The management of produced water has become a main issue in petroleum industry due to the huge quantities to be dealt with. Produced Water Re-Injection (PWRI) allows combining disposal with Improved Oil Recovery (IOR) opportunities. Nevertheless, PWRI could damage the formation, eventually compromising field performances. Well impairment is a complex phenomenon depending on several aspects. This paper focuses on the impact of Oil In Water (OIW) in injectivity performance, validating lab predictions and field evidences. A comprehensive review of field cases has been carried out, analyzing injection well performance with Production Data Analysis (PDA) tools together with water quality data. On a selection of field cases where OIW has been identified as main impairment reason, a workflow was established performing core flooding experiments to measure experimental loss of permeability with different content of OIW. Obtained results have been integrated by bibliographic research. Field evidences showed a direct relationship between permeability reduction and hydrocarbon content; moreover, injectivity impairment measured on field data has been found to be comparable (same order of magnitude) with the permeability reduction measured on core flooding. Combining all data together, a common trend of injectivity reduction vs. OIW content has been extrapolated passing through the definition of Injectivity Index (II). New water injection concept depends on several elements that could affect the overall project value, water quality requirements is one of them. Water quality rule of thumb can be found in literature, but each project basis of design is different. The proposed approach allows to preliminarily quantifying the expected well impairment as function of water quality. It can be used as a first tool to dimension treatment facilities in function of the reduction of injectivity that we can accept for any given reservoir. Obtained trend is representative of a subset of real field cases, where OIW content is the main impacting parameter on PWRI well damage.