Polymer Flooding Cost Optimization Using Electrodialysis Reversal EDR for Produced Water Desalination

Abstract

Polymer flooding projects require hundreds of ppm of polymer (often HPAM) to viscosify the injection water. It is well known that the required dose of HPAM 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. 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. A lower concentration of polymer leads to significant reductions of CAPEX (storage tank, pump size) and OPEX (polymer cost, transport and handling). But there are also indirect advantages 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 Total, MemBrain and MEGA. The development of suitable membrane and stack withstanding up to 80°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 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 (CAPEX higher but lower OPEX) compared to a base case without any desalination. The next step is to qualify the technology on a site pilot with real produced water.