Introduction to Oilfield Produced Water

Oilfield produced water is formed during the extraction of crude oil from underground reservoirs, where it undergoes initial processing stages such as demulsification and separation. This wastewater contains a mixture of suspended solids, oil, dissolved gases, and dissolved salts. The impurities in the oilfield produced water mainly consist of five categories: suspended solids (SS) and colloidal particles, dispersed oil, floating oil, emulsified oil, and dissolved substances.

The Treatment Process of Oilfield Produced Water

In the field of domestic oilfield produced water treatment, there are two main technological approaches: conventional treatment and advanced treatment. For high-permeability oil reservoirs, conventional treatment processes are commonly adopted. Specifically, methods such as sedimentation for oil removal and single or dual-layer media filtration are typically employed.

Oil removal is a critical step in the produced water treatment process, encompassing four primary types: gravity oil removal, pressure oil removal, flotation oil removal, and hydrocyclone oil removal. As for the filtration process, it mainly relies on single or dual-layer media filtration.

After treatment, the effluent that meets the standards will be reinjected into the formation, which has multiple significances. On the one hand, it can inhibit the swelling of clay minerals and significantly reduce potential damage to the reservoir. On the other hand, from an environmental perspective, it helps minimize the risk of pollution. At the same time, it conserves water resources, yielding substantial economic and environmental benefits.

In conclusion, adopting advanced and efficient produced water treatment technologies is a crucial and indispensable task for the development and sustainable growth of oilfields.

The Characteristics of Tuha Oilfield

Most developed oil reservoirs in the Tuha Oilfield are characterized by low porosity and low permeability. The reservoirs exhibit multi-layer systems with significant heterogeneity, accompanied by serious issues such as water sensitivity, salt sensitivity, and velocity sensitivity. Given the “low-porosity and low-permeability” reservoir physical properties, the Tuha Oilfield imposes stringent standards on injected water quality. If substandard water is injected into the formation, it will reduce the water absorption ability of the formation, adversely affecting the effectiveness of water flooding and the normal progress of water injection development.

If the oilfield's water injection ability cannot meet the requirement of injection-production balance, it results in low formation pressure levels, preventing the full realization of the production potential. During the process of reinjecting treated produced water, water quality gradually deteriorates. SS, corrosion products, bacteria, and other impurities are reinjected into the formation along with the water, leading to the blockage of the formation pore. The water absorption capacity of injection wells shows a declining trend, with some wells consequently in a state of under-injection. Once the dissolved oxygen content, bacterial content, and carbon dioxide content in the injected water exceed standards, the corrosiveness of the injected water increases. Corrosion of water treatment equipment mainly manifests as perforation corrosion at weld connections.

Oil Removal Technology for Oilfield Produced Water

Water injection plays a critical role in oilfield development, with its core purpose being to replenish formation energy and maintain a stable balance in reservoir pressure, thereby ensuring long-term, high, and stable production from the oilfield. However, if untreated produced water is injected into the reservoir, formation damage will occur. Such damage primarily manifests as: proliferating bacteria, mechanical impurities, and iron precipitates, which can clog the reservoir. This leads to increased injection pressure, reduced injection rates, and severely impacts the efficiency of water flooding in displacing crude oil. Precisely for these reasons, the purification of water intended for reservoir injection is imperative.

Given that the wastewater originates from reservoir production, the primary objectives of treating produced water for reinjection are oil removal and SS removal. In general, this treatment process can be divided into two stages. The first stage is oil removal, which utilizes the density difference between oil and water along with the demulsification and flocculation effects of chemicals to achieve oil-water separation. The second stage is filtration, which relies on the adsorption and interception capabilities of filter media to capture suspended solids, oil, and other impurities on the surface of the media or prevent them from passing through the filter bed. The specific oil removal method employed must be determined based on the properties of the crude oil in the produced water, such as its density and solidification point. Currently, the primary technological methods used in the oil removal stage, both domestically and internationally, include gravity oil separation tank technology, pressure settling oil removal technology, air flotation oil removal technology, and hydro-cyclone oil removal technology.

1. Gravity Oil Separation Tank Technology

This technology relies on the density difference between oil and water to achieve oil removal. When oily wastewater enters the separation tank, larger oil droplets naturally rise to the surface due to buoyancy, while emulsified oil coalesces into larger droplets under the action of demulsifiers (or coagulants). After a certain retention time, most of the crude oil floats to the upper part of the tank and is removed.

The characteristics of this technology include the great volume of the separation tank and the long retention time of the wastewater. Even if the inflow rate or water quality undergoes sudden changes, the effluent quality remains relatively stable. However, its drawbacks lie in the large footprint required and its limited effectiveness in removing 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 oily wastewater passes through the coalescing material layer, small oil droplets coalesce into larger ones, thereby accelerating the rise rate of the oil. This shortens the wastewater retention time and reduces the equipment volume. A key feature of this technology is the integrated application of coalescing inclined plates, which significantly enhances oil removal efficiency. However, compared to gravity oil separation tanks, it is less adaptable to fluctuations in inflow volume and water quality.

3. Air Flotation Oil Removal Technology

This technology generates a large number of small bubbles in oily wastewater. Oil droplets with sizes ranging from 0.25 to 25 μm and SS adhere to these bubbles and rise together to the water surface, thereby removing oil and SS from the wastewater. The application of air flotation technology significantly increases the rising velocity of oil droplets and SS, shortening the treatment time. Its characteristics include high treatment capacity and efficiency, making it suitable for oily wastewater from heavy oil fields and wastewater with high emulsified oil content.

4. Hydro-cyclone Oil Removal Technology

This technology utilizes the density difference between oil and water. When the liquid flow undergoes high-speed rotation, oil and water are separated due to the differential centrifugal forces acting on them. Its characteristics include compact equipment size and high separation efficiency. However, this technology is less adaptable for treating oily wastewater with crude oil relative density greater than 0.9.

In recent years, due to advancements in science and technology and the relentless efforts of oilfield water treatment experts, produced water treatment technologies have continuously evolved and innovated. Many new treatment technologies have emerged, offering high efficiency, superior quality, and enhanced reliability.

5. Sinokle’s Oilfield Produced Water Treatment Technology

Sinokle has creatively integrated hydro-cyclone separation with the latest micro-bubble generation technology, and dissolved gas flotation to develop a novel and efficient technology—Cyclonic Dissolved Gas Flotation Unit (CDFU).

In 2018, two CDFU units were deployed at the Tuha Oilfield for treating produced water. Under influent conditions with oil content ≤ 1000 mg/L and suspended solids ≤ 500 mg/L, the effluent achieved oil content ≤ 10 mg/L and suspended solids ≤ 10 mg/L. This resulted in an oil removal rate of 99% and a suspended solids removal rate of 98%.

On-Spot Equipment

Treatment Effect