Establishing Best Practicable Environmental Option Practice for Produced Water Management in the Gas and Oil Production Facilities

Abstract

Abstract With the increased volume of produced water handled in the North Sea, the environmental issues are becoming a major concern in the industry, especially with the possibility of further reduction in the oil content allowed in the discharged water, as well as the fact that produced water contains a number of undesirable toxic components. These components are either added in the process of treating the produced fluid or some are already present naturally in the water produced from the reservoirs. Using most of the alternative options available to handle produced water, a procedure has been established to determine the Best Practicable Environmental Option (BPEO) for handling produced water both in mature fields and future developments in general terms. The BPEO takes into consideration the nature of the water, technology limitations, both emission to the atmosphere and discharges into the sea, nature of the discharges, safety concerns and cost, as well as establishing any environmental gains in each case. In this procedure a systematic empirical ranking and indicator tool has been applied to the different aspects of the alternative options considered. In addition, the strengths and limitations of BPEO procedure will be addressed for specific cases offshore, in terms of oil production facilities and gas / condensate production facilities. Introduction In the North Sea, the most common conventional method for pressure maintenance and increased oil recovery is by means of injecting seawater into the different reservoirs. With the gradual depletion of North Sea reservoirs, there is more water recovered, partially formation water (the water naturally present in the reservoir) and partially the seawater or flood water which has been injected over the years for pressure maintenance. Produced water composition is quite complex and highly variable. Produced water consists of a number of minor amounts of organic and inorganic constituents. It contains dissolved and dispersed oils, is often highly concentrated in salts and it is normally free of oxygen. The salt content in the North Sea produced water may vary from fresh water in some gas / condensate gas fields to about 10 times that of the sea water. Although there are heavy metals in the produced water, the inputs of heavy metals to the North Sea from produced water are minor compared to the total influx of heavy metals to the North Sea. Additionally, produced water may contain low levels of naturally occurring radionuclides, primary 226Ra and 228Ra at the part per trillion levels. Again this amount of radium represents less than 0.04% of the total input of radionuclides into the North Sea. Produced water is a secondary product out of the oil and gas production industry both onshore and offshore. The most common route and method for disposal of this by-product in the oil and gas industry onshore in Western Europe and North America has been ground disposal. However, the offshore oil and gas industry in the North Sea has not taken the same approach for disposal over the years. Until recently, discharging the produced water overboard into the sea has been the most common method of disposal and is still the preferred route. Alternative options of disposal in the North Sea have not been considered to be viable, due to flexibility, cost and availability of suitable reservoirs and wells for downhole disposal. For a number of reasons, re-injection or disposal of produced water into suitable reservoirs has not been considered in the North Sea and in a number of offshore operations elsewhere. These reasons include, risk of plugging wells and reservoir souring, which were among others in considering to use produced water for re-injection, as well as the additional cost of providing dedicated wells for disposal in an offshore environment.