Water Flood Front Matching by Advanced Modelling Strategy of Imbibition Capillary Pressure Applied to Oil-Wet Carbonate Reservoir in Offshore Abu Dhabi

Abstract This study was conducted to improve the history-matching quality of injected water frontal advancement speed and shape in the dynamic model of a major carbonate oil field in offshore Abu Dhabi. Similar to many other carbonate reservoirs in Abu Dhabi, the modelled reservoir of this field started to observe water break through at the earlier timing than that indicated by the existing previous reservoir models. In order to improve the model’s predictability of injected water frontal advancement, geological model was newly re-generated through close communication between geologists and reservoir engineers resulting in the launch of the new dynamic model considered as suitable for the evaluation of upcoming field development plan which includes change of water injection scheme. After completing the history-matching of the new full-field dynamic model, though overall history matching quality was improved, it still remained as a big challenge that the actual injected water flood front is still moving much faster than the predicted reservoir simulation. It resulted in optimistic forecast for water cut build-up profile and well production potential, which can potentially lead to sub-optimal field development decisions. In this study, we investigated the effectiveness of newly introduced method to this field to accelerate the injected water frontal advancement in the dynamic model without losing the achieved good quality of history-match of reservoir pressure in the completed new model. The modelling challenge was identified as the difficulty in reproduction of earlier water advancement through specific higher permeability layer because this reservoir is geologically characterized as thick, homogeneous and vertically well-connected nature without existence of clear impermeable sub-layers which can work as baffles or barriers to confine the water movement inside the high perm streak. Several geologically possible scenarios were investigated by using simplified element model extracted from the full-field model by combining the following uncertainties: i) the ratio between vertical and horizontal permeabilities; ii) quality of high permeability streak; iii) imbibition capillary pressure setting, etc. After several sensitivities, it was concluded that a proper setting of imbibition capillary pressure model was most appropriate to prevent the injected water from moving from higher permeability layers to lower permeability layers with maintaining the established pressure matching. It can significantly accelerate the advance of the water flood front in the model. It was identified that properly adjusted set of imbibition negative capillary pressures of different rock types can effectively further improve the water cut matching quality of the completed full field dynamic model without losing the established pressure matching. Though utilization of capillary force barrier concept by proper setting of negative imbibition capillary pressure is becoming common for carbonate reservoirs these days, it should be highlighted that, for this particular reservoir, the location of the high permeability streak was at the bottom of the reservoir, though the locations of high perm streaks of many other carbonate reservoirs are located at the top of the reservoir. Note that it prevented many previous reservoir engineers involved in the modeling of this reservoir to find out the non-negligible impact of imbibition capillary pressure on water frontal advancement because the effect had been mixed with and masked by the water slumping due to gravity effect.