Long-term simulation of process and hydrogeochemistry of gas field produced water reinjection into a limestone reservoir

Abstract

Large water volumes have been injected to enhance oil and gas recovery. However, the generated oil or gas-produced water (GPW) may contain undesirable and harmful substances. Reinjection of GPW into suitable subsurface formations is considered an effective disposal method. In this study, a numerical model of vertical-radial two dimensional well flow was developed to explore the long-term process and hydrogeochemistry of GPW reinjection into a limestone formation, which is considered as a homogeneous equal thickness reservoir. The obtained results indicated that there was an increase in the reservoir pressure at the reinjection well from 150 to 251.4 Bar at a reinjection rate of 300 m³/day. The pressure propagation range extended up to approiximately 3300 m during the injection. Calcite volume caused a maximum volume fraction change of −0.91, corresponding to a dissolution rate of 0.083/year, which increased the reservoir porosity to over 0.9. Reservoir pH values were altered within 90 m of the reinjection well as a result of water-rock interaction. Additionally, concentrations of Ca²⁺, Mg²⁺, Na⁺, Cl⁻, HCO₃⁻, and SO₄²⁻ in the reservoir were affected within 2000–3000 m from the reinjection well due to injection pressure, water-rock interactions, and diffusion. This study provides insights into assessing the environmental behaviors of GPW in reservoirs, and ensuring safe, and effective long-term GPW reinjection; though accuracy and reliability of the model requires further validation using practical monitoring data in the future.