|
|
|
What are oil sands? Oil sands are naturally occurring deposits of bituminous sand. These are grains of sand surrounded by a film of water and clay, with bitumen (petroleum) filling the pore spaces between grains.
How were the oil sands formed? When oil migrates to shallow depths on the margins of basins, it is attacked by bacteria, which remove the light ends, leaving behind sticky viscous materials known variously as bitumen, asphalt and tar. These substances grade into heavy oils, and there have been difficulties in knowing how to classify them precisely. Their characteristics vary, depending on the composition of the oil from which they were derived and the subsequent alteration processes (Campbell 1997). This material moved into sands originally deposited as the delta of an ancient river which flowed into a primeval sea located in what is now northern Alberta. Today bitumen can be found seeping from the sand on southfacing (sun-warmed) exposed faces such as those which occur along river banks .
Natural bitumen seep on the bank of the Horse River (a tributary of the Athabasca) west of Fort McMurray, near the site of the the old Abasands oil sands plant.
What is reclamation? Reclamation is the process of reconverting disturbed land to its former or other productive uses. This is different than restoration which is the process of restoring site conditions to what they were prior to the land disturbance. Reclamation involves all practicable and reasonable methods of designing and conducting an activity to ensure that when it is finished the site involved will have: (1) stable, non-hazardous, non erodible, favorably drained soil conditions (2) equivalent capability to pre-disturbance conditions
How are these specialized terms being defined? The terminology being used here follows as closely as possible the definitions laid out by Alberta Environment in the publication: Glossary of reclamation and remediation terms used in Alberta. The definition of reclamation listed above was taken from this source.
What do all these acronyms mean? There are many acronyms which are relevant to oil sands. These stand for both organizations (government, industrial, and nonprofit), and technical terminology. For an alphabetical listing of these acronymns and their definitions, click here.
Where are oil sands found? Oil Sands deposits have been identified in several locations around the world including North America, South America, and Asia (See below). The largest are the Canadian and Venezuelan deposits, but there are many others around the world, including two large ones, known as Aldan and Siliger in the Former Soviet Union (Campbell 1997). The main North American deposits are located in northern Alberta (see map). Canada posseses 3/4 of the world's oil sands deposits. One of these, the Athabasca deposit, is located close to the surface and much of it can be accessed through open pit mining.
How much oil is in the oil sands? There are an estimated 1.7 trillion barrels of oil in Alberta oil sands deposits, which is 1/3 of the world's known oil reserves. Of this approximately 7-8% can be accessed by surface mining. Oil sands mines provided approximately 20% of Canada's oil needs in 1993.
Where are oil sands being mined? Currently there are three commercially productive operations which are all located in the Athabasca deposit, north of Fort McMurray (see map). These mines are operated by Syncrude Canada Ltd., Suncor Energy Inc., and most recently Albian Sands Energy Inc. All projects involve open pit mines, with the Syncrude and Suncor operations undergoing expansion. Plans exist for additional projects to begin operations in the near future, including CNRL (Canadian Natural Resources Limited) and OPTI Canada Inc./Nexon (The Long Lake Project). There are also experimental subsurface mining operations occurring at several locations. These were developed under the Alberta Oil Sands Technology and Research Authority (AOSTRA) now known as the Alberta Energy Research Institute (AERI).
Why are the surface mines located where they are? The mines are located near the Athabasca River as that is where the oil sand deposits are closest to the surface (see geological cross section), and hence the most economical area to mine. The more overburden material which must be removed to access the ore, the more the mines cost to operate. Only 7-8% of the ore can be accessed in this way, which is why subsurface mining techniques are also being developed.
What was present before mining started? Prior to mining, the leases supported a mixedwood boreal forest (Rowe 1972). The area is part of the Central Mixedwood section of the Boreal Forest Region (Alberta Environmental Protection 1994). The forests surrounding the mines are dominated by associations of aspen (Populus tremuloides) and balsam poplar (Populus balsamifera), mixed with white spruce (Picea glauca) and black spruce (Picea mariana) (Stringer 1976). Stands of jack pine (Pinus banksiana) are also common. White spruce is the climax species on relatively well drained sites, however aspen is the forest cover of greatest extent due to its ability to regenerate rapidly following disturbance (Rowe 1972). Jack pine is dominant on sandy areas but mixes with other species on dry glacial till soils. Black spruce occurs mixed with jack pine in the uplands and with tamarack (Larix laricina) in poorly drained areas and low lying bogs. Balsam fir (Abies balsamea) and paper birch (Betula papyrifera) are also found in the area.
Ells River (a tributary of the Athabasca) northwest of Fort Mackay
How big an area has been affected? The cumulative disturbance to date is over 15,000 hectares or 150 square kilometers. The image below provides an indication of the extent of the disturbance. In addition to the mines, large areas are affected by storage of both tailings sand and fine tails. In fact this effect of oil sands mining far exceeds that of any other form of mineral processing (Marshall 1982). For example, the tailings pond immediately north of Syncrude's extraction facilities covers over 22 square kilometers. Much of the expansion which is either planned or underway will occur to the north and south of the image below. Some estimates suggest that by the year 2023 the impacted area may be as much as 10 times the area affected to date, potentially exceeding 1,406 square kilometers (AEP 1998).
