A Surface Flow Constructed Wetland System (called a Reed bed) is used as a disposal means for produced water (PW) containing hydrocarbons in a field located in the south of Oman. The Reed bed system is a farm of plants that remove oil from water, followed by evaporation ponds where the water is evaporated.
In the same field, it is planned to undergo HPAM Polymer Flood. One of the risks envisaged with this activity was the capability of the Reed bed system to handle back produced water contaminated with polymer. Therefore, a series of tests were conducted to understand the impact of polymer contaminated produced water on the Reed beds.
The experiment was carried out in two phases with a small scale experiment in batch mode followed by a long term field trial at a full scale. The first phase aimed to introduce by batches HPAM into small Intermediate Bulk Containers planted with reeds and monitored for 5 months. It was observed that HPAM resulted in an increase in the growth rate and evapotranspiration rate in some of the plants. It was clear that the HPAM did not cause any negative effects on the plants during the short-term duration of the study and the results were very encouraging.
A long term field trial was then conducted to verify the results observed from the batch experiment. To mimic the large scale Water Treatment Plant reed bed, four pilot scale (40m × 40m) surface flow wetlands were built and planted with five types of plants similar to the plants currently available, all receiving produced water with different HPAM concentrations (0, 250, 500 and 1000ppm), 0ppm serving as a control. The trial was conducted for duration of one year. The Figure 1 shows the long term field trial.
The success criteria evaluated in this pilot were divided into two categories with critical and non-critical criterias. A critical criteria was defined as one for which a negative outcome requires significant system modifications to process produced water containing polymer. The criterias are Oil Removal, Above Ground Dry Biomass and Necrosis. The non-critical criterias such as polymer removal, plants toxic symptoms, plants health, Acrylamide accumulation and water loss imply potential minor design modifications maybe requiredto the existing Reed Bed. These criterias were developed and assessed by the company responsible for the design, operation and monitoring of the Long Term Field Trial.
The outcome was positive for all critical criteria with the exception of plant necrosis at 1000 ppm polymer. Two plants species out of five showed necrosis in the 1000ppm wetland higher than the 0ppm wetland. The Necrosis was determined to be inconclusive at 1000ppm as there were no signs of Necrosis at 500ppm and below and other factors not related to polymer are highly suspected to be responsible for this behavior. All the non-critical criteria were highly positive except for polymer removal. The wetlands did remove some of the HPAM from the produced water but not all of it. There are some uncertainties surrounding the long-term fate of the HPAM in the system for reusing the treated produced water from the wetlands. Currently, the water is evaporated after the reed beds, however the presense of polymer in it limits any further use of that water. The positive results seen during the trials have demonstrated that there is no risk on reed beds when processing up to 500ppm HPAM.