TAR MAT Formation on Baffles in the Middle of the Oil Column

Tar mat formation has been poorly understood but is now being addressed in revealing case studies employing new asphaltene thermodynamics using the Flory-Huggins-Zuo equation of state along with the Yen-Mullins model of asphaltenes. Tar mats are associated with a large concentration of asphaltenes. Immature oil and biodegraded oil often have enriched asphaltene content, but these origins alone are generally insufficient to explain tar mat formation. In particular, tar mats often result from an asphaltene phase separation due to incompatible fluids being charged into reservoirs. This latter process falls within the purview of ‘reservoir fluid geodynamics’, the discipline heretofore largely ignored which explicitly treats in-reservoir fluid processes in geologic time. This case study addresses adjacent fault blocks all with a late light hydrocarbon charge into an oil reservoir; that is, all fault blocks have related reservoir fluid geodynamic processes taking place thereby supporting a robust interpretation. After the light hydrocarbon rises to the top of the reservoir without mixing, light components diffuse down into the oil, increase the solution gas ratio and destabilize the asphaltenes. In close proximity to the charge point, the charge may also sweep laterally through the reservoir. All three fault blocks under investigation are at different realizations of these processes, largely due to variability in vertical permeability and proximity to the charge point. In a fault block where low permeability and baffles slows down the process, the solution gas and asphaltene content grade continuously throughout the liquid phase. In this fault block, these fluid gradients are in gross disequilibrium. In a fault block where the diffusion process is fast (high vertical permeability), the emigration of both gas and asphaltene towards the bottom of the oil column is complete; all reservoir oil has high solution gas, and the asphaltenes have accumulated and phase separated at the base of the reservoir in a large tar mat. In the third fault block the top part of the reservoir is subjected to top down diffusion of light hydrocarbon, resulting in a tar mat deposited on a baffle. In the lower part the charge is sweeping laterally through the reservoir as the well is believed to be close to the charge point. This sweep process is much faster than diffusion. The asphaltene is destabilized and deposited locally in the pore space throughout the reservoir while preserving some of the rocks permeability. The asphaltene distribution is determined by Downhole Fluid Analysis, PVT, and core extracts. Well test data confirms the baffle in the middles of the reservoir is not pressure sealing. Moreover the fluids are equilibrated above and below the baffle indicating its lateral extent is limited. Nevertheless, the baffles imped the downward migration of asphaltenes. When the light hydrocarbon migrates down through the oil column, the asphaltenes undergo a phase transition forming a layer with very high asphaltene content on top of the baffle. This trapping of asphaltenes by baffles is expected to occur elsewhere in this reservoir away from the well bore. This case study ties together recent scientific revelations with the use of new technology and new workflows to understand asphaltene distributions in reservoirs. The unique comparison of three fault blocks in the same petroleum system allows insight into how the same process can yield different results as a function of baffles and reservoir quality.