Oil Recovery Optimisation in High Watercut Heterogeneous Confined Channel Sandstone Reservoirs

Abstract

This paper delves into an examination of reservoir management and Enhanced Oil Recovery (EOR) strategies for optimising oil recovery in highly heterogenous confined channel reservoirs characterized by high watercut evolution. It includes the identification of reservoir performance challenges and their root causes while evaluating dynamic reservoir simulation options for selected EOR techniques. The Enyenra Oil field, located offshore Ghana, serves as the case study for this research.

The study focuses on the optimisation of sweep efficiency in such reservoirs, using diagnostic plots to identify root causes of high watercut evolution. Analogue field studies were conducted to benchmark potential techniques for improving oil recovery. Two EOR methods (polymer flooding and surfactant flooding) emerged as the most suitable options from the benchmarking. Subsequently, dynamic reservoir simulation was employed to determine the optimal EOR technique and implementation strategies for mitigating aggressive watercut evolution and enhancing oil recovery.

Results suggest that poor reservoir sweep efficiency and potentially poor recovery factor may be mitigated by avoiding preferential flooding of high permeability streaks by the injected water, thereby improving oil recovery in low permeability streaks. The study demonstrates the effectiveness of these chemical-based EOR techniques over other methods. Simulation outcomes show that cyclic surfactant flooding notably increased the recovery factor by 3% and reduced watercut by 12%. However, cyclic polymer flooding increased the recovery factor by 1.8% and reduced the watercut by 4%. An economic evaluation of the improved profiles revealed a marginal gain in cyclic polymer flooding compared to cyclic surfactant flooding.

Findings from the study offer practical strategies for reservoir management in highly heterogenous sandstone reservoirs and provide valuable guidance for practising Engineers seeking to optimise sweep and production efficiency and sustain operations in similar challenging reservoirs. The workflow employed in this study could be applied to evaluate similar problems in existing fields.