Electrodialysis Reversal for oil & gas application: produced water desalination

Olivier F. Garnier, Ph.D, TotalEnergies SE

To guarantee a responsible use of water resource which is necessary for the oil production, TotalEnergies is trying to both minimize surface water use and production water disposal. Offshore sea water offtake and disposal at sea of produced water is still widely used while in onshore facilities, produced water full reinjection and minimal disposal are a paradigm. In Enhanced Oil Recovery (EOR) mechanisms, low salinity water injection has proven its effectiveness in many aspects:  Its potential of improving oil production has been proved through laboratory experiments, low-salinity water flooding can change the wettability of the reservoir rock to increase oil recovery. It is a simple injection, low cost, and low capital investment EOR method. Use of low salinity water in Polymer flooding projects is also advantageous as it is well known that the required dose of polymer powder to obtain a targeted viscosity will decrease by reducing the salinity of the inlet water. When the water salinity is low enough, desalination of water for reducing the required polymer concentration brings effective cost savings. A lower concentration of polymer leads to significant reductions of CAPEX (storage tank, pump size, polymer preparation unit) and OPEX (polymer cost, transport, and handling). But there are also indirect advantages of produced water desalination and reinjection: lower consumption of anticorrosion or desemulfier chemicals and lower investment in water treatment facilities.

The application of produced water desalination should be restricted to low to moderate salinity as the purge water concentrating the salt should not go beyond 25-30% of the inlet flowrate. With the presence of oily components and chemicals in the produced water, the desalination of such water is not possible with reverse osmosis as the membranes are very sensible to dissolved organics. Evaporation could be a technical solution but is not applicable offshore and is energy intensive. TotalEnergies is looking for produced water desalination solutions which are robust with oil, chemicals and eventually reproduced polymer, with a moderate energy consumption (and related GHG emissions).

In a scenario where the produced water has a salinity of 6 g/L, desalination of this water down to 1 g/L before polymer injection would reduce by half polymer consumption (from 1300 ppm down to 700 ppm). Such low salinity can be found in many existing polymer flooding projects in sandstones reservoirs. and cost savings impact of low incoming Polymer concentration in polymer flooding projects. Polymer flooding technology increases and accelerates the oil production by a so-called piston effect pushing an oil bank and enhancing conformance in the reservoir. But there are issues relative to polymer production such as lower separation efficiency, thermal clogging of the polymer in the heat exchangers and poor performance of produced water treatment due to the presence of polymer. It was proven that the impact on water treatment performance is directly related to the concentration of polymer in the produced water. To reduce this impact, existing technical solutions (such as mechanical or chemical degradation, separation by centrifugation) are costly. The presence of polymer is very detrimental to any filtration technologies (membrane fouling) and therefore Oil in Water reduction below 20 ppm is becoming challenging. Waiting for suitable cost-effective water treatment technologies, existing polymer flooding projects have adopted a different strategy aiming at reducing or stopping polymer solution injection when the back produced polymer concentration was about to reach a limit known to impact the existing water treatment. Using the EDR technology to reduce required polymer concentration will thus reduce the back produced polymer concentration and could allow the existing water treatment technologies to handle back produced polymer without additional modification and cost. EDR adaptation to desalination of produced water in presence of polymer, dispersed oil, and production chemicals was performed by TotalEnergies, MemBrain and MEGA. The development of suitable membrane and stack withstanding up to 60°C was engineered by MemBrain and tested during a few weeks on synthetic produced water on a semi-industrial scale pilot treating 10 m3/h synthetic water (in closed loop) with an EDR stack containing 29.2 m2 membrane area. After a few reference tests for characterizing the EDR stack performances, the pilot was operated during 1 month in presence of a salt matrix representative of the case study: 6 g/l of salt, 600 mg/l HPAM polymer, 20 mg/L crude 2 oil, 50 mg/L corrosion inhibitor and 20 mg/l anti-scalant. Voltage was set at 1 V/pair (100 V). The temperature was set at 60°C with no impact on the membrane stack reliability during the test. The presence of HPAM slightly decreases desalination rate but no fouling was observed. Cost and environmental evaluations showed that EDR improves all the indicators. The total technical cost of the project is lower with EDR compared to a base case without any desalination. The next step is to qualify the technology on a site pilot with real produced water.

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