New ways to exploit oil sands
It takes a very rich prize to attract serious research dollars, but the bitumen (oil sands) resources of northern Alberta (and Saskatchewan) are just that. According to 2004 figures from the Alberta Energy and Utilities Board, this region contains a resource of 270 billion m3 of bitumen–equivalent to the petroleum resources of Saudi Arabia. This dwarfs Alberta’s conventional petroleum industry, which has 2.42 m3 of light and heavy oil in resources. At current production rates, the oil sands will last 436 years, while the light and heavy oil in the province will run out in just eight years or sooner.
But there’s a catch. The reason there’s still so much bitumen in place is because it’s not easy to recover. While much of the oil sands resource is amenable to surface mining techniques or in situ production (steam-assisted gravity drainage or cold bitumen production), almost 90% of Alberta’s bitumen resource is currently considered not to be recoverable with commercial technologies. That’s the equivalent of 1.5 trillion barrels of unrecoverable oil. And if oil is selling for US$50/bbl, that’s worth… an awful lot. Finding a way to recover those trillions of barrels is the prize worth going for.
Need for new methods
The location and extent of the bitumen resource has been known for some time, so exploration and discovery are not the limiting factors. Over its brief history, the development of the oil sands business has depended upon the testing and proving of new technologies for both recovery and processing.
The oil sands business got its start in the 1960s and 1970s with the startup of the Suncor and Syncrude surface mining operations, and they have grown step-wise in both size and improvements. The next push was to find ways to produce oil from the deeper bitumen deposits, which resulted in the in situ techniques (SAGD and cold) developed in the 1980s and implemented in a number of operations today. Mining and in situ production are today’s proven technologies.
The targets of research now are the deposits that are too deep for surface mining, or too shallow or thin or too geologically uncontained to use current in situ methods. In addition to the oil sands areas of northern Alberta, there are bitumen resources in the “Carbonate Triangle” west of Fort McMurray, and small oil sands deposits in Saskatchewan. The application of conventional methods to the latter two resources would be very energy- and water-intensive, generating an unacceptable amount of greenhouse gas emissions. Therefore, the future recovery of these bitumen resources demands the development of new technologies or step-change improvements to existing technologies.
Low GHG recovery scenarios
Many of the new R&D directions for oil sands production have been identified in a study by Petroleum Technology Alliance Canada (PTAC) entitled, “Expanding heavy oil and bitumen resources while mitigating greenhouse gas (GHG) emissions and increasing sustainability.” Commissioned by Natural Resources Canada and described as “a high-level roadmap,” the study was published last May, and is available at www.ptac.org. The lead author is Bruce Peachey of New Paradigm Engineering Ltd.
In February, PTAC presented a workshop outlining the progress on the first “exploratory study” on bitumen in carbonates and conventional heavy oil, which is intended to gather more detailed ideas to build on the high-level report. “In time, favourable economic conditions will drive the exploitation of challenging deposits using incremental adaptation of existing thermal technologies,” he said in the workshop. The problem with this is that it would result in significantly increased energy use as well as GHG emissions and water demands. The driving force behind the study is to bypass this negative result by evaluating alternative technologies to recover the hard-to-reach bitumen, which would take less energy and emit lower GHG amounts.
The second PTAC report (to build scenarios for a production of Bitumen in Carbonates and Conventional Heavy Oil) should be available in April 2007, and will also be available at www.ptac.org. A series of additional “exploratory studies” is being proposed that will eventually lead to a new and expanded detailed oil sands roadmap covering all conventional and oil sands resources in Canada.
Longwall and borehole mining
Most intriguing are the underground mining methods being considered for bitumen production at intermediate depths, between 150 and 300 m below surface. Scott Dunbar of the Department of Mining Engineering, University of British Columbia, discussed two methods–longwall mining and borehole mining–in a proposal submitted in March last year to PTAC.
The coal industry is the traditional user of longwall mining, a highly mechanized system of underground mining. However, geological similarities between the intermediate depth oil sands and coal fields have led Dunbar to consider longwall mining in the oil sands. He writes: “A layer of oil sands would be selected and blocked out into a … panel. Tunnels would be excavated along the length of the panel to provide access and to place a conveying system to transport material out of the mine. Entry tunnels would be constructed from the passageways along the width of the panel. The longwall system would mine from these passageways between entry tunnels.”
Dunbar points out three potential problems in applying this method to oil sands extraction:
* the potential for surface subsidence
* the need for a strong enough reaction surface for the hydraulic jacks that hold up the roof supports. “Likely oil sands are not strong enough for this,” he writes.
