Developments in Environmental Protection Related to Produced Water Treatments and Disposal (Produced Water Re-Injection)
Authors: Evans R. C. 
Published: December 2007

Produced water re-injection (PWRI) has become an increasingly important topic in the oil industry in the last few years, especially in mature oil producing areas such as the North Sea. It even has its own acronym. The prime causes for this include the increasing volumes of produced water from mature oil fields, the continued drive to reduce CAPEX and OPEX, and the greater concern over environmental protection. In the latter stages of the economic life of an oil field up to 90 % of the produced fluid volume may be water. Detailed planning for the treatment and disposal of produced water is required at an early stage in the development of an oil field to avoid it becoming a bottleneck to production.

Among the perceived benefits of produced water injection are; a reduction in the overall volumes of water treated offshore; that the water should not need deaeration if a closed system is maintained; that it could may reduce the potential operating problems associated with scaling and reservoir souring; that the injection zones may be able to accept a poorer water quality than is specified for surface disposal.

Subsurface produced water disposal has taken place for a number of years onshore where a typical strategy is to pump the produced water, after various levels of treatment, into a high permeability sand or fractured limestone. However produced water re–injection for pressure maintenance will be a more exacting engineering challenge as operational problems will have a direct impact on production and the profitability of the development.

The uncertainties and decisions which will have to be addressed for each produced water reinjection project include; the injection water quality specification; contingency for the surface disposal or storage of the produced water should injection have to periodically halted; the selection and integration of the additional water source which may be required during the early stages of production when the volumes of produced water are insufficient for pressure maintenance; and the design of the produced water treatment and system.

Due to this multidisciplinary nature, a produced water re–injection project will obviously require the formation of an integrated team of reservoir engineers, production chemists, geologists and process engineers.

The key factor in the design of the produced water rejection system will be the injection water quality specification, particularly in comparison to the specification for surface disposal. Considering the continuing debate, after two decades of experience, on water quality for seawater injection, research into water quality specification for produced water re–injection should be targeted as a key area of research if produced water re–injection is to be confidently and successfully applied given the constraints of particularly offshore production. Each field will require its own studies to determine the water quality for produced water re-injection and the appropriate water treatment system design, due to the variations in reservoir geology and the characteristics of produced fluids.