The Lower Burgan reservoir in the Abdali field bears extra-heavy oil of viscosity ranging from 8,000 to 20,000 cP. In view of its depth, the LBAB reservoir is not amenable to thermal steam-based EOR methods. Non-thermal methods such as polymer-flooding or liquid solvent injection have been investigated. Demonstrating the feasibility of polymer-flooding using ~260,000 ppm TDS effluent water, under high temperature conditions, can lead to major cost savings in relation to water sourcing and treatment.

Extensive lab evaluation was performed to qualify polymers that can withstand harsh salinity and temperature conditions to generate important data for numerical modeling in preparation for potential field implementation. Polymers from four manufacturers were evaluated through viscometry, long-term stability under anaerobic reservoir conditions, resistance to mechanical degradation and injectivity. The reservoir rock structure and mineralogical composition were analyzed before capillary pressure and relative permeability data were generated using procedures adapted to unconsolidated reservoir sand and high-viscosity oil. Finally, coreflood tests were performed to assess the quality of the in-depth polymer propagation and determine the relevant parameters for the simulation dataset.

Polymers were evaluated at the reservoir temperature (i.e. 190°F / 88°C) using brine with a TDS and divalent cations concentration of ~260,000 ppm and 19,000 ppm, respectively. For such conditions, data available in the literature clearly indicate that ATBS-acrylamide copolymer chemistry is required. Different ATBS levels were tested ranging from 10 to 70 mol% to optimize techno-economic feasibility. While all polymers exhibited comparable performances in terms of thickening ability, resistance to mechanical degradation and injectivity in permeabilities representative of that of the reservoir, the long-term anaerobic aging tests revealed that 55 mol% was the minimal ATBS content to establish stability over at least 6 months. Reservoir rock analysis and characterization revealed that the rock is slightly oil-wet, homogeneous and composed of monodispersed quartz grains with marginal clays and no cementation, thereby resulting in a favourable permeability of ~1000 mD, with no consolidation post oil removal. Polymer injection coreflood tests demonstrated good in-depth propagation, with Resistance Factors matching the injected relative viscosity, low adsorption (40 to 80 μg/g) and almost no rheo-thickening at near-wellbore velocities, despite the relatively high polymer concentrations investigated (~4000 mg/L).

This study proves the feasibility of using ATBS-based polymers for harsh reservoir conditions in terms of temperature, salinity and hardness. ATBS levels must be tuned to ensure polymer stability while establishing favourable economic feasibility. Coreflood tests also demonstrated the technical feasibility of polymer-flooding to unlock massive reserves from a deep extra-heavy oil reservoir.

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