Nanofiltration of oil field-produced water for reinjection and optimum protection of oil formation

摘要

In view of the increasingly stringent norms of disposal of effluent from petroleum activities and interest in water conservation, the treatment of the oilfield-produced water (PW), the largest wastewater (WW) stream associated with oil and gas industry in the Suez Gulf, was investigated for injection in the oil formation to enhance oil production and other environmental-friendly reuse purposes. Long-term laboratory and pilot testing of treatment of PW and Gulf water (GW) by coagulation/filtration and coagulation/filtrationanofiltration (NF) aimed to avoid blocking of the porous oil formation upon injection and to inhibit the detected corrosion and biomass accumulation on the internal wall of the injection piping, which was shown to re-contaminate the treated injection water. Analysis of PW showed the concentrations of TDS, organics including hydrocarbons, oil droplets, sulfate, silica, Boron, and suspended solids (SS) of 96472.6, 268.2, 120.4, 7087.5, 134.4, 29.3, and 20.2, respectively. The high sulfate content of both PW and GW would explain the observed hardness scale, on the well casing and pipelines. Only trace concentrations of U-238 as complex carbonates and heavy metals as copper, vanadium, nickel, and lead were detected in GW. The thick biofilm detected inside the injection pipes consists of biomass of 92% water, extracellular polymer substance (EPS) of mainly anaerobic sulfate-reducing bacteria (SRB) of 1.8 x 10(9) MPN/gm, and iron compounds due to steel corrosion. The dry film includes high concentrations of iron, sulfur, and a remarkably high radioactivity of uranium 238 of 6,740 pCi/gm, heavy metals such as copper, chromium, lead, and vanadium at concentrations much higher than in GW. Results confirmed that SRB enzymatically reduced the trace uranium and the other soluble cations in PW and concentrated them in the biofilm with parallel depolarization of the cathodic-controlled corrosion of steel to produce ferric sulfide and other iron compounds. Coagulation of PW efficiently removed SS, organics including hydrocarbons and oil. Only partial removal of uranium took place, which was too pH dependent. However, since coagulation did not suppress the biofilm formation and the related phenomena of microbial corrosion and accumulation of radioactivity, the release of these components recontaminated the treated PW. On the other hand, the proposed process of "intermittent chlorination/coagulation/NF" of PW efficiently rejected sulfate, uranium, and other metal cations and polished the removal of SS, bacteria, and organics. This process inhibited the formation of scales and biofilm as well as the related undesirable phenomena and, therefore, stopped the recontamination of the PW prior to injection. Only poor accordance was observed between the experimentally determined percent rejection of PW components by NF and that computed according to the solution of the extended Nernst-Planck equation.