Heavy oil is being recovered from bitumen reserves deeper than 100 meters mainly through cyclic steam stimulation (CSS) and steam assisted gravity drainage (SAGD) processes. Both these processes use high pressure steam to mobilize the heavy oil. The steam to oil ratio used is about 2.5 to 4.0 m3(cold water equivalent)/m3 of heavy oil produced. The steam generation pressures are about 8 to 12 MPag and saturated steam is used for injection. Steam generation at this high pressure requires good quality boiler feed water. Due to competing demands on the use of fresh water, oil producers are looking at using brackish water as a source of makeup water. The use of brackish water along with recycled produced water poses challenges for water treatment design, material selection and waste disposal.
In this paper, the quality of brackish water and its impact on materials of construction is reviewed and available technologies for its treatment to obtain boiler feed water are reviewed. An example of the quality and quantity of effluents is included. The final treatment and disposal methods are studied based on current technology and provincial regulation for disposal. The monitoring and control requirements for wastewater disposal have been studied.
The total recoverable crude bitumen resources in Alberta are estimated to be1 50 billion cubic meters. Of these, only 4% of the total crude bitumen is expected to be surface mined2. The remaining subsurface bitumen has to be recovered using in-situ processes such as cyclic steam stimulation (CSS) and steam assisted gravity drainage (SAGD). In both these processes, makeup water is required to generate sufficient steam as well as to operate the surface facilities treating the oil/water emulsion produced from the CSS and SAGD wells.
In Alberta, the fresh water resources for an industry are allocated based on diversion licenses as per the Water Act. Since in-situ bitumen extraction processes compete with other users of fresh water, there is a growing opposition in allowing fresh water diversion for oil production3. When the oil development takes place in the white area of the Province, such as Cold Lake, the developer is required to investigate methods of minimizing fresh water use and consider alternate sources prior to applying for water allocation license and environmental clearances.
Brackish water from deeper aquifers is an alternate water resource being considered by the oil producers as makeup water for steam generation. This water can be treated to satisfy the boiler feed water specifications and combined with recycled produced water to meet the water requirements. There is no competition for the use of brackish water with higher than 4000 parts per million total dissolved solids, from other users and a diversion license is not required to use this water. However, the brackish water requires more treatment than fresh water due to its salinity. The brackish water also requires special corrosion resistant materials for construction of the treatment facilities due to high chloride content.
Brackish water, available from deeper aquifers in Alberta, is defined4 as water containing more than 1,000 parts per million of total dissolved solids (TDS).