Abstract
AbstractTar mats in reservoirs can cause severe issues in oil production such as preventing aquifer pressure support and water sweep. Therefore, it's important to understand the fluid geodynamics that occur in geologic time that give rise to heavy oil gradients and to tar mats. A common event in reservoirs especially in deepwater is a late gas charge into an oil-filled reservoir. In this case, the gas can quickly migrate to the top of the reservoir through fault planes without mixing (except locally) with the existing reservoir fluid. This newly charge gas can then diffuse down into the oil column thereby significantly altering solution gas and dissolved asphaltene content and their gradients. In particular, an increase in solution gas causes a reduction of asphaltene solubility. This process can result in significant variations in asphaltene concentration throughout the column. Moreover, a density inversion can be created due to the combined effects of methane diffusion and diffusion and expulsion of asphaltenes. This results in convective flow of asphaltene-rich fluids which can yield large asphaltene concentrations and tar mats at the base of the oil column. In addition, rapid gas charge can overwhelm the ability of asphaltenes to migrate thereby leading to upstructure tar deposition. Both scenarios can be associated with Flow Assurance concerns over part or all of the field. In this paper, we discuss case studies that exhibit such potentially problematic fluid columns. Simulated cases are also modeled to provide guidance for understanding the fluid geodynamics. The ability to model density inversion and asphaltenes gravity currents is seen to help significantly in understanding of reservoir fluids dynamics.
Published Version
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