* The largest height currently using a longwall mining system in the United States or Australia is 4.8 m, while some deep oil sands deposits are 30-60 m thick. This could result in very low recovery.
The advantages of using longwall mining would be high productivity, and extracting the oil sands without removing overburden.
The report suggests that ground freezing could be used to strengthen the material underlying the roof support system, by circulating chilled (-20 to -30C) calcium chloride brine through pipes installed in the ground. Backfilling the mined-out areas with dewatered tailings from the plant could reduce surface subsidence. A problem would arise, however, if both freezing and backfilling were needed: the system would have to freeze the material in front of the longwall miner, while at the same time allowing tailings deposition immediately behind. This might not allow for reliable, continuous operation.
There is current research going on into applying longwall mining to thick coal seams. One such scheme that might be applicable to thick oil sands deposits is the multi-slice longwall method with sand backfill. The thick layer is mined in slices starting from the bottom of the layer. The area behind the roof supports is backfilled with sand, providing the surface for another slice to be mined. Writes Dunbar: “What is interesting is that the sand backfill provides sufficient support for the longwall system. This suggests that strengthening the material underlying the longwall mining system may not be required.”
Dunbar describes a series of trials for extracting bitumen using another method, borehole mining, from deposits in the Cold Lake area. A well was drilled to about 480 m deep and water pumped down the well, so the jet at the bottom formed a pancake-shaped cavern. The bitumen slurry rose up the annulus between the drill string and the drill hole. Although the concept worked, it became evident that a cavern volume of more than 100,000 m3 was needed to offset the drilling and construction costs. If the bitumen layer is 40 m thick, the span of the shale roof covering the cavern would need to be 80 m. It was found that, without support, no rock would be a
ble to remain intact over this span, but would fail.
Brainstorming ways to strengthen the overlying rock are needed, but there is one fundamental problem for boreholing: the difficulty of controlling the jetting and cavern formation from surface. “The less a borehole mining system relies on unpredictable or uncontrollable processes in geological materials, the better,” concludes Dunbar. For the full report see “Unconventional heavy oil and bitumen extraction, upgrading and GHG emissions – A technology roadmap (review of tag areas 2.3 and 2.4)”, which is available at the PTAC website.
Other mining technologies
Another proposal submitted to PTAC, this one authored by Clayton Deutsch and Jeff Boisvert of Clayton V. Deutsch Consultants Ltd. in Edmonton, Alta., summarized various underground mining techniques that may in future help with recovery of currently inaccessible hydrocarbon resources. As they point out, traditional underground methods such as longwall mining may be difficult to implement because of ground control issues. However, there are other proven methods for underground extraction, like hydraulic jetting and caving that “may be able to take advantage of the geotechnical weakness of the oil sands.” Some of their ideas are mentioned below.
A Continuous Miner could be used, based from surface or from an underground drift. The machine would advance through the deposit while supporting the roof. Once the miner was withdrawn, the roof could be allowed to fail, or could be supported by tailings.
Block Caving would involve developing drifts below the deposit and drawing material out of the drifts. The surrounding material would be allowed to cave in behind the extracted ore. The feasibility of this method would depend on the dip of the deposit (it cannot be flat), thickness, angle of repose of the material, and keeping the material from caving into the production opening.
Placer mines traditionally use Hydraulic Jetting, but this could be applied to oil sands because of its geotechnically weak ore. High pressure water would be fired at the oil sand causing it to wash down or cave into a collection area underground, from which it could be piped out for processing.
A Hybrid Drainage method could be used by developing drifts beneath the deposit to make drainage points for oil extraction. Fan drilling from the drift would give access to the deposit and provide conduits for steam injection or blasting. Oil would then be extracted through the drill holes. This would be particularly beneficial in the carbonate setting where the rock has low permeability.
Once the strip ratio makes surface mining uneconomic, the additional oil sands could be extracted from the bottom of a pit using Auger Mining. The auger could be operated remotely from surface to extract oil sands below the highwall. This method could be modified to be used in an underground setting, augering from drifts to draw material down into the production opening.
Inclined Room and Pillar could be used in shallow dipping or horizontal deposits. It would extract the ore using conventional mining equipment but would leave behind support pillars. This would be difficult to implement in oil sands, as the roof would have to be supported for long periods.
All of these mining methods would require some type of Roof Support. This could be done by freezing the oil sand, as in room and pillar mining or block caving. Another option would be to use a cementing agent mixed with sand tailings for support. Hydraulic supports could be used for temporary support where necessary.
These and other “brainstorming” ideas can be found in the report by Deutsch and Boisvert entitled “A preliminary review of underground mining of currently inaccessible hydrocarbon resources,” Feb. 27, 2006 at www.ptac.org.