The coalescence between crude oil droplets is a major factor influencing the efficiency of most produced water treatment processes. As the droplets grow bigger in size, it is easier to remove them from the continuous water phase, which will improve the quality of produced water and help meet increasingly stricter environmental and process regulations. Here, we investigate the coalescence process of crude oil drops in water with the use of previously reported microfluidic tools. It was shown that the composition of both oil and water phases heavily impacts the merging between droplets, both outcome (final droplet size distribution) and the kinetics of the phenomena (coalescence time). In the droplet aging experiments, the coalescence was always most extensive for the droplets with the shortest aging time, while the coalescence typically decreased with increasing droplet size distribution; however, this was oil-specific. Overall, the results underline the importance of crude oil and produced water chemistries during the water treatment process.