What is the key difference between interwell tracer tests using conservative and partitioning tracers?

The key difference between interwell tracer tests using conservative and partitioning tracers lies in their behavior and the type of information they provide about the subsurface. Both types of tracers are injected into one well and monitored for their arrival at another well to understand flow paths and reservoir characteristics. Conservative tracers are designed to be chemically inert and do not interact significantly with the reservoir rock or fluids. They travel with the water phase and provide information primarily about flow paths, flow velocities, and connectivity between wells. Examples include deuterium oxide (D2O) or certain fluorescent dyes. Partitioning tracers, on the other hand, are designed to partition between the water phase and another phase, such as a non-aqueous phase liquid (NAPL) or the rock matrix itself, through processes like adsorption or absorption. This partitioning behavior provides information about the presence, saturation, and distribution of these other phases. By comparing the arrival time and concentration of a partitioning tracer to a conservative tracer, one can infer the extent of partitioning and, therefore, estimate properties related to the phase with which it interacts. For instance, a partitioning tracer will arrive later and at a lower concentration than a conservative tracer because some of it has been retained in the other phase. So, while conservative tracers map water flow, partitioning tracers provide insights into the presence and properties of other phases within the reservoir, offering a more comprehensive understanding of the subsurface.