Improving the quality of produced water discharged in open waters by incorporating a microbiological treatment on operating offshore platforms

Abstract

Produced water is the water that comes along with oil during the reservoir extraction and is the main subproduct of the petroleum industry, accounting for 3 to 5 times the global production of oil. The produced water separated from the oil still contains some oily fractions in its composition, making it necessary for some treatment before its discard (most common practice adopted on offshore platforms) or reuse. The traditional method of treatment is based on the physical separation of oil and water, driven by their density difference. This type of treatment is unable to remove dissolved or small diameter particles dispersed, which turned out to be an issue in face of some new legislation worldwide that lowered the maximum amount of total oil and grease (TOG) the water can have for its discharge. Biological treatment is a widespread method for treating wastewater in several different industries due to its relative simplicity and low operational costs. Nevertheless, as it is a treatment that normally requires large-volume vessels, it is not used on offshore platforms due to a lack of space for such high dimension equipment. The objective of this research was to carry out a literature review using the article and patent search bases to find technical options that allow the use of biological treatment for water produced on offshore oil production platforms. Next, propose a model to incorporate biological treatment and estimate the performance of this produced water treatment model. An arrangement was proposed to convert a cargo tank of an operating offshore oil platform into a microbiological reactor. The conversion could be useful to treat the produced water that has a concentration of oil above the maximum limit allowed to discharge or to promote the additional extraction of a fraction of oil that could not be removed by the conventional treatment plants. The objective of this research was to perform a literature review to find technical options that could allow the use of biological treatment for the produced water in offshore oil-producing platforms, to propose a model to incorporate the biological treatment, and to estimate the performance of this model treating produced water. After researching articles and patents in the literature, it was seen that the Upflow Anaerobic Sludge Blanket (UASB) was the best type of reactor to be incorporated into an offshore platform. The reactor was chosen since it had good performance in treating produced water anaerobically and low sludge production. The UASB reactor most resembled the arrangement of a cargo tank, which was a paramount factor for the feasibility of the process's success. It is expected that the cargo tank converted to an UASB could remove at least an extra 30% of the total oil and grease (TOG) when compared to the traditional physical removal process existent for the produced water. However, it is still necessary to perform some laboratory tests to better predict the removal rate that would be observed in using this microbiological treatment with the high salinity-produced water from pre-salt Brazilian fields.