Effect of Outrunning the Aquifer in Heterogeneous Water Drive Gas Reservoirs

Water-drive gas reservoirs (WDGRs) are often affected by early water breakthrough and reduced recovery, particularly in heterogeneous and stratified systems. Producing gas at elevated rates outrunning the aquifer has been proposed to delay water encroachment and enhance recovery factor (RF). This study conducted nearly 1,000 reservoir simulation runs to evaluate the effects of aquifer size, heterogeneity (Lorenz Coefficient), stratification, reservoir dip, vertical permeability, and gas type (dry and wet) on outrunning performance. Gas condensate systems were excluded due to condensate dropout and liquid accumulation below the dew point, which can distort production behavior. Results indicate that outrunning can increase RF by up to 15% in moderately heterogeneous systems (LC ≈ 0.3), while highly heterogeneous cases (LC ≈ 0.9) exhibit more variable responses, including modest RF reductions. Stratification introduces additional complexity, with RF reductions of up to 6.45% at low aquifer sizes; however, this negative effect diminishes as aquifer strength increases, reflecting a strong interaction between layering and aquifer support. Intermediate aquifer volumes consistently provide the highest RF improvement, whereas very large aquifers reduce effectiveness due to water compressibility effects. Reservoir dip also influences performance, with low-dip systems showing the highest gains, while gas type has minimal impact. Vertical permeability sensitivity shows no consistent trend, indicating the need for dual-porosity–dual-permeability representations. Formal probabilistic uncertainty quantification (e.g., Monte Carlo analysis) was not included in this study. Overall, outrunning is conditionally effective and strongly governed by coupled reservoir and aquifer characteristics.