SAGD (Steam Assisted Gravity Drainage) is an Enhanced Oil Recovery (EOR) process common in oil sands deposits such as those found in central Canada. SAGD is extremely water intensive thus proper treatment is
essential to eliminate corrosion and scale build-up. Analytical measurements such as pH and dissolved oxygen have traditionally been done in the lab if they were performed at all. The cost of ignoring or guessing
about water quality has lead to costly piping failures and downtime.
Oxygen Measurement Challanges
Dissolved oxygen should be one of the final measurements of the feedwater to the boiler. This measurement serves two purposes. When an oxygen scavenger is used, the measurement will determine the effectiveness of the chemical in removing oxygen. When the scavenger is properly dosed and has enough reaction time readings should be <10ppb. A dissolved oxygen reading is also very helpful in determining leaks in upstream valves, heat exchangers and pumps. These leaks will allow the ingress of oxygen which can be detected by the increasing reading.
pH Measurement Challanges
In WLS process high particulates in the clarifier can lead to coating and plugging problems with the pH sensor. Residual hydrocarbons will also be present. These unrefined hydrocarbons often have traces of hydrogen sulfide (H
2S) and heavy metals. H
2S is well known to poison the Ag/AgCl element in the pH sensor. The elevated temperature of the WLS process also tends to shorten sensor life. All three issues combine to make this a difficult pH measurement. In the evaporation process the pH sensor is used to control caustic addition. This sensor has to withstand 11 to 13 pH. Such high pH can dissolve the glass measurement electrode and shorten the sensor’s life. This sensor may also have issues with residual hydrocarbons entrained in the produced water. In this application temperature is less of an issue since the measurement is upstream of the preheater.
The second pH application in the evaporator process is the final treatment of the brine for environmental compliance. The pH is remains high to keep the dissolved salts in solution. At the clarifier these salts will precipitate out as the pH is lowered to neutral levels. The resulting sludge must be disposed of. Once again coating and plugging of the sensor due to high solids can become an issue.