Overview of Technologies for Oilfield Produced Water Treatment
Introduction to Oilfield Produced Water
Produced water is a multiphase system extracted from oil reservoirs along with crude oil, after undergoing initial processes such as crude oil demulsification and separation. It contains suspended solids, oil, dissolved gases, and dissolved salts. The impurities in produced water mainly consist of five categories: suspended solids, colloidal particles, dispersed oil, free oil, emulsified oil, and dissolved substances.
Oilfield Produced Water Treatment Process
In the field of oilfield produced water treatment in China, two main processing methods exist: conventional treatment and deep treatment. For high-permeability oil reservoirs, conventional treatment is the common choice. Typically, gravity separation for oil removal and single or double-layer filter media filtration are employed.
For low-permeability oil reservoirs, deeper treatment processes are generally implemented to meet higher standards. These deep treatment processes mainly involve secondary or tertiary filtration operations.
In the specific treatment process of produced water, the oil removal process is the key stage. It mainly includes four types of oil removal methods: gravity separation, pressure separation, flotation, and hydrocyclone separation.
After treatment, produced water that meets the required standards is reinjected into the reservoir. This process has significant implications: on one hand, it effectively suppresses clay mineral expansion, significantly reducing potential damage to the reservoir; on the other hand, it helps protect the environment by reducing pollution risks and saving freshwater resources, thereby offering significant economic and environmental benefits.
In summary, the adoption of advanced and efficient produced water treatment technologies is a critical and indispensable task for the development and sustainable operation of oilfields.
Oilfield Produced Water Oil Removal Technology
Water injection plays a crucial role in oilfield development. Its primary purpose is to replenish the reservoir energy and maintain a stable balance of reservoir pressure, ensuring long-term high and steady production. However, injecting untreated wastewater into the reservoir can cause significant damage. This damage is primarily due to the presence of bacteria, mechanical impurities, and iron precipitates, which can block the reservoir, increase injection pressure, and reduce injection volume, severely affecting the efficiency of waterflooding in oil recovery. Therefore, it is essential to purify the water injected into the oil reservoir.
Since the wastewater originates from the oil reservoir, the main goal of oilfield produced water treatment is to remove oil and suspended solids. In general, this treatment process can be divided into two stages: the first stage is the oil removal stage, where oil-water separation is achieved using density differences between oil and water, as well as demulsifying and flocculating agents; the second stage is the filtration stage, where filter media adsorb and intercept suspended solids, oil, and other impurities in the wastewater, preventing them from passing through the filter layer. The oil removal stage requires selecting the appropriate treatment methods based on the physical properties of crude oil in the wastewater, such as density and freezing point. Currently, both domestic and international technologies used in the oil removal stage include gravity oil separation tanks, pressure settling oil removal, flotation oil removal, and hydrocyclone oil removal.
1. Gravity Oil Separation Tank Technology: This technology relies on the relative density difference between oil and water to achieve oil removal. When oil-contaminated wastewater enters the oil separator, larger oil droplets naturally float to the surface due to buoyancy, while emulsified oil is broken down by a demulsifier (or flocculant) and coalesces into larger oil droplets. After a certain retention time, most of the crude oil floats to the top of the separator and is removed. This technology is characterized by a large tank size and long retention time, which ensures that even if there are sudden changes in inflow or water quality, it will not significantly affect the effluent quality. However, it has the drawback of occupying a large area and limited ability to remove emulsified oil.
2. Pressure Settling Oil Removal Technology: This technology involves filling the oil removal equipment with materials that promote the coalescence of oil droplets. When the oil-contaminated wastewater passes through the coalescing material layer, small oil droplets coalesce into larger ones, which accelerates the oil rise speed, shortens the retention time, and reduces the equipment size. This technology combines the coalescing plate technique, significantly improving the oil removal efficiency. However, compared to the gravity oil separator, it has poorer adaptability to changes in water inflow and quality.
3. Dissolved Air Flotation Oil Removal Technology: This technology generates numerous fine air bubbles in the oil-contaminated wastewater, which cause suspended oil droplets (ranging in size from 0.25 to 25 μm) and solid particles to adhere to the bubbles and float to the water surface, thus removing the oil and suspended solids. The application of flotation technology can significantly increase the flotation rate of suspended oil droplets and solid particles, reducing treatment time. This method is characterized by large treatment capacity and high efficiency, making it suitable for heavy oil fields and wastewater with high emulsified oil content.
4. Hydrocyclone Oil Removal Technology: This technology uses the density difference between oil and water and relies on centrifugal forces created by high-speed liquid rotation to separate oil from water. Its advantages include a small equipment size and high separation efficiency. However, it has limited applicability to oil-contaminated wastewater with crude oil having a relative density greater than 0.9.
In recent years, with the advancement of science and technology and the unrelenting efforts of oilfield water treatment experts, produced water treatment technologies have continued to develop and innovate. Many new treatment technologies with high efficiency, good quality, and strong reliability have emerged.
