Natural fractures can have a significant impact on fluid flow by creating permeability anisotropy in hydrocarbon reservoirs. They can also play an undesirable role on reservoir subsidence and compaction during depletion, with important consequences for production strategy and well and surface facility equipment. The investigation of these possible fracture effects motivated a comprehensive integrated fracture study of three reservoirs from a giant gas-condensate field in Abu Dhabi. The main objective of the study was to build representative 3D fracture models and compute fracture properties of each reservoir, to be used in future accurate dynamic simulations. The results of the fracture study were also used to define the risk associated with the geomechanical integrity of the reservoirs.
The integrated workflow included several approaches, all contributing towards the global understanding of the fracture distribution and flow impact in the reservoirs. The static fracture characterization involved detailed seismic inversion and fracture characterization, core and borehole image analysis. It focused on the identification of the fracture components occurring in the reservoirs and their geometrical properties and spatial distribution. This was complemented by a study of the well dynamic data (e.g. fluid injection/production data, well tests, flowmeters, static pressure data), carried out to evaluate the dynamic impact of the fractures by identifying wells showing anomalous dynamic behaviors. The integration of static and dynamic data allowed the identification and quantification of which fracture components played a role on fluid flow in the reservoirs.
The results of the static and dynamic data analysis were integrated to develop a 3D Discrete Fracture Network (DFN) model of each reservoir, reflecting the fracture organization identified during the characterization stage. The hydraulic properties of fractures (aperture and conductivity) were determined using flowmeter and well test data. The calibrated fracture models were then upscaled as to compute equivalent fracture properties (fracture porosity, permeability tensor and equivalent matrix block sizes or shape factors) to be used in further full-field reservoir simulation models.
The results of the study concluded that the reservoirs are dominated by the presence of large scale fracture corridors that were modelled deterministically in the DFNs based on the integration of well and seismic data. There is also a negligible, unconnected small scale diffuse fracturing in the reservoirs, with no flow impact. The fluid production is mainly controlled by matrix support with very limited contribution from fracture corridors. Finally, the very limited fracturing detected suggests no major risk in terms of reservoir integrity.
The integrated study carried out allowed the development of accurate, consistent fracture models for the three studied reservoirs. The uncertainty associated with the fractures and their impact was properly addressed, allowing building better field development plans and defining risk-free reservoir depletion strategies.