Scale avoidance during waterflooding by optimized effluent mixing

Waterflooding through the injection of seawater into petroleum reservoirs is a well-established secondary recovery method to maintain reservoir pressure. The incompatibility between injection and formation water at subsurface requires cost-intensive chemical treatment methods for seawater to reduce scale risk and clogging of pore space. This paper presents an alternative method to improve fluid injectivity by mixing SO 4 2− -enriched seawater with Ba 2+ -enriched produced water to trigger mineral precipitation. In contrast to the standard procedure of chemical pre-treatment of reservoir-compatible seawater to avoid subsequent scaling in the reservoir, the present technology triggers the precipitation and removal of sulfate minerals prior to injection. Geochemical analyses of major, minor, and trace elements on fluid phases and mineralogical analysis on solid precipitates were coupled with thermodynamic modeling and laboratory experiments in order to define optimum fluid mixing ratios for waterflooding strategies. Geochemical reactive modeling with thermodynamic data revealed the lowest residual SO 4 2− concentrations (< 100 mg/L) in commingled fluids at a mixing ratio of 10:90 between seawater and produced water. A peak removal of 1.1 g of celestite (SrSO 4 ) and 0.5 g barite (BaSO 4 ) is achieved for mixing ratios of 40:60 and 10:90, respectively, per liter of injection fluid water. XRD and ESEM-EDS analyses reconfirmed the formation of homogenous, agglomerated oval-shaped mineral precipitates of 2.5 µmm average size, mainly composed of barite (67 wt%), celestite (10 wt%), and gypsum (14 wt%). As a practical benefit, non-reactive characteristics of the desulfated fluid-mixing product will contribute to minimize scaling of operational wells and to enhance the performance of waterflooding projects. The proposed combination of sulfate removal from seawater and subsequent microfiltration represents a cost-efficient alternative to the energy-intensive nanofiltration technology. The volume and treatment costs of injected seawater will significantly be reduced by the usage of produced water as major component of the total injection fluid mix. Additionally, the enhanced utilization of produced water for waterflooding as a commingled injection fluid component will reduce the amount of produced wastewater and treatment costs.