Inline process refractometer based on image defocusing

In oil and gas production, it is important to determine the type and chemical composition of the formation fluids in the pipeline in real time. Different types of the hydrocarbon formations present in the downhole environment have distinct optical characteristics, such as spectral transmittance, diffusion, fluorescence and refractive index. In the formation sampling and testing, knowing the refractive index of a downhole fluid makes it possible to identify the fluid (water, gas or oil), estimate various properties, such as salinity and crude oil density, and monitor the cleanup processes. The paper describes a new design for an on-axis inline process refractometer for continuous fluid measurements using a defocusing imaging technique. Design principles for the refractometer are discussed, the dependence of the defocus effect on the refractive index is calculated. Experimental energy distributions for different liquid samples are obtained and analysed.