If you asked plant operators at Cenovus Energy what they do for a living, you might be surprised at the answer. More than one has quipped that they “run giant water treatment plants that produce a bit of oil.”
For an in situ oil sands producer like Cenovus, water – and proper water management – is critical. About 80% of Canada’s oil sands resource is too deep to be mined using the big trucks and shovels that most people are familiar with. This oil must be recovered using specialized techniques to drill and pump the oil to surface. However, there is a catch.
“Conventional oil has a consistency similar to olive oil. It flows easily,” said Mark Bilozir, director of technology development at Cenovus. “Oil in the oil sands is much thicker. In some areas it can be as hard as a hockey puck. We have to stimulate it to make it flow.”
Cenovus uses a technology called steam-assisted gravity drainage (SAGD) to produce its oil sands assets. Two horizontal wells, one approximately 1.5 m (5 ft) above the other, are drilled into the reservoir. Steam is injected in the top well and spreads out into the reservoir to create a steam chamber. The oil is heated and flows downward to the producer well. It is then pumped to surface.
Water use
Water is obviously an integral part of the process. Its use also is one of the most important metrics for measuring the efficiency of in situ oil sands projects. Companies measure their steam-to-oil ratio (SOR), which simply represents how much steam is required to produce one barrel of oil. Lowering the SOR reduces water use, which in turn reduces emissions and potentially the surface footprint by reducing the size of the water treatment facilities required.
“Water is essential to our business,” Bilozir said. “In fact, the success of our operations depends on our ability to use water efficiently. We’re investing a lot of resources into developing technologies to help minimize our water use and make our processes more efficient.”
Cenovus has an average SOR of about two at its Christina Lake and Foster Creek oil sands projects. This is one of the lowest SORs in the industry, Bilozir said.
The majority of water used in the oil sands is non-potable saline water drawn from underground aquifers. And that water is recycled. In fact, in Alberta it is mandatory to achieve a produced water recycle ratio of 90% for SAGD projects.
One of the ways Cenovus is achieving a high recycle rate is through the use of its patented blowdown boiler technology. “Big” is an understatement when it comes to steam generators in the oil sands. The boilers Cenovus uses produce 175 MMBtu to 250 MMBtu of steam. As with all industrial equipment, they are configured to
run under certain conditions with certain specifications. The average boiler runs at about 80% efficiency. Conventional wisdom states that the remaining 20% leftover water (blowdown water) has to be treated before it can be reused. This adds cost and is energy-intensive.
A new way of thinking
Cenovus was not satisfied with conventional wisdom. “We could take blowdown water and spend a lot of money, chemicals, and equipment to treat and then run it again,” Bilozir said. “But it’s a lot more economic and simple if we can just figure out how to go from one boiler right into another.”
The blowdown water is highly concentrated. Cenovus engineers ran models to determine if using blowdown water as feed water – without treating it at all – would damage the boiler.
“There was a pretty competitive betting pool,” Bilozir said. “There were a number of people that didn’t believe it would work. They were putting money on when the boiler would fail.”
A lot of work was done before the test even started. Engineers discussed the idea with industry experts and vendors, visited other operators, and conducted detailed engineering studies. The trial started at Cenovus’s Foster Creek oil sands project, jointly owned with ConocoPhillips, in 2007 – and is still going strong.
The company initially tested the process on a smaller steam generator capable of generating about 50 MMBtu/hr. Over a test period of 166 days more than 59,000 cu m (2 MMcf) of steam was generated, along with nearly 150,000 b/d of oil using only untreated water.
Mechanical inspections were conducted throughout the test and at the end of each testing stage. Each inspection showed similar results: The boiler was in good shape with minimal scaling. It worked.
Following the pilot, Cenovus ran a successful commercial test for 100 days in late 2010 with a 175-MMBtu steam generator. The company has since incorporated blowdown boilers into future phases of its oil sands projects.
Major benefits
The benefits are clear. By “reboiling” the blowdown water, Cenovus is able to convert more than 90% of the initial feed water into steam. Only 2% to 5% of the original feed water is disposed of with this process; without it about 20% of the original water is disposed of.
Using blowdown boilers also decreases the need for makeup water by about 50% and reduces capital and field costs.
Water management plants are typically one of the largest capital costs associated with in situ oil and gas facilities. By reducing the amount of blowdown water the plant has to treat and handle, Cenovus can reduce the size of the plant – and reduce operating costs by about 15 cents per barrel.
“It’s really a win-win,” Bilozir said. “We don’t need to add chemicals or do further treating. We can reduce capital and operating costs. And we can get more steam with less energy and less water required from the environment.”