Water treatment for the largest oil and gas producer in China

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Introduction
The largest oil and gas producer in China explores several heavy oil production oilfields by means of Steam Assisted Gravity Drainage (SAGD). This is an enhanced oil recovery technology for producing heavy crude oil. Steam is injected into the upper well to lower the oil viscosity in the reservoir. The oil mixed with hot water flows to the lower well, from which it is then pumped to the surface. This process results in large volumes of water being returned to the surface with the oil with water to oil ratios of more than 4. The wastewater produced after oil separation, the so-called produced water, is hot and saline and cannot be discharged and needs to be re-injected or reused for the process. For this location the client has chosen to reuse the water, app. 20.000 m3/day, meaning that the water needs to be treated up to a very stringent quality for the steam generators.

System design
The strict local treatment target requirements for feed water to steam generators were particularly challenging. According to this standard, it is necessary to achieve <2 mg/L oil, <2 mg/L total suspended solids (TSS) and <0.1 mg/L total hardness (as CaCO3) before steam generators. Besides that the system should also be capable of handling fluctuations in water quality and flow.

Treatment Sequence

  • Buffer tank, for buffering temporary fluctuation in water flow rate and composition.
  • Skimmer tank, for removal the majority of the oil from the water.
  • Dissolved air flotation (DAF), these units removes large quantities of oil and suspended solids to low levels.
  • Sludge dewatering, the sludge, which is mainly created and collected from the two DAF units, is dewatered with filter presses.
  • Mechanical Clarifiers, these units were designed to remove silica by reaction with magnesium salts.
  • Walnut Shell Filters, to remove most of the remaining oil and suspended solids after DAF.
  • Media filters, the media filters were designed to act as polishing filters to achieve the final objective.

Summary
After completion and commissioning the system has proven to be very reliable and simple to operate compared to other systems. The DAF units, which remove the bulk of the TSS and oil and the micro-media filters, achieved a very high removal efficiency (>98%) under the most extreme conditions.


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