Centre of excellence
Produced water re-injection (PWRI), the risk of reservoir souring and the need for understanding the mitigation process.
By Anthony Dinning PhD, Frøydis Garshol MSc, Carsten Schwermer MSc
The drive for produced water re-injection (PWRI) in the Norwegian Sector has been aided by the expert competence of Aquateam AS. Aquateam has been active with respect to offshore systems for more than 20 years, aiding in the optimisation of water injection, calibrating and optimising produced water treatment, sand removal, solids handling and performing technology efficiency tests. The company was originally developed as a centre of excellence in technology transfer. Now in its 25th year Aquateam has a team of highly experienced scientists and engineers dealing with every aspect of water treatment from drinking water purification to produced water re-injection. Since 1996, Aquateam has been active in R&D within PWRI and has been used by operators from Brazil to the Norwegian Sea, including both onshore and offshore installations.
Authorities within OSPAR have tightened the discharge limits from production facilities. This is mainly due to the development of oil production from environmentally sensitive areas (Arctic, Lofoten, Barents Sea) and increased water production from mature fields. These are some of the main factors behind the demand for the implementation of PWRI.
It is commonly known that reservoir souring occurs due to seawater injection over a long period of time. The observation of this is dependant upon the pore volume of the field and the degree of scavenging minerals within the reservoir. With the implementation of PWRI, the observed onset of souring increases as the concentration of available nutrients increase within the produced water being reinjected. This has been the cause of stoppage of PWRI in some fields.
The risk of souring is one of the main problems with PWRI and mediates the demand for a mitigation programme. Aquateam has been involved with souring mitigation, and has been used in projects, which have utilised both nitrate and nitrite based mechanisms. In the process of souring, sulphate-reducing bacteria (SRB) produce sulphide from sulphate. This either remains as iron sulphide complexes in the produced water or partitions into the gas phase as H2S. With the addition of nitrate or nitrite, the Nitrate reducing Bacteria (NRB) utilise nitrate as an electron source, and in the process oxidise available sulphide to sulphate. This has shown to be successful in seawater systems and has been widely used by StatoilHydro and BP.
Implementation of nitrite based mitigation on Ekofisk showed that nitrite could be added batch wise and maintain control over souring for up to a period of eight days. These studies were performed offshore in a souring mitigation cabinet (SMC). Ekofisk is a high temperature oilfield and the mechanism of mitigation in the bacterial population is not entirely understood with the implementation of nitrate / nitrite. On Draugen, (moderate temperature oilfield) the utilisation of nitrate mediated souring mitigation showed a good effect. However, the PWRI system experienced elevated corrosion rates during nitrate application. This was further monitored using the SMC, fitted with a corrosion monitoring sidestream. With the application of nitrate, corrosion rates increased dramatically. Upon the addition of biocide to the system, corrosion rates were shown to reduce. This suggested that the corrosion mechanism with the addition of nitrate was microbiologically influenced.
The characterisation of the microbial community and how this changes as a result of the mitigation strategy is the key to understanding and successfully solving the challenges of reservoir souring and microbial influenced corrosion (MIC) during PWRI. Our knowledge of the mechanisms causing reservoir souring and MIC is still limited. Through the characterisation of microbial communities and microbial activities it might be possible to enhance this knowledge further. Aquateam has initiated a research project in collaboration with the industry and the Norwegian Research Council entitled: ‘Sustainable PWRI through application of the PWRI bio-toolbox’, in order to investigate the mechanisms causing reservoir souring and MIC, and to establish safe mitigation methods.
To assess the mechanisms for reservoir souring and reservoir souring mitigation it is important to perform laboratory experiments that correlate to the situation in situ. Since a continuous flow-through system will provide a more realistic means of simulating offshore operation than static bottle tests, flow-through tests in packed bed columns are currently being performed in our laboratories.
In addition, experiments are set up to investigate the mechanisms for MIC using microsensors. Microsensors are needle shaped devices that can be applied to detect physiochemical gradients (O2, H2S, NO3-, NO2-, N2O, pH, etc), in biofilms on metal surfaces biofilms. Due to their small tip-diameter (1-200 μm), concentration profiles can be precisely measured with minimal disturbance of the sample structure. From these, fluxes and local volumetric rates of biochemical transformations can be deduced. Microsensors are applied combined with molecular methods to monitor processes in MIC. This is being used at Aquateam to develop and give a better understanding to the application of nitrate in PWRI systems, which may be prone to elevated MIC as a result of souring mitigation. The information received from the processes occurring between the biofilm and the metal surface will allow for an effective PWRI implementation with a guarantee towards maintaining a sweet reservoir and good integrity in the PWRI system and associated flowlines.
Although valuable microbial information can be gathered in growth experiments, it is only possible to grow a small portion of the population in the laboratory. Therefore information about the un-culturable organisms is lost. Despite the fact that these organisms cannot be cultured, their DNA is still present and intact. By analysing the DNA it is possible to gather information about the total microbial community. We can find out who they are, their population number, and more importantly what they do. In addition to the flow-through and biofilm/corrosion/microsensor experiments DNA analyses including clone libraries and DNAfingerprints will be performed.
The achievement of this project and the 12 years experience that Aquateam has with respect to the broader aspects of PWRI implementation (solids handling, effect on environmental impact, erosion problems, equipment optimisation, compatibility testing etc) and with the ongoing experience from the field, has helped to make Aquateam a centre of excellence for PWRI consultancy and reservoir microbiology.
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