The purpose of this paper is to present the results of a comprehensive subsurface study of a field in South of Iran. The reservoir is typical of several carbonate fields in south west Iran, comprising low porosity carbonate, but highly fractured. After several years of natural depletion a gas injection was started at a time that reservoir became saturated at the crest to prevent further shrinkage of the oil and keep the reservoir oil pressure constant. The source of gas was a deeper reservoir where the gas was produced and injected by its own pressure through one producer and one injector.
A three dimensional model was set up using a 3D static model as the basis for geological input. All available data including, fluid losses during drilling, well test and flowmeter surveys were used to define the fracture properties. A good history match was obtained for pressure and fluid levels with a commercial black oil simulator using the model in dual porosity mode. The calibrated model was then used in prediction mode to forecast the reservoir behavior under gas injection pressure maintenance and also repressuring close to original crestal pressure. The simulation results show large increase in ultimate recovery due to gas injection as compared with natural depletion.