Using high-salinity effluent water for polymer flooding is a strategic endeavor to mitigate water-handling constraints and debottleneck oil production. This study sheds light on the workflow that was adopted to select appropriate polyacrylamides with AMPS/ATBS monomers to improve polymer stability, particularly under harsh salinity, hardness and H2S levels. Molecular weight optimization was important to rationalize polymer selection for two stacked reservoirs with different permeabilities to reduce polymer retention and maximize oil gains.

Extensive laboratory evaluation followed by a one-spot EOR pilot and a regular 5-spot pattern confirmed the effectiveness of polymer flooding for the Umm Niqa Lower Fars (UNLF) sour heavy oil reservoir, using high-salinity effluent water with Polymer A, mainly targeting the F2 layer. This instigated further investigation on the feasibility of polymer-flooding for the UNLF F1 layer (i.e. F1), overlying the UNLF F2 layer (i.e. F2), using the same wells, configuration, inlet water and surface facilities. Two new polymers with different molecular weights, but similar chemistry to that of Polymer A were lab-evaluated on a fast-track basis covering rheology, injectivity, oil recovery, polymer retention, resistance factor, residual resistance factor and polymer stability.

F1 has higher oil viscosity, lower permeability and shallower depth compared to F2. This necessitates optimizing polymer molecular weight and concentration to establish good injectivity, optimize polymer concentration and reduce polymer retention. Following F2 field implementation using polyacrylamide Polymer A, two lower molecular weight polymers were identified for F1. Laboratory results using high-salinity effluent water and reservoir temperature showed that polyacrylamide Polymer B is better-suited for F1. This was evident from polymer retention and injectivity data. The optimal polymer concentration for field implementation has been defined using reservoir simulation to ensure commercial throughput and maximize oil gains, while safeguarding caprock integrity.

Polymer-flooding is time-sensitive and the incremental benefits of it tend to diminish with time. This study demonstrates the feasibility of fast-tracking laboratory evaluation to select appropriate polymers for expeditious field implementation. Future work can focus on further optimization of ATBS/AMS content, polymer concentration, water treatment requirements, and well completion to concurrently target multiple reservoir layers with reduced surface footprint.

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