In a high number of deepwater fields, water injector and producer wells are being drilled with synthetic or oil-based drilling fluid (SBM or OBM). The filter cake and drilling fluids residue left on all of the well surfaces after the drilling phase is a water-impermeable layer. If not removed, they will compromise project objectives. To avoid excessive operating costs and to remove near-wellbore damage, a quick and efficient removal of SBM or OBM with properly-designed clean-up fluids, such as microemulsion fluids, is required. Properly designed microemulsion spacer fluid formulations can be used to displace the SBM or OBM to a water-based fluid and completely water-wet all the surfaces in less than one circulation. The displacement fluids are formulated with customized surfactant blends (CSB) that exhibit equal affinity for the oil and water phases. They promote a very thorough microemulsification of the oil residue from the SBM or OBM during the displacement operation resulting in an aqueous external phase at the end of the process. The microemulsion displacement fluids are formulated to: (a) prevent viscous sludge at the fluids interface; (b) remove all of the oil from the surfaces; and (c) make the wellbore surfaces and near- wellbore region water-wet. In addition to spacer fluid formulations, customized microemulsion fluids can remediate wells and increase water injectivity and well productivity. These fluids contain a customized surfactant blend (CSB), a small amount of acid, and an optional co-surfactant. They form an in-situ microemulsion and produce a uniform wellbore cleanup in horizontal and highly-deviated open-hole wells. Customized microemulsion fluids are able to cleanup in a broad range of temperature, density, and salinity. They restore the water-wet condition of the rock and increase injectivity or productivity of the wells. This paper presents laboratory evaluations and field data of treatment fluids for wellbore displacement and near-wellbore cleanup based on the microemulsification mechanism. Data from systematic evaluation tests performed prior to field application (e.g. interfacial tension, detergency tests, fluids compatibility, filter cake removal and regain permeability) demonstrates performance results that were beyond the capabilities of conventional treatments. Based on the key performance indicators (turbidity of completion brine, volume used and resulting cleanliness of tools), displacement of SBM or OBM with microemulsion cleaners in deepwater applications was achieved in one circulation during the wellbore displacement process. Successful removal of SBM or OBM in the near wellbore region is evidenced by comparing predicted and achieved injection and production rates.