PWS board member Chip Westaby discusses key considerations operators should make when choosing between water treatment systems, as produced water quality and discharge regulations vary widely.
Reading reviews of water treatment methods and attending conferences will expose people to details of many treatment methods. Invariably, attendees will see a review or hear discussion of new or novel technologies used to solve a particular water treatment issue.
One thing to keep in mind is that every oilfield is unique. Production methods, water properties and water discharge requirements can vary widely around the world and even from site to site within a local region. Novel water treatment systems are often developed where oil production and discharge standards are more homogenous throughout an area. These treatment systems may have better efficiency and performance at local sites than those used in fields elsewhere. For instance, the process to inject and recycle high-temperature water in the Canadian oil sands is completely different than that used to treat produced water from an offshore gasfield for overboard discharge.
Raw produced water characteristics and desired post-treatment water quality must be known to adequately evaluate different water treatment technologies. Analytical methods and testing frequency must be known when collecting data on water characteristics. With enough data, a treatment system can then be evaluated for use at a specific oilfield.
Some soluble compounds in the water tend to be stable. However, oil concentrations can vary significantly and change quickly. Oil and water do not mix. Oil can be sticky and can form abnormal globs, large droplets and coat solids. Conditions in a treatment system can quickly change when large droplets or globs of oil pass through differently than ‘normal’ free-floating oils. Soluble compounds should be tested regularly to understand whether a process is changing, especially with a change in water chemistry. Oil concentrations should be tested more frequently to catch process upsets that can occur suddenly.
Due to the nature of produced water, analytical technologies must be carefully considered. For instance, oil can coat optical and electrochemical sensors, causing errors in soluble compound measurement and resulting in the need for high levels of maintenance. Additionally, all oil measurement technologies respond differently to various oil types. For instance, gravimetric technologies have higher responses to heavy oils than light oils. To make things more complex, different solvents used to prepare oil-in-water samples will extract different oils, causing a change in oil measurement.
Once the details of a study are known, a decision can be made as to how to integrate a water treatment system into a particular production area.