No abstract

The maximum use of existing surface produced water treatment (PWT) facilities is a prerequisite for an economic chemical enhanced oil recovery (cEOR) in mature fields, as the erection of additional dedicated polymer treatment facilities can seriously harm the project's business case. These existing facilities often exhibit a reliable design, but do not necessarily fulfill the requirements of treating back-produced polymer. An optimization of installed facilities based on prior assessment of limitations is a way to upgrade facilities with regard to future EOR operations. Since its start-up in 2015, the main PWT plant comprised three separation stages: corrugated plate interceptors (CPIs), dissolved gas flotations (DGFs) and nutshell filters (NSFs). The plant processes up to 1,200 m3/h of conventional produced water at the Matzen field in Austria. Additionally, in 2009 a polymer injection pilot was initiated, with continuous polymer injection started in 2012, and now produces a segregated water stream containing back-produced polymer. Prior field tests with a pilot scale water treatment plant indicated operational issues with the existing set-up of facilities and the flotation chemicals used, with increasing polymer concentrations. At the end of 2018, severe injectivity issues were observed at injectors which were supplied with commingled conventional and polymer containing produced water. These were caused by a chemical interaction between the partially hydrolyzed polyacrylamide (HPAM) and alumina-based water clarifiers, which were applied in the dissolved gas flotation, finally leading to a loss of production. Therefore, a strict segregation of polymer and conventional streams at the common well network has been developed and established, where the separated streams could be injected into different parts of the injection system without any issues. This experience pointed out the future risks and hurdles of an economic cEOR full field roll-out where up to 200 ppm back-produced polymer at all surface treatment facilities is expected. Several studies were performed to identify alternative technologies able to treat polymer containing water. A business case driven option was to initiate an optimization program to develop smart upgrades and ensure maximum use of the existing PWT facilities. The main task was to substitute or stop the current poly-aluminum chloride-based coagulant in the DGF with a dosage of 40 to 60 ppm due to its unfavorable interactions with the back-produced HPAM. A technology assessment, comprehensive measures and economic retrofits of the installed gas dissolving units, the circulation cycle and bubble injection points resulted in a 200% higher flotation bubble bed density. Thanks to these improvements, the dosage of water clarifiers could be stopped, accomplishing similar or even better PWT performance values. In addition to the operational savings achieved, the existing treatment plant can now be used to treat cEOR fluids, as first tests with up to 59 ppm of back-produced polymer proved. Considering this new opportunity, a customized and economic modular cEOR debottlenecking concept was developed.

The implementation of highly effective polishing filter technologies plays a key role in polymer EOR where reservoir characteristics require extremely good water qualities. Few filter technologies are capable of handling volatile inlet fluid conditions such as back-produced polymer concentrations up to 420 ppm and OIW contents up to 70 ppm while targeting treated water qualities of less than 5 ppm OIW. A fully integrated field trial was set up in the Matzen field in Austria where producing wells from EOR operations provide sufficiently high polymer concentrations for technology evaluations. A three-phase separator and a multi-chamber flotation unit were used as upstream treatment to supply produced water to the different filtration technologies under evaluation. With this setup, a broad range of entry criteria like OIW, solid content, polymer concentration and oil droplet size could be adjusted to challenge each filtration technology to its limit. None of the tested technologies such as nutshell filters, conventional multi-media-filters, coated media, or vendor specific technologies were able to handle the challenging inlet water conditions while providing the required water qualities for polymer flooding projects. A new concept was developed, combining conventional multi-media filtration design with a type of media that, thus far, was not common in the oil and gas industry. A system-specific operating envelope was developed, describing the strong impact of inlet water conditions such as oil droplet size D50 values down to 15 micron, OIW concentrations up to 70 ppm, solid contents up to 10 mg/l and viscosities up to 3 cp. Where state-of-the-art technologies did not fulfill the performance criteria, it was possible to achieve less than 5 ppm at the filter outlet. Not only the filter performance but also the crucial aspect of effective media regeneration was solved with a novel filter backwash design, since known backwash systems completely failed with the newly implemented media.