Produced-water-chemistry history matching using a 1D reactive injector/producer reservoir model

Understanding connectivity between injection and production wells is valuable information for reservoir management. Typically, connectivity might be evaluated by use of 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 with this method have identified barriers to flow in reservoirs. Information on connectivity is also beneficial to scale management, particularly that in which sulfate (SO4) -rich seawater is injected into reservoirs containing formation water rich in divalent cations [i.e., barium (Ba), strontium (Sr),calcium (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 (Wright et al. 2008).

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 performed this history matching with a 1D reactive transport model connecting an injector and a producer through a number of noncommunicating layers characterized by permeability, porosity, and height (completion interval)(Vazquez et al. 2009). The model simulates the injection of seawater, the mixing of the seawater with reservoir-formation brine, and the subsequent deposition of sulfate 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, similar to the approach presented by McCartney et al. (2012), Tjomsland et al. (2010), and Wright et al. (2008).

The model has been used to demonstrate the way in which produced-water compositions vary as the physical properties of the layers between the wells are modified, and, particularly, the way in which they vary in the presence of thief zones. Finally, a stochastic method was applied--in particular a particle-swarm-optimization (PSO) 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 interwell connectivity.