Treatment of SAGD Produced Waters Without Lime Softening

Abstract

Abstract The produced waters from Steam Assisted Gravity Drainage (SAGD) facilitiesare characterized by low concentrations of both total hardness (TH) and totaldissolved solids (TDS) and high silica concentrations. Treatment of thesewaters to provide feed-water suitable for Once Through Steam Generators (OTSG)has traditionally included either hot or warm lime softeners (HLS/WLS) and ionexchange. The primary function of the lime softening process is for silicaremoval. A proposed alternate process that utilizes ion exchange only and depends onthe premise that provided the divalent and trivalent cations are reduced to theparts per billion (ppb) level, will allow the OTSG to operate at high silicaconcentrations. This process eliminates the need for a lime softening stage andsavings in capital and operating costs are realized. Introduction The feed-water quality for OTSGs, the most common form of boiler found inoilfield enhanced oil recovery projects and used in generating steam atpressures up to 15,400 kPa with treated produced waters was establishedtwenty-five years ago and has changed little. SAGD facilities typically operateat lower steam pressures (in the 8400 to 11,200 kPa range) but use the samequality criterion. Currently the accepted quality for the major parameters is: TH= <0.5 mg/L as CaCO3 (calcium carbonate) Silica = <50 mg/L TDS = <10,000 mg/L Oil = <10 mg/L An appropriate de-oiling system located upstream of the water treatmentplant is required to reduce the oil concentration to <10 mg/L. A skim tankfollowed by gas flotation and filtration is the standard de-oiling process. The traditional produced water treatment plant installed to obtain the abovewater quality for a SAGD facility is shown in Figure 1. A HLS or WLS reduces the silica and in certain cases the TH concentrations. The afterfilters remove the sludge carryover from the lime softeningprocess. Primary and polishing strongly acidic cation units (SAC)operating in the sodium form reduce the TH to <0.5 mg/L as CaCO3. The OTSGgenerates steam at 80% quality and a steam separator removes the 20% waterphase from the steam which is then sent to the reservoir as 100%quality. In the lime softening process, lime, magnesium oxide and a flocculant areadded to either a HLS or WLS operating at a pH of 9.5 to 9.8. The lime causes areduction in the temporary hardness i.e. the calcium and magnesium combinedwith the bicarbonate alkalinity and the magnesium oxide facilitates the removalof the silica. The flocculant aids the floc formation so that a sludge thatsettles more readily is formed.