The Challenges of Using Brackish Water in Thermal Facilities
Article by Franklin Foster, Ph.D.
In some areas, one key component in the production of heavy oil is the thermal facility. These large plants produce large quantities of steam which are injected into the pay zone to reduce oil viscosity and make it easier to produce. There are still lots of treatments needed by the produced oil but there are also many challenges on the steam production side. One of these challenges is to reduce the amount of fresh water used to make steam.
One example of ongoing developments in this area can be found at the Canadian Natural Resources Ltd. (CNRL) facility near Bonnyville, Alberta and known as the Wolf Lake Thermal Facility. Phase I of this facility was constructed in 1984. At that time, fresh water was the exclusive source of water feedstock for the making of steam. This fresh water came from a deep well aquifer 100 meters below the surface. Even this water, though, had to be treated and softened in order to be suitable for use in the boilers. The initial system installed in 1984 consisted of three softener trains. A train or fresh water treating vessel configuration consists of an ion exchanger called a SAC (Strong Acid Cation) primary vessel followed by a SAC polisher vessel. These ion exchangers soften the fresh water down to an outlet effluent hardness of < 0.1 ppm (parts per million). Once the train is exhausted, which occurs when either the polisher leakage is > 0.1 ppm hardness or the primary leakage is > 3 ppm hardness, then the vessels must be regenerated.
By 1990, concerns about the potential environmental impacts of large draws on deep aquifers prompted attempts to use alternate sources of water. The next alternative was produced water. This is the water that is produced along with the oil and, from the oil production point of view, is referred to as the water cut. This water requires much more treatment than fresh water. A stage added was the a Warm Lime Softener (WLS) followed by Weak Acid Cation (WAC) primary and then WAC polisher exchangers. The WLS reduces the produced water effluent hardness to < 20 ppm and the silica to < 50 ppm. A 10% hydrated lime (CaOH2) slurry and 4% magnesium oxide (MgO) slurry are added to the WLS reaction zone to remove the hardness and silica, respectively. The partially softened produced water is then filtered in anthracite pressure filters and Lime Softener Filters (LFS) before entering the WAC/WAC ion exchangers. This multi-stage process again brings the water to the parameters needed for the boilers, namely < 0.1 ppm soluble hardness and < 0.5 ppm Total Acidified Hardness (TAH).
Given these requirements for boiler use, imagine how much treating must be done to transform brackish water which arrives with over 8000 ppm. This brackish water, the remnant of ancient oceans, comes from the deepest wells of all – some 580 meters below the surface. Beginning in 2005, brackish water began to be used as a source of steam generation. Larger vessels, longer series of treaters, and more rapid regeneration cycles wee just some of the requirements. Costs of treating brackish water rose 30 to 40 per cent over treating produced water, and 300 to 400 per cent over treating fresh water. As well, brackish water is enormously corrosive of vessels and pipelines, and its variability produces new challenges in measuring and monitoring its hardness and other constituents.
Despite all these challenges; CNRL continues to make progress in using brackish water as an important feedstock.
Table 4. Summary of Fresh, Produced, Brackish Water Streams Key Parameters
Table courtesy of Melonie Myszczyszyn, B.Eng.. CNRL