<p>As oilfield development advances, the destination of oilfield operating waste liquid (OWL) has become a significant challenge. OWL is typically presenting a complex composition that is challenging to treat. If OWL is not treated, it will cause serious harm to the environment. Currently, OWL treatment technologies focus on reuse and reinjection, this study adopts both laboratory and field experimental methods to investigate the compatibility issues between processed OWL (POWL) and Yan-9 layer produced water (Y9W) during reinjection. The laboratory experiment mainly carried out scale inhibitor (SI) screening, and compatibility assessment. The field experiments primarily validated the laboratory screening results. The results indicate that when Y9W and POWL are mixed at a 2:8(V/V) ratio, both the scaling amount (SA) and the Ca²⁺/Mg²⁺ loss rates reach their minimum values. Using SI, reducing POWL hardness, or a combination of both, the SA can be controlled within 30&#xa0;mg/L, meeting the SA threshold for oilfield water injection. An economic analysis revealed that adding SI is the most cost-effective approach, with a cost of 0.69 USD/m³. However, this study is limited to the specific conditions of the Yan-9 reservoir and the SI evaluated; thus, its findings require further validation under different reservoir chemistries.</p> Graphical abstract <p></p>