By combining existing technologies in a new way, National Oilwell Varco(NOV) came up with a system that recovers more oil from produced water, meets strict discharge specifications, and does it without the use of chemicals. Jennifer Pallanich has the details.
Some produced water treatment systems are not able to meet stringent discharge specifications without relying on chemicals. By adding chemicals such as polymer-based demulsifiers and flocculants to the process, it is possible to grow oil droplets into sizes large enough for the system to pull the oil out of the water.
Mark Wolf, NOV’s director of integrated facilities, onshore, for process and flow technologies, says using de-oiling and de-sanding hydrocyclones in series is more efficient. The setup has the added benefit that it does not rely on chemicals for oil recovery from produced water.
NOV’s Water Wolf Dynamic Oil Recovery (DOR) system is based on the hydrocyclone, but with a more refined set-up than previous systems. Hydrocyclones use pressure-generated centrifugal force to spin out solid particles and oil droplets more rapidly than gravity-based tanks. “We didn’t really reinvent the wheel,” Wolf says. “The problem was not really with the hydrocyclone itself but with the controls. They made it inefficient.”
NOV eliminated the control valves and swapped a conventional pump with their Moyno progressing cavity pump to “pump the water without blending it”, making the whole system more efficient. “Hydrocyclones work very well when pushing a lot of flow and pressure through them, but when that flow and pressure drops, they’re not nearly that efficient,” says Wolf, a petroleum engineer who has been involved in steam flood projects since the beginning of his career. “The faster it spins, the more centrifugal force on the water and the more the oil droplets will separate out. Higher pressure means better spin, which means better separation.”
Sending the oil droplets through the progressing cavity pump creates larger droplets that are easier to separate out. The key, Wolf says, is that the Water Wolf system, sitting on an eight-foot by 26-foot (2.5 by eight metres) skid can accomplish more efficient oil separation from water in a single stage than the typical two or three-stage process, which takes up much more space.
Conventional treatment methods use polymer-based chemicals to help grow oil droplets. These chemicals act like oil magnets and hold, or agglomerate, the droplets together in a flock. The resulting “oily sludge” is more of “a waste stream than a product stream,” he says.
Wolf says conventional tanks will remove oil droplets that are 40 microns（μm） and larger, while the WaterWolf system will remove oil droplets as small as 10 microns.
The combination of the NOV Moyno pump and the de-sanding cyclone reduces the fraction of oil droplets under 10 microns and increases the proportion of molecules that are over 10 microns. The result is a “clean, uncontaminated oil that can be sold”, Wolf says.
The WaterWolf DOR concept had been floating around at the UK’s Merpro Group since about 2010. NOV acquired Merpro in April 2011, and a few months later agreed to fund construction of a pilot WaterWolf skid. The skid was tested in Ventura, California, for a year starting in September 2012.
NOV added a de-sanding cyclone to the de-oiling cyclone and then moved the pilot skid to a heavy oil field outside Bakersfield, California, in October 2013. It later moved to Wyoming for a third field test, which started in April 2014.
SPIN CYCLE: The WaterWolf Dynamic Oil Recovery system combines de-oiling and de-sanding hydrocyclones with the non-shearing action of NOV Moyno progressing cavity pumps to treat water.
In the Wyoming field trial, an operator chose to use the WaterWolf system for environmental reasons. The operator had been spending about $0.08 per barrel for chemical treatment to take outlet oil concentrations from between 400 and 500 parts per million down to 150 ppm.
In addition, the operator had high dissolved carbon dioxide and hydrogen sulfide in the produced water, and these gases came out of solution when treated at atmospheric pressure. The WaterWolf system reduced the oil concentration to 20 ppm without the use of chemicals, according to NOV.
Further, Wolf says, the system was able to treat the water without bringing it to atmospheric pressure, thereby making it possible to re-inject the gasses into the ground.
Earlier this year, a system started operating in Terrebonne Bay, Louisiana. That system is reducing oil in produced water to 30 ppm from a starting level of 200-300 ppm. The operator opted for the WaterWolf system because the existing tank and chemical method was leaving 200 ppm of oil in the water, and injecting that water was fouling disposal wells, Wolf says.
NOV is manufacturing the skids at its facility in Harvey, Louisiana. Wolf expects NOV to offer three different sizes of the system to treat 6000 barrels per day of water, 16,000 bpd, and 50,000 bpd.
Currently, only the 16,000 bpd size is available in a system NOV refers to as the DOR175.
The DOR500, designed to treat 50,000 barrels of water per day, “includes some really neatpump technology”, Wolf says. “Our Moyno progressing cavity pump group stepped up with a pump that can give us three times the capacity in a smaller footprint.”
Wolf says the system is simple to operate — requiring only the pressing of a button to switch it on or off — and to maintain. He sees the WaterWolf DOR as being useful for standalone treatment or as a pre-treatment on produced water before additional technologies such as desalination are applied.
“The produced water market is huge,” Wolf says. “Water produced globally is around 350 million bpd of water and it’s growing at an incredible rate. Even when drilling and completion activity slows down, the wells in service produce water, and they continue to produce more water every year.”
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