it is one of the biggest and growing challenges in oil industry and in all oilfields all over the world, many researches and organizations tried to study this important matter and to give solutions about “how to deal with produced water, how to treat it and how to create new technologies to deal with this challenge”.
What is Produced Water?
It is the largest byproduct stream from oil industry, it contains associated water “which is mainly reservoir water or water injected to the well”, and the water used in treatment such as wash water used to reduce the salt content in wet crude stream, according to the mentioned data; we can define produced water in this brief equation:
Produced Water = Associated Water + Wash Water
It is to be mentioned that some of the produced water called “Free Water” will be disposed before wet crude treatment facility, using Three – Phase separators or while settling in storage tanks before treatment.
Produced water contains soluble and insoluble organic compounds, dissolved solids, production chemicals (corrosion inhibitors, surfactants etc.) and solid particles due to leaching of rocks and corrosion of pipelines. The methods available for treating produced water are physical, chemical, biological and membrane treatment processes.
Produced water may also contain chemicals used during drilling or processing crude oil.
In this sketch, you will understand the produced water stream.
Why Removing Produced Water?
Removing produced water will help allowance of more weight and space in equipment designs and installations production, storage and offloading units, and may reduce the retention time.
But the most important thing that produced water will cause further problems when react with acidic gases such as Hydrogen Sulfide H2S, or Carbon Dioxide CO2.
One other thing is that the salts in produced water may settle in furnace or heat exchanger pipes, which will reduce heat transfer efficiency; thus, reducing the amount of heat transferred to the crude oil, this thing will also lead to a damage in these pipes due to high pipe skin temperature.
Another disadvantage is that the existence of water in oil will lead to increase the energy required to heat crude oil
see our Produced Water Treatment Books section
How Much Produced Water we produce?
The US Department of Energy’s Argonne National Laboratory estimates the volume of produced water in the United States at 21 billion bbl a year, Additional production from the rest of the world is estimated at a volume of more than 50 billion bbl a year, these amounts are will continue to increase globally.
Produced Water Treatment Cost
Estimated costs for produced water treatment vary widely across regions. MacLeod presented cost comparisons with the caveat that there are inconsistencies in the criteria used to define treatment: US onshore, USD 0.05 to USD 0.30 per bbl; North Sea, USD 0.19 to USD 3.40 per bbl; and Poland shale gas, USD 80 per bbl. Clearly, as the market prices for oil and gas fluctuate, the ability to contain these costs remains a high priority concern to producers aiming to optimize recovery from reservoirs, extend field life, and maximize returns.
How to deal with Produced Water
Some of the options available to the oil and gas operator for managing produced water might include the following:
- Avoid production of water onto the surface – Using polymer gels that block water contributing fissures or fractures or Downhole Water Separators, which separate water from oil or gas streams downhole and reinject it into suitable formations. This option eliminates wastewater and is one of the more elegant solutions, but is not always possible.
- Inject produced water – Inject the produced water into the same formation or another suitable formation; involves transportation of produced water from the producing to the injection site. Treatment of the injectate to reduce fouling and scaling agents and bacteria might be necessary. While wastewater is generated in this option, the waste is emplaced back underground.
- Discharge produced water – Treat the produced water to meet onshore or offshore discharge regulations. In some cases, the treatment of produced water might not be necessary.
- Reuse in oil and gas operations – Treat the produced water to meet the quality required to use it for drilling, stimulation, and workover operations.
- Consume in beneficial use – In some cases, significant treatment of produced water is required to meet the quality required for beneficial uses such as irrigation, rangeland restoration, cattle and animal consumption, and drinking water for private use or in public water systems.
Produced Water Treatment Process
Choosing the appropriate process is another challenge in this matter, but in general; it depends on certain factors such as company policy, capital cost, weight and space limitations, amount of produced water, environmental regulations. However, each company tries to use the best available technology.
Here, we will not explain each process of produced water treatment, but will specify the main points in each process.
The selection of water treating technology and chemical treatment requires knowledge of the water to be cleaned and the contaminants to be removed across the whole reservoir.
The general objectives for operators when they plan produced water treatment are:
- De-oiling – Removal of free and dispersed oil and grease present in produced water.
- Soluble organics removal – Removal of dissolved organics.
- Disinfection – Removal of bacteria, microorganisms, algae, etc.
- Suspended solids removal – Removal of suspended particles, sand, turbidity, etc.
- Dissolved gas removal – Removal of light hydrocarbon gases, carbon dioxide, hydrogen sulfide, etc.
- Desalination or demineralization – Removal of dissolved salts, sulfates, nitrates, contaminants, scaling agents, etc.
- Softening – Removal of excess water hardness.
- Sodium Adsorption Ratio (SAR) adjustment – Addition of calcium or magnesium ions into the produced water to adjust sodicity levels prior to irrigation.
- Miscellaneous – Naturally occurring radioactive materials (NORM) removal.
Produced water is typically treated in stages, the primary treatment is to separate it from oil by any separation equipment such as three phase separators or cyclones.
Then, produced water enters the second stage, which is filtration, from the name we can assume that filters are used in their different sizes and shapes, some other methods are used for the same purpose such as coalescence or Induced Gas Floatation known as IGF Unit.
Coalescence are done using a vessel with 2 or 3 types of wire mesh in different sizes and heights, these structures traps oil droplets making them in one layer on the surface of the produced water, then overflow over a weir to the oil section.
If you want to re-inject the produced water to the reservoir, you have to add some chemicals to enhance the quality and to prevent problems, these chemicals include:
- Corrosion or Scale Inhibitor: to prevent corrosion in equipment and pipes due to the reaction between the acidic gases and produced water.
- Biocide: kills the bacteria and mosses, these may cause plugging in the reservoir and reduce the porosity.
- Oxygen Scavenger: which expel the oxygen droplets; these droplets may hit the internal surface of the pipe, explode and cause damage day after day.
Resources:
- Produced Water Treatment: Yesterday, Today, and Tomorrow workshop – RFC Roundup.
- Technical Summary of Oil and Gas – Produced Water Treatment Technologies / J. Daniel Arthur, Bruce G. Langhus and Chirag Patel.
- State of the Art Treatment of Produced Water – Rangarajan T. Duraisamy, Ali Heydari Beni and Amr Henni
