|Oily water treatment schematics. Click to enlarge.
ABB has developed a water treatment technology that cleans oily wastewater—by far the largest waste product in oil and gas production—quickly, cost-effectively and energy-efficiently.
Oily wastewater, also known as produced water, is basically water that is trapped in underground formations that is brought to the surface during oil and gas production. About seven to ten barrels of produced water are generated for each barrel of oil. The water can be highly toxic and typically contains oil, grease and other hydrocarbons, as well as high levels of salts, metals and trace elements. Its management presents considerable environmental challenges and costs to operators.
|Water before, during and after treatment at one of the ABB de-oiling plants. Click to enlarge.
In addition to the formation water, produced water from gas operations also includes condensed water and has higher contents of low molecular-weight aromatic hydrocarbons such as benzene, toluene, ethylbenzene and xylene (BTEX) than those from oil operations. Studies indicate that the produced waters discharged from gas/condensate platforms are about 10 times more toxic than the produced waters discharged from oil platforms.
Standard oily-wastewater remediation relied for decades on gravimetric lagoon separation, then reprocessing the recovered floatable oil portion and using holding-pond clarification of the wastewater portion before ‘land-farming’ discharge. This approach led to substantial groundwater and air pollution.
Although new configurations of separation technology have expanded oily-wastewater treatment options, and offer better process response, according to ABB, they are incapable of meeting proposed European environmental protection legislation (and also risk noncompliance with the ATEX Directive for processes operating in explosive environments).
Developed on behalf of a leading national oil and gas company, the ABB solution is installed at three oil and gas fields in North Africa where it has exceeded the customer’s specifications for hydrocarbon content and suspended solid concentration in the outlet water by seven and 55 times respectively.
The ABB process comprises three separate treatment cycles:
Water treatment cycle. Produced water is collected in a storage tank. The water is passed through a corrugated plate interceptor (CPI) and then to a flocculation unit, where specific chemicals (flocculant and coagulant) are added. Water is transferred to a flotation unit and the cleaned water is passed through a filter unit before underground injection.
Oil treatment cycle. Floating hydrocarbons on the surface inside the storage tank and the CPI are recovered by oil skimmers (disc-oil) and collected in a recovered oil tank before being sent to an oil production unit.
Mud treatment cycle. Flocs developed inside the CPI and the flocculation unit are sent to the flotation unit. Flocs grow until they become mud, which is recovered by the scraper inside the flotation unit and sent to the thickening machine. Mud collected at the bottom of storage tank, flotation unit and flocculation unit are also sent to the thickening machine.
Compared to the existing technologies for treating produced water, the ABB solution has proved more effective not only in terms of process results but in critical areas like cost, energy efficiency, footprint, speed of installation and ease of operation, according to the company.
Other benefits of the solution include compact footprint, the use of chemicals that can easily be produced on site using inexpensive base ingredients, ease of installation (it is built on skids and then transported to the site for installation and commissioning), and ease of operation (it uses standard equipment and requires no complex skills or operator experience). The solution also saves energy through its compact footprint, minimal use of chemicals and innovative use of gravity.