Polymer-enhanced-oil-recovery (EOR) operation has been implemented for the production of oil from difficult mature oil fields in Oman. The polymer used to sweep oil toward production wells in this EOR technique is resulting in the generation of polymer-flood produced water (PFPW) of increasing viscosity. Current methods of treating oilfield produced water must be reconsidered for the effective treatment of PFPW of such changing quality.
In a previous study, the use of polyaluminum chloride (PAC) was proposed for the coagulation of oil in produced water to be separated by flotation and filtration. As such, laboratory tests were conducted to evaluate the applicability of PAC and other chemicals for treatment of PFPW with higher viscosity than ordinary oilfield-produced water. These tests indicated clearly that aluminum sulfate (AS) was more effective for treatment of such higher--viscosity water.
A pilot plant developed during the earlier study was used to conduct coagulation/flocculation-, flotation-, filtration-, and adsorption-treatment trials for PFPW from an oil field at which polymer EOR was under way. For the final trial, the inlet PFPW viscosity was 1.4 cp at 40°C and oil concentration was greater than 200 mg/L. AS was applied for the coagulation/flocculation and flotation stages, and was found to be effective in reducing oil concentration to 1 mg/L. Filtration and adsorption stages resulted in further improvement of water quality. Most of the polymer used for EOR was believed to have been removed along with oil and suspended solids.