Abstract: A disclosed system for determining sources of water in a downhole fluid includes one or more downhole sensors that measure at least one analyte concentration in the downhole fluid, and a computer having analyte concentration characteristics for water from multiple sources. The computer uses the analyte concentration characteristics and the at least one analyte concentration measurement to determine an amount of water from at least one given source. A described method for determining sources of water in a downhole fluid includes associating with each of multiple sources of water a characteristic concentration of at least one analyte, obtaining measured concentrations of the at least one analyte with one or more downhole sensors, and deriving for at least one source of water a fraction of the downhole fluid attributable to that at least one source. The deriving may also be based upon measurements from distributed pressure and/or temperature sensors.

Abstract: A disclosed downhole optical sensor system includes at least one optical sensor positioned in a borehole and coupled to an interface via a fiber optic cable. Each of the optical sensors includes a waveguide for conducting light, and a reagent region positioned between the waveguide and a fluid in the borehole to absorb a portion of the light from the waveguide, the portion being dependent upon a concentration of a chemical species in the fluid. A described method for operating a well includes deploying one or more downhole optical sensors in a fluid flow path in the well, probing the one or more downhole optical sensors from a surface interface to detect concentrations of one or more chemical species, and deriving a rate of scale buildup or corrosion based at least in part on the detected concentrations.