The carbonate reservoir in question is located in the northwest of the Sultanate of Oman and was developed first in depletion mode since 1970. From the year 2000 until today a horizontal water flood scheme has been implemented. The reservoir is made up of 2 carbonate layers of 27 and 13 meters thickness intercalated with several meter thick shale layers. They form the deepest reservoir layers of the Cretaceous Natih Formation. The reservoir layers are composed of laterally continuous, microporous, low permeability (5-10mD) limestone that is interpreted to be heterogeneously but overall sparsely fractured. The implemented water flood in this field is considered to be well behaved with a stable oil production and low water cuts of around 20 to 25%.
An integrated field study was carried out for a planned horizontal infill development. The main objective was to obtain a representative set of static and dynamic models that match historic production. One of the principal challenges was the unknown impact of fracturing and faulting on the intensified water flood development in the reservoir layers and on the potential vertical communication within and with overlying reservoir layers.
Seismic, geological, petrophysical, and reservoir data were integrated with drilling and production information to produce a detailed matrix and fracture description of the reservoir. Several iterative workflows that included numerous feedback loops with reservoir simulation results were applied to achieve an appropriate history match and confidence into the predictive capabilities of the reservoir model and the simulation forecasts.
The main achievements of the applied workflow are a major reduction of the uncertainties related to the impact of faults and fractures on reservoir behavior. Key was the close integration of simulation results of the dual porosity permeability model and field data. The modeling workflow of the matrix and fracture models and their implementation in the reservoir simulator were optimized in such a way that uncertainty evaluation was entirely handled in the simulator and simulation times were reduced significantly.
This study has clearly shown that even in reservoirs that appear to be relatively simple and well behaving with respect to the chosen development option may require a much deeper level of understanding and may reveal significant complexities. In the presented case the reservoir formerly believed to be "simple matrix dominated reservoir" shows a significant heterogeneity in fracturing across the area of interest. Only detailed understanding after comprehensive data integration, construction of a dedicated continuous fracture model and a dual porosity permeability simulation model allowed achieving reliable predictions on reservoir behavior. The study has led to improved well planning and well and reservoir management practices in response to sudden increase in water production.
The applied workflows may serve as an example for comparable carbonate reservoirs with apparent sparse fracturing that, however, may impact water flood development.