Diagnostic Tracking of Produced Gas and Condensate for Gas-Oil Communicating Reservoirs Via Comprehensive Reservoir Modeling Workflow

Abstract The paper presents a deeper saturated oil reservoir with gas cap and a shallower gas reservoir, with a communication area due to fault juxtaposition. Gas from the upper reservoir, which is not under development, is being produced along with some oil producers from the oil reservoir as per acquired surveillance data. Pressure depletion is observed in observer wells of the upper reservoir, which substantiate both reservoirs communication. The oil reservoir has been on production for 30 years, under miscible gas injection, imposing additional complexity to the problem definition. In this study, a complex full field and EOS-based simulation tracer workflow is developed to track the originating source of produced gas from four sources - (1) dissolved gas in the original oil in place of the deeper oil reservoir, (2) free gas in the gas cap of deeper oil reservoir, (3) injected gases (hydrocarbon and CO2) into the deeper reservoir, and (4) the shallower gas condensate reservoir. A commercial reservoir simulator is used to describe the entire two-reservoir systems composing 58 million grid cells with a compositional PVT treatment of a single, 8-component EOS model. The full-field model runs use five tracers to quantify the four sources of produced gas - four methane tracers and one CO2 tracer. Validation of the five-tracer solution is made using (1) test runs with only two sources, where each source has its own separate 8-components (e.g. C1-U, C1-L⋯) and (2) models using tracers for every component in all four sources. The logic used in this study to track the amount of surface products (surface gas or stock-tank oil) from various sources is shown to be indicative and accurate, mimicking the available surveillance. A smaller subset of tracers could be used when the surface products consist mainly of key components, e.g. methane and/or CO2 for produced gas, and C7+ for produced oil. The ability to validate a given tracer solution using "duplicate" components from each source is recommended and straightforward. To avoid extensive run time, a sector model could be used, representing all of the main geological and flow dynamics relevant to the full-field model behavior. The adopted methodology can apply to tracking produced stock-tank oil (STO) in any field in instances where differentiation is needed between STO from the oil zone as one source and STO from the gas cap as another source, or where produced STO from different layers needs to be quantified in commingled wells. The findings of using the tracer solution in this study also give an additional QC tool to the existing development plan and provide recommendations for optimal co-development options of the communicating reservoirs.