A technique facilitates evaluation of a fluid, such as a fluid produced from a well. The technique utilizes a modular and mobile system for testing flows of fluid which may comprise mixtures of constituents, and for sampling fluids thereof. The multiphase sampling method includes flowing a multiphase fluid comprising an oil phase and a water phase through a first conduit, the oil phase and water phase at least partially separating in the first conduit, mixing together the oil phase and water phase to form a mixed bulk liquid phase by flowing the multiphase fluid through a flow mixer toward a second conduit downstream the flow mixer, sampling a portion of the mixed bulk liquid phase at location at or within the second conduit, wherein the sampled portion of the mixed bulk liquid phase has a water-to-liquid ratio (WLR) representative of the pre-mixed oil phase and water phase.