Reinjection of Produced Water into Formations in Unconventional Gas Reservoirs

This paper provides a comprehensive analysis of gas field produced water from four perspectives: water sources, chemical composition, treatment methods, and application scenarios. It identifies critical challenges in current formation reinjection practices, including poor containment performance for injection layers, difficulties in optimal layer selection, and uncertainties in injection volume determination. To address these issues, systematic selection criteria for reinjection layers were established. Taking a depleted gas reservoir in the Ordos Basin as a case study, we conducted a geological analysis of candidate formations based on previous research findings. We set up three groups of schemes regarding injection wells, injection rate, and permeability inhomogeneity and studied reservoir reinjection water volume, reinjection formation pressure, reinjection waves and range, and reinjection safety using three-dimensional numerical simulation technology. Finally, we selected the preferred scheme of reinjection well location in consideration of permeability inhomogeneity, with a cumulative reinjection volume of 1554.3 × 104 m3 and a change in reinjection formation pressure of 0~20 MPa. The pressure change in the upper overburden of the reinjection layer was kept within 3 MPa, a value consistent with actual historical reinjection data, confirming again the accuracy of this layer selection strategy and the aforementioned layer selection analysis and providing a basis for layer selection and reinjection safety for the assessment of recovered water reinjection in other unconventional gas reservoirs.