Image of oil sands plants north of Fort McMurray
What will the area be like once reclaimed? The long-term plan for affected sites is to establish a mosaic of self-sustaining forests (50%), grasslands (20%), wetlands (10%), and lakes (20%) (Syncrude 1995). These sites are intended to protect and maintain water quality, provide habitat for wildlife, create recreational opportunities, and produce commercially useful timber. Restoration of the area is not possible due to the extreme nature of the disturbance. However, this does not prevent the rehabilitation of the site through establishment of self sustaining native plant communities capable of meeting these objectives. In time a stable ecosystem should develop in the site which is similar to that of the surrounding areas in terms of species composition and functioning, and which should integrate successfully into the local landscape. Vegetation on these reclaimed end-of-lease landscapes will be exposed to relatively high levels of salt resulting from contact with tailings (Renault et al 1998). Such conditions will impose short, and perhaps, long-term constraints on regeneration, depending upon the topography and natural recharge of the sites. Success of reclamation will depend in part upon the ability of the vegetation to tolerate these salts, which is one of the factors driving the Network's research.
Artist's rendition of planned reclamation of the Syncrude mines
How is reclamation progressing? As the figure below illustrates, currently a much larger area has been disturbed than has been reclaimed. However a significant increase in reclamation work is planned over the next several decades in order to close this gap. This planned rapid increase in reclamation, and the (in some cases) challenging materials (Saline/Sodic Overburden, Mature Fine Tails etc.) involved, require additional research to improve and refine reclamation practices.
What kinds of disturbance does oil sands mining cause? The surface mining of oil sands carried out by Syncrude and Suncor involves excavation to remove overburden and provide access to the bituminous sands below it. Consequently everything above the sands: forest, soil, and overburden is removed, and a large open pit is created. However, the disturbance caused by these operations is a result of more than just excavation. The amount of land required to dispose of oil sands tailings far exceeds that of any other form of mineral processing (Marshall 1982). Although procedures for handling and disposing of tailings are simple, the volume to be dealt with imposes practical constraints on reclamation planning (Monenco 1983). Subsurface mining such as that done by test plants operated by AERI involves much less disturbance of the surface, though there is still subsurface disturbance associated with this process.
Syncrude's North Mine
What is overburden? Overburden is all surface material above the mineable oil sands (Hardy BBT 1990). The quantity of overburden is a function of the depth at which the bituminous sands deposit was found, which varies locally. Closer to the river there may be less than 20 meters of overburden, while further away from it ore deposits may be found beneath 60-150 meters of such material. Overburden is nonhomogenous, containing peat, shale, clay tills and glaciofluvial sands (Monenco 1983). In general the overburden is non-saline and slightly alkaline, though materials from the Clearwater formation, which includes much of the shale overlying oil sands deposits north of Fort McMurray, can be quite saline.
What are tailings? Oil sands tailings consist of the whole ore body plus net additions of water used in processing, less the recovered bitumen product. Until now, tailings have been allowed to segregate into tailings sand, and fine tails, and then disposed of separately. Techniques for handling and revegetating tailings sand exist (Monenco 1983; Techman 1983). The biggest challenge is disposal of fine tails. These settle slowly and tend to form a highly voluminous sludge (Marshall 1982). One of the main factors preventing settling is the salinity of fine tails, which causes clay particles to repel each other (FTFC 1995), keeping them suspended. Because of the high water content of the sludge (as much as 85%), it retains fluid characteristics and must be stored behind dykes with little possibility of using it as a solid substrate for plant establishment (Marshall 1982; FTFC 1995). A possible solution to dealing with this material is the production of CT (called Composite Tails by Syncrude, and Consolidated Tails by Suncor). Composite Tails are composed of 30% fine tails, and 70% tailings sand, mixed with gypsum (Sheeran 1998). Through ion exchange, the calcium from the gypsum displaces much of the sodium from the clays, and causes the fines to flocculate and settle out (MacKinnon 1998).
What is tailings sand? Tailings sand is made up of the granular mineral deposits left after water and sludge removal, and is relatively easy to revegetate. Tailings sand deposits consist of approximately 95% sand, 4% silt, and 1% clay (HBT AGRA 1994). Most tailings sand is slightly alkaline as a result of sodium hydroxide added during processing to separate oil from the sand (Lesko 1974). Available nitrogen and phosphorus are low, due to the low organic matter content of tailings sand (<1%). All other chemical and physical properties are within acceptable limits for plant growth (Monenco 1983).
What are fine tails? Fine tails are 5 - 10% of the tailings (suspended silt and clays). Syncrude's fine tails are about 85% water, 13% clays, and 2% bitumen (FTFC 1995). Fresh fine tails are considered to be acutely toxic for aquatic organisms due to the presence of a number of organic compounds such as naphthenic acids which are derived from the bitumen. However, toxicity declines with time as these compounds degrade (Hunter et al. 1989). It is generally thought that the extremely slow consolidation of this material is related to the dispersed nature of the fine and ultrafine particles as well as the ionic chemistry of the process water (FTFC 1995). The protracted retention of fluid characteristics by this material offers little possibility of establishing vegetation over its surface.
What are composite / consolidated tails (CT)? Production of CT (called "Composite Tails" by Syncrude and "Consolidated Tails" by Suncor) reduces the volume of fine tails by removing most of the water component. The process used presently the involves mixing of tailings sand, gypsum and fine tails so that calcium from the gypsum acts as a flocculent, causing the fine clays to settle out with the sand and release the water (Sheeran 1998). Composite Tails are composed of 30% fine tails, and 70% tailings sand, mixed with gypsum (Sheeran 1998). Through ion exchange, the calcium from the gypsum displaces much of the sodium from the clays, and causes the fines to flocculate and settle out (MacKinnon 1998). However, CT deposits produced by this method are more difficult to reclaim than tailings sand. Composite Tails retain a high water content and high concentrations of sodium, chloride, and sulfate (Renault et al., 1998). The development of anaerobic conditions in the root zone has also been observed in CT, and may exacerbate the effects of salt and other stresses induced by tailings (Croser and Zwiazek 2000). Oil sands ore naturally contains significant amounts of NaCl, and this becomes concentrated in tailings during extraction due to recycling of process waters. The initial amounts of NaCl contained in the ore vary. The deposits that Syncrude is mining are significantly more saline than those of Suncor, and as a consequence Syncrude's tailings materials have higher concentrations of ions than those produced by Suncor (Renault et al. 1998) The high gypsum dosages required for production of CT (800-1200 g/m3 of tailings) result in significant addition of ions as sodium displaced from the clays associates with sulphate from the gypsum to form sodium sulphate (Renault et al. 1998). As a consequence, the main ions occurring in CT are Na, Cl, and SO4. These ions are present in CT in concentrations resulting in electrical conductivities (EC) averaging 4.5 dS/m (Golder Associates 2000), when 4 dS/m is generally considered the threshold at which such materials impose a significant constraint to plant growth (Singer and Munns 1996). Excess water is slowly released from the CT as it consolidates. Composite tails become trafficable within a year or two, but as the high EC indicates, this material can be quite saline, which continues to create significant problems for revegetation (and thus erosion prevention and reclamation). Given that existing technologies for dealing with tailings sand work well, conversion of tailings sand to CT can be seen as a complication and contamination of otherwise benign material. However, some means of disposal of fine tails must be found, and production of CT is one of the most promising options.
Why produce CT? The production of CT will reduce the large volume of fine tails which has built up behind dykes over the years, and allow management (see figure below) of production of fine tails to prevent similar accumulation in the future (Sheeran 1998). Release of the water from this material reduces the overall volume of tailings, even though by incorporating tailings sand it increases the proportion of material that will be difficult to revegetate.
In order to understand the magnitude of the fine tails problem and the quantities of material involved, it is illustrative to construct a model of tailings production based upon operational numbers, even though ongoing enlargement of the operation has rendered these particular numbers obsolete. When the Syncrude plant opened in 1978, it was designed to produce 125,000 barrels per day of synthetic crude oil (Smith 1979). To meet this level of production 231,000 tons of bituminous ore were required per day (ALCRC 1982), which is approximately 1.85 tons per barrel. The void space created by extraction of one ton of bituminous ore is 0.4 cubic meters and the tailings which result from processing the ore occupy 0.6 cubic meters (Simpson-Lewis et al. 1979). This means that Syncrude's daily use of 231,000 tons of ore opened up 92,400 cubic meters of space but produced tailings which required 138,600 cubic meters of storage space - a volumetric discrepancy of 46,200 cubic meters per day of operation. This is in spite of the fact that between 7 and 17% of the ore was recovered as bitumen. A large proportion of this discrepancy is due to the process waters locked up in the fine tails.
Are there alternatives to CT? Several alternatives to CT exist. Where significant quantities of fine tails already exist, these can be disposed of by conversion to paste rather than CT. Another alternative is extraction using the Counter Current Drum Seperator Process (CCDS) developed by Bitmin Resources Inc., rather than the current froth treatment technology. If fines in the feed are below 16 percent, then use of Bitmin's CCDS process at the time of extraction can completely eliminate the neccesity of producing fluid fine tailings for storage in a tailings pond. Instead it results in relatively dry tailings sand with a slightly higher fines content than the current norm. Adding less water allows a significant reduction in the volume of tailings produced, and thus the area affected by storage and disposal of such tails. Syncrude's new Aurora mine will use this technology.
|
