Capturing the Subsurface Blowout for Unlocking Remaining Potential in a Mature Water Flooded Naturally Fractured Carbonate Oil Reservoir: A Case Study of Potwar Region in Pakistan.

Abstract Reservoir modeling has evolved into a potent tool with the recent introduction of sophisticated computing capabilities, allowing it to capture minute reservoir features much more accurately. This study focuses on one such case history, in which a mature water-flooded field had a subsurface blowout, with naturally fractured carbonates serving as the main reservoirs. Developing an optimal field development strategy required a significant challenge: matching production history to the simulation model while faithfully capturing the subsurface blowout and its aftermath on the reservoir flow dynamics. The geologically complex reservoirs have 84 faults and four distinct sublayers with different petrophysical properties. More than thirty years of production and injection data are available for these reservoirs. Almost halfway through the production period, a subsurface blowout occurred in one of the wells, which was thought to be rectified. Building the geological model and adding reservoir properties marked the start of the investigation. The simulation model was initialized, and a history match was obtained. This match showed accuracy prior to the blowout, however there were differences in the match after the blowout period. This implied that, contrary to the first hypothesis, the consequences of blowout had persisted. Due to the blowout, there was a great deal of uncertainty about the type and extent of ongoing subsurface communication. The issue now was to manage this uncertainty and establish a strong historical match for the post-blowout period without sacrificing the pre-blowout match. Layer-by-layer and whole field history matches were obtained by varying parameters like skin, fault transmissibilities, permeability modifiers, and relative permeability adjustments. Restart runs for the post-blowout phase and entire history runs were both taken into consideration. After more than 350 simulation runs, a good match was achieved for pressures and the production rates in the post blow out period, without significantly affecting the pre blowout history match in the field. Initially the issue was most significant in two of the seven wells that were in close vicinity to the blowout well. The matches for these wells were checked for each layer separately to assess the quality of the overall match. The well histories were significantly improved from the base case, yielding a high-quality match. Based on the history matched model, increased production potential was evaluated considering areas of bypassed oil. This study had a unique case where the reservoir behavior was altered by manmade intervention, while the exact behavioral change remained uncertain. Strategies for dealing with such uncertainties were devised in a complex geological setting and encouraging results were achieved, creating a model that can used with confidence for exploring field development scenarios.