Treating oil-field co-produced water by forward osmosis for low-salinity water injection and enhanced oil recovery

Co-produced water re-injection is a mature recovery technique for oil fields. Co-produced water that is not re-injected is the largest wastage stream in the oil industry. Handling, treatment and management (especially re-injection back into the reservoir) is an expensive operation. PWRI is a secondary oil recovery method with a small recovery factor in the range of 15-25% and contributes to many surface and subsurface issues, e.g., scaling and reservoir plugging, resulting in the decline of water injectivity, and thus lower oil recovery. This reduction, of course, impinges significantly on the revenue stream of major oil corporations. However, low-salinity (LowSal) water injection is an emerging method that boosts oil recovery by reducing the downsides of produced water re-injection. Using forward osmosis to produce low-salinity water for injection is a novel idea, in which the co-produced water will be the draw solution. In this concept, low-salinity water from water wells (brackish water) is used as the feed to dilute the co-produced water. The diluted co-produced water will then be re-injected as LowSal water. The obviously cheaper alternative of direct dilution of the co-produced water with the brackish water might not produce a water compatible with the oil reservoir in both ionic composition and strength. Data have been collected from different oil fields with various co-produced water and formation characteristics. Different co-produced water treatments were observed in each oil field due to differences in co-produced water chemistry. The water sample for analysis was taken at the skim tanks prior to the water injection stage. A theoretical resistance-in-series model for the forward osmosis stage is presented, which has been adapted from the literature, which incorporates the mass transfer equations, in which the boundary layer and thin-film theory for the membrane intrinsic layers are integrated. An improved shell mass transfer correlation is introduced in addition to the incorporation of a modified reflection coefficient into the resistance-in-series model. The collected data were then incorporated into the theoretical model to calculate and evaluate the forward osmosis performance and, in turn, the water chemistry before re-injection. A forward osmosis rig has been erected to use the latest hollow fibre membrane supplied by the Toyobo Company (Japan). Water and solute flux were measured to validate the model estimations. The model estimated results were at 95% confidence to the measured values. Analytical investigations (ion analysis) for the membrane filtrate at various flowrates and applied pressures were performed to determine the forward osmosis filtrate ion composition. The FO filtrate compositions were then simulated using ScaleChem studio software from OLI for scaling tendency. The software predicted a remarkable reduction in the scaling tendency in the injection water infrastructure (including the oil reservoir) and by more than 50% compared to conventional co-produced water re-injection. Parallel to the ScaleChem predictions, the FO filtrate water was experimentally investigated for scaling using the Differential Scaling Loop rig, in a third-party lab. The DSL results are in good agreement with the ScaleChem predictions. The experimental scaling tendency results show that the injection of forward osmosis filtrate has the minimum occurrence of scaling both in the surface and subsurface. This new concept to produce LowSal produced water re-injection has the potential to improve oil recovery by minimizing the oil reservoir plugging due to scaling.