The Impact of Hierarchical Fracture Networks on Flow Partitioning in Carbonate Reservoirs: Examples Based on a Jurassic Carbonate Ramp Analog from the High Atlas, Morocco

Agar, Susan ¹; Geiger, Sebastian ²; Matthäi, Stephan ³; Alway, Robert ⁷; Tomás, Sara ⁴; Immenhauser, Adrian ⁵; Shekhar, Ravi ¹; Paul, Jonathan ⁸; Benson, Gregory ¹; Karcz, Zvi ¹; Kabiri, Lahcen ⁶
(1) ExxonMobil Upstream Research Company, Houston, TX. (2) Heriot Watt University, Edinburgh, United Kingdom. (3) Montan Universität Leoben, Leoben, Austria. (4) Universität Potsdam, Potsdam, Germany. (5) Ruhr Universität Bochum, Bochum, Germany. (6) Errachidia University, Errachidia, Morocco. (7) ExxonMobil Development Company, Houston, TX. (8) Imperial College, London, United Kingdom.

Hydrocarbon reservoirs commonly contain an array of fine-scale structures that are below the resolution of seismic images. These features may impact flow behavior and recovery, but specific impacts may be obscured by the upscaling process for reservoir simulations. It is therefore important to identify those situations in which subseismic structures can introduce significant departures from full-field flow predictions.

Using exposures of Jurassic carbonate outcrops near the village of Amellago in the High Atlas Mountains of Morocco, we have developed flow simulations to explore the interactions of a hierarchical fracture network with the rock matrix of carbonate ramp strata. Model geometries were constructed in CAD software using field interpretations and LiDAR data of an outcrop area that is 350 m long by 100 m high.

The impact of water injection on oil recovery between an injector and producer pair was investigated. Simulations were performed by a single medium reservoir simulator. The effects of changing scenarios for rock permeability and porosity as well as facture permeability distributions were investigated. First-order results show that the best recovery was achieved by a model with a high permeability, homogeneous matrix combined with a heterogeneous fracture network.

The worst recovery scenario was given by a model with low, homogeneous permeability and high fracture permeabilities. After approximately 450 days there is a divergence in recovery profiles. Three models continue to recover oil while average oil saturations for the other 4 models start to plateau. The divergence captures a threshold between high and medium fracture-permeability scenarios that determines whether recovery continues in late time or the well starts to water out. The results highlight the importance of the permeability contrasts between the matrix and the fractures for overall recovery and the very significant impact that fractures can have on recovery by creating shadow zones and providing critical connections between permeable layers.

Strong fingering developed even in homogeneous matrix cases and velocity patterns reveal competing fluid pathways among matrix and fracture routes. Such models can help to develop production strategies to improve recovery from fractured carbonate reservoirs and evaluate different populations of conductive and baffling structures, spatial variations in wettability and capillary pressures and well positions.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.