Understanding connectivity between injection and production wells is valuable information for reservoir management. Typically, connectivity might be evaluated using downhole pressure information or by injecting tracers into the reservoir. A less established but inexpensive technique is to history-match produced water compositions. For example, previous studies using this method have identified barriers to flow in reservoirs. Information on connectivity is also beneficial to scale management, and particularly where SO4-rich seawater is injected into reservoirs containing formation water rich in divalent cations (i.e. Ba, Sr, Ca) because in these cases the sulphate mineral scaling conditions in production wells are a function of the physical properties of the flow paths connecting the injection and production wells.
In this study, we have considered this latter relationship from a reverse perspective and explored the potential of history-matching produced water compositions to understand the physical properties of flow paths connecting injection and production wells for reservoirs under seawater flood. We have done this using a 1-D reactive transport model connecting an injector and a producer through a number of non-communicating layers characterized by permeability, porosity and height (completion interval). The model simulates the injection of seawater, its mixing with reservoir formation brine and the subsequent deposition of sulphate scales (barium and calcium sulphate among others) in the reservoir under equilibrium and kinetic conditions. The results of interest are the predicted produced water compositions over time from the model.
The model has been used to demonstrate how produced water compositions vary as the physical properties of the layers between the wells are modified and particularly how they vary in the presence of thief zones. Finally, a stochastic method was applied, in particular a Particle Swarm Optimisation algorithm, for the automatic history-matching of actual produced water compositions from individual wells in fields under seawater flooding. The results are promising and show that this technique can provide valuable information about the nature of inter-well connectivity.