New Process For Reusing Produced Wastewater

Introduction:

The Liaohe oilfield is located in Liaoning province, in north-east China. It is the largest heavy oil production oilfield in China with 10,000 oil wells and a production capacity of 250,000 BPD (12,500,000 T/year).

The steam stimulation process, used for heavy oil recovery at the Liaohe oilfield, results in large volumes of water being returned to the surface with the oil. The water to oil ratio is greater than 4. The wastewater produced after oil separation, the socalled produced water, is hot and saline and cannot be discharged to surface watercourses according to the newly established (1998) Environmental Protection Law of the Chinese Government. It must be either reused for steam generation, or injected into abandoned wells. Reuse offers several advantages: Steam generation fuel costs are reduced since the water is preheated to 60°C. Moreover, the oil content is also of value. This approach was also very attractive in this case, since there is a limited supply of fresh water available for use as boiler makeup.

More than ten produced water treatment plants have been installed and commissioned over past decade in China, but all of them are only capable of meeting the "double ten" water standard. This means that the oil and suspended solids in the discharged water may be as much as 10 mg/L each with no reduction in COD or SiO2 levels. As a result, the treated wastewater can only be injected into underground disposal wells.

The produced wastewater at the Liaohe oilfield has a number of characteristics which make effective treatment difficult. The strict treatment target requirements specified in the Chinese National industrial standard for feedwater to steam generators are 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.

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