modelled deterministically as effective properties. Given the uncertainties associated with data and sub-surface knowledge, coupled with the classification of all reservoirs in the stacked sequence being fracture-assisted matrix zones, the objective of each and every fracture model approach was to create a representative, simple and fit-for-purpose model that can be modified and updated easily during history-match iterations and that serves business needs.