Technologies for Oil and Gas Produced Water Treatment a report by Ahmadun Fakhru’l-Razi and Ali Reza Pendashteh Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia
Produced water is the largest waste stream associated with crude oil and natural gas production processes. It is the water brought up from the production well together with hydrocarbons. Oil- and gasfield wastewater contains a number of hazardous substances, including dissolved and suspended oil, aromatics, polynuclear aromatics, phenols, production chemicals, heavy metals and salts. Produced water is a major constraint in the production of oil and gas.1
separation, treatment and disposal, and the environmental impacts of discharging are the main, substantial issues.
Origin of Produced Water
In subsurface formations, naturally occurring rocks are permeated with water, oil and/or gas. In most oil-bearing formations, the rock can be completely saturated with saline water prior to the invasion by and trapping of petroleum. The less dense hydrocarbons migrate to trap locations, displacing some of the water from the formation in becoming hydrocarbon reservoirs. Thus, reservoir rocks normally contain both petroleum hydrocarbons (liquid and gas) and water.1
Produced water is any water that is present in a reservoir with the hydrocarbon resource and is brought to the surface with the crude oil or natural gas.1
• • • • •
The costs of
the method of well drilling; the different types of completion;
whether it is a single or co-mingled zone; the type of water separation technology used;
whether water injection or water flooding is used to enhance oil recovery;
• poor mechanical integrity; and • underground communications.
Global Onshore and Offshore Produced Water Global produced water is estimated at around 250 million barrels/day, compared with around 80 million barrels/day of oil (see Figure 1). The water to oil ratio is therefore around 3:1; i.e. the water cut is about 75 %. The global water cut has risen for the last decade and continues to rise. Produced water is driven up by the maturing of old fields, but is also driven down by better management methods and the introduction of new oil fields.3
To enhance oil recovery, additional water is usually injected into the reservoirs to force the oil to the surface. Both formation and injected water are eventually produced along with the hydrocarbons and, as an oil field becomes depleted, the amount of produced water increases as the reservoir is filled with injected water.
Factors Affecting the Volume of Produced Water Different factors can affect the amount of produced water over the life of a well. These include:2
•
whether the well is located within homogeneous or heterogeneous reservoirs;
Ahmadun Fakhru’l-Razi is Dean of the Faculty of Engineering at Universiti Putra Malaysia, which he joined in 1981. From 2009 to 2010, he was Chair Professor of the Prince Khalid Bin Sultan Chair for Water Research at King Saud University in Riyadh, Saudi Arabia. Dr Fakhru’l-Razi’s research interests are in the areas of environmental engineering, nanotechnologies, and safety and emergency management. He has a BSc Hons degree in chemical engineering and an MSc in environmental pollution control from the University of Leeds, as well as a PhD in environmental engineering from Newcastle University, UK.
E:
fakhrul@eng.upm.edu.my
Ali Reza Pendashteh is a PhD student at the Faculty of Engineering of Universiti Putra Malaysia. In 2000, he joined the Environmental Research Institute of the Iranian Academic Center for Education, Culture and Research as a member of the educational board and a designer of industrial wastewater treatment plants. Previous to that he worked as an environmental inspector in the Iranian Department of the Environment in Rasht. He has a BSc degree in chemical engineering from the Petroleum University of Technology in Ahwaz, and an MSc in chemical engineering from Shiraz University, Iran.
E:
arpendashteh@yahoo.com
Produced water is a mixture of organic and inorganic materials. Its characteristics depend on the nature of the producing/storage formation from which they are withdrawn, the operational conditions and the chemicals used in the processing facilities.4
produced water from different sources can vary by orders of magnitude (see Table 1).
The major compounds of produced water include:5 •
The composition of
dissolved and dispersed oil compounds (dissolved oil compounds: organic acids [formic acid and propionic acid]; benzene, toluene, ethylbenzene and xylenes [BTEX]; and phenols);
• • •
dissolved formation minerals: cations and anions, heavy metals, naturally occurring radioactive materials (NORMs);
production chemical compounds: scale and corrosion inhibitors, biocides, emulsion breakers, antifoam and water-treatment chemicals;
production solids (including formation solids, corrosion and scale products, bacteria, waxes and asphaltenes); and
• dissolved gases: CO2, O2 and H2S.
Environmental Problems Associated with Produced Water Produced water can have different impacts on the environment depending on where it is discharged. For example, discharges to small streams are likely to have a larger environmental impact than discharges made to the open ocean because of the dilution of the chemicals after discharging.
Salinity is the major toxicity contributor of produced water for aquatic environments. Dispersed oil and droplets do not precipitate at the bottom of sea, but rise to the surface of the water. Volatile and/or toxic
104 © TOUCH BRIEFINGS 2011
HSE – Produced Water